Article

Structural, thermodynamic, thermal, and electron transport properties of single-crystalline LaPt 2 Si 2

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

LaPt2Si2 in a single-crystalline form was subjected to structure, thermodynamic, thermal, and electron transport studies with a special emphasis on the structure phase transition appearing at T=85 K. X-ray diffraction proves the noncentrosymmetric tetragonal structure of CaBe2Ge2-type (space group P4/nmm). The transition manifests as a small step of opposite sign in both lattice parameters, leaving almost no volume change. c decreases and a increases in the low-T phase, but the change of the c/a ratio does not exceed 0.1%. Additional periodicity, related to the supposed charge density wave (CDW) state, can be related to satellites corresponding to the wave vector q≈(0.36,0,0), which start to grow with temperature decreasing below 175 K and almost vanish (or relocate from the investigated ab plane) below the 85 K transition. Electrical resistivity reveals that the 85 K transition is hysteretic in temperature, with the difference between heating and cooling being almost 10 K, proving the first-order type of the transition. The transition dramatically enhances resistivity in the low-T state, pointing to a formation of a pseudogap. This, however, does allow a superconducting state, arising below Tc=1.6 K. The temperature dependence up upper critical field is not compatible with the weak coupling BCS theory. Strong anisotropy of electronic structure and its dramatic changes at the structure transition are manifest also in thermal expansion and thermoelectric power. The Sommerfeld coefficient γ=7.8mJmol−1K−2 and Debye temperature of 205 K could be derived at low temperatures, but the specific heat has a strongly non-Debye like T dependence, which can be ascribed to a low-energy Einstein mode.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... LaPt 2 Si 2 belongs to the MT 2 X 2 (M = rare earth/alkaline earth, T = transition metal, X = Si or Ge) family and crystallizes in the CaBe 2 Ge 2 -type structure similar to the APt 2 Pn 2 (A = Sr, Ba; Pn = As, Sb) superconductors. In this compound, superconductivity (SC) and charge density wave (CDW) coexist [19][20][21][22][23][24]. It has similar physical properties to SrPt 2 As 2 . ...
Article
Full-text available
We have investigated the electrical and magnetic properties of the compounds LaPt2As, LaPt2As2, and BaPt2As2 prepared by conventional solid-state reaction, in the temperature range of 5–300 K, up to the field of H = 9 T. The zero-field cooled magnetization (MZFC) and field cooled magnetization (MFC) split into two branches at the bifurcation temperatures due to large thermal hysteresis effects. A huge diamagnetic magnetization has been manifested on the MZFC branches in both samples. We describe different possible scenarios of the negative magnetization-like behavior. First, this huge diamagnetic signal may arise from some local nano-sized ferromagnetic clusters subjected to the negative internal field, which is originated from the local distortions in the electronic structure due to the inhomogeneous Pt atoms distributions. The second is suggested as an alternative model, so that the present phenomenon was not a true negative magnetization, the observed negative magnetization, in fact, is relative change in the magnetization. It may arise from the freezing spins at lower temperatures. The magnetic analysis reveals the development of superconductivity with a filamentary character at temperatures below 10 K for the sample LaPt2As and 22 K for the sample LaPt2As2. In order to prove the observed superconductivity, their critical current density, Jc(H), and the normalized pinning force, Fp/Fp,max, are obtained as a function of the applied field, and the types of flux pinning centers are identified using the conventional Beans and Dew-Hughes models. We report the resistivity data in the same temperature interval. The resistivity curves are fitted to the form \(\uprho\)(T) = ρ0 + ρ1Tα + ρ2exp (− To/T) over the entire range of the measurement temperatures. The last term presents a magnon-assisted inter-band electron–phonon electron scattering mechanism. In addition, we observe an anomaly around 115 K in LaPt2As2 associated with the charge density (CDW) phase transition.
... Recently, both superconductivity and CDW order were found in the isostructural compound LaPt 2 Si 2 . The presence of CDW order was confirmed by selected area electron diffraction [16] and x-ray diffraction results [21]. In contrast to BaPt 2 As 2 , LaPt 2 Si 2 shows a superconducting dome near the critical pressure P c = 2.4 GPa where the CDW order suddenly vanishes [18]. ...
Preprint
We have studied the superconducting gap structure of LaPt$_2$Si$_2$ by measuring the temperature dependence of the London penetration depth shift $\Delta\lambda(T)$ and point contact spectroscopy of single crystals. $\Delta\lambda(T)$ shows an exponential temperature dependence at low temperatures, and the derived normalized superfluid density $\rho_{s}(T)$ can be well described by a single-gap s-wave model. The point-contact conductance spectra can also be well fitted by an s-wave Blonder-Tinkham-Klapwijk model, where the gap value shows a typical BCS temperature and magnetic field dependence consistent with type-II superconductivity. These results suggest fully gapped superconductivity in LaPt$_2$Si$_2$, with moderately strong electron-phonon coupling.
... Such an interpretation is in agreement with the description for the Fermi surface nesting associated with the CDW phase transition. Similar analyses of κ e have been employed in various CDW materials such as SmNiC 2 , Rb 0.3 MoO 3 , and LaPt 2 Si 2 [21,[45][46][47], for providing insights into the electronic origin for the CDW formation. ...
Article
Full-text available
Copper monotelluride CuTe is of current interest due to the discovery of the quasi-one dimensional charge density wave (CDW) behavior below the transition temperature TCDW≃335K. To explore the transport and thermodynamic properties and provide experimental insights into the underlying origins of the CDW, we have carried out a combined study by means of the electrical resistivity, Seebeck coefficient, thermal conductivity, as well as specific heat measurements on single crystalline CuTe. The CDW phase transition has been characterized by marked features near TCDW from all measured physical quantities. In particular, the observed Seebeck coefficient and electronic thermal conductivity exhibit a pronounced reduction as cooling the temperature across TCDW, indicative of the partially gapped Fermi surfaces associated with the CDW formation. From the examination of the excess specific heat in the vicinity of TCDW, we obtained evidence for the strong-coupling character of the CDW, suggesting that the electron-phonon coupling plays an important role for the CDW instability in CuTe.
Article
We have studied the superconducting gap structure of LaPt2Si2 by measuring the temperature dependence of the London penetration depth shift Δλ(T) and point contact spectroscopy of single crystals. Δλ(T) shows an exponential temperature dependence at low temperatures, and the derived normalized superfluid density ρs(T) can be well described by a single-gap s-wave model. The point-contact conductance spectra can also be well fitted by an s-wave Blonder-Tinkham-Klapwijk model, where the gap value shows a typical BCS temperature and magnetic field dependence consistent with type-II superconductivity. These results suggest fully gapped superconductivity in LaPt2Si2, with moderately strong electron-phonon coupling.
Article
High temperature crystal structure of UPt 2 Si 2 determined using single-crystal neutron diffraction at 400 K is reported. It is found that the crystal structure remains of the primitive tetragonal CaBe 2 Ge 2 type with the space group P 4/ nmm . Anisotropic displacement factors of the Pt atoms at the 2 a (3/4 1/4 0) and Si atoms at the 2 c (1/4 1/4 z ) Wyckoff sites are found to be anomalously large.
Article
This work is focused on the structural and physical properties of CePt2Al2, an intermetallic compound. At room temperature, the modulated orthorhombic structure Cmme(a00)000, with q → = (0.481, 0, 0) has been determined by single-crystal X-ray diffraction supplemented by dependence of lattice parameters above room temperature for which the X-ray powder diffraction was used. The compound undergoes a structural transition to a tetragonal structure above room temperature. This transition exhibits 50 °C hysteresis and creates a domain structure in the sample. The magnetic behavior has been studied by specific heat, magnetization, and transport measurements in the temperature range between 0.5 and 300 K. Specific heat and susceptibility shows an antiferromagnetic order below 2 K. On the basis of electrical resistivity and other bulk measurements, CePt2Al2 can be considered a Kondo lattice material. The presence of a modulated crystal structure opens the possibility of a charge density wave state in CePt2Al2 as observed for (Re)Pt2Si2.
Article
Our study is focused on superconductivity in LaPt2Al2 compound. Specific heat and electrical resistivity measurements reveal a superconducting state below 0.57 K. Superconducting behaviour is in agreement with the weak coupling limit of BCS theory. The electronic Sommerfeld coefficient and equilibrium volume were described by first principles methods. The calculated electron phonon constant, λ=0.48, using the McMillan formula is small. The crystal lattice of LaPt2Al2 is described by the orthorhombic space group Cmme. Polarisation contrast microscopy reveals a domain structure, which is the result of a symmetry lowering structural transition above room temperature.
Article
Using single-crystal neutron and x-ray diffraction, we discovered a charge density wave (CDW) below 320 K, which accounts for the long-sought origin of the heat capacity and resistivity anomalies in UPt2Si2. The modulation wave vector, Qmod, is intriguingly similar to the incommensurate wave vector of URu2Si2. Qmod shows an unusual temperature dependence, shifting from commensurate to incommensurate position upon cooling and becoming locked at aproximately (0.42 0 0) near 180 K. Bulk measurements indicate a crossover toward a correlated coherent state around the same temperature, suggesting an interplay between the CDW and Kondo-lattice-like coherence before coexisting antiferromagnetic order sets in at TN=35 K.
Article
Results of structural, magnetic, thermodynamic, and electron transport measurements on single-crystalline NdPt2Si2 are reported. Temperature variations of crystal structure show two different charge density wave states. The structure modulation with the propagation vector q1=(0.323,0,0) arises below T=300 K. An additional modulation, with propagation vector q2=(0.163,0.163,0.500), appears below T=50 K in a first-order phase transition. This situation was found analogous to isostructural LaPt2Si2 with similar (but not identical) propagation vectors and characteristic temperatures. Nd magnetic moments order antiferromagnetically at TN=1.5 K. Temperature dependence of electrical resistivity for current j applied along the a, b, and c axes is strongly affected by the first-order transition, inducing a Fermi surface gapping.
Article
Full-text available
The superconducting phase transition at \(T_\mathrm{c} = 2.3\) K was observed for the electrical resistivity \(\rho ({T})\) and magnetic susceptibility \(\chi (T)\) measurements in the ternary compound La\(_{5}\hbox {Ni}_{2}\hbox {Si}_{3}\) that crystallizes in the hexagonal-type structure. Although a single-phase character with the nominal stoichiometry of the synthesized sample was confirmed, a small trace of the La–Ni phase was found, being probably responsible for the superconducting behaviour in the investigated compound. The magnetization loop recorded at \({T} = 0.5\) K resembles a star-like shape which indicates that the density of the critical current can be strongly suppressed by a magnetic field. The low-\(T _{\rho }(T)\) and specific heat \({C}_\mathrm{p}({T})\) data in the normal state reveal simple metallic behaviour. No clear evidence of a phase transition to any long- or short-range order was found for \(C_\mathrm{p}(T)\) measurements in the T-range of 0.4–300 K.
Article
Full-text available
A common way of speeding up powder diffraction measurements is the use of one or two dimensional detectors. This usually goes along with worse resolution and asymmetric peak profiles. In this work the influence of a straight linear detector on the resolution function in the Bragg-Brentano focusing geometry is discussed. Due to the straight nature of most modern detectors geometrical defocusing occurs which heavily influences the line shape of diffraction lines at low angles. An easy approach to limit the resolution degrading effects is presented. The presented algorithm selects an adaptive range of channels of the linear detector at low angles, resulting in increased resolution. At higher angles still the whole linear detector is used and the data collection remains fast. Using this algorithm a well-behaved resolution function is obtained in the full angular range, whereas using the full linear detector the resolution function varies within one pattern which hinders line shape and Rietveld analysis.
Article
Full-text available
The intriguing coexistence of the charge density wave (CDW) and superconductivity in SrPt2As2 and LaPt2Si2 has been investigated based on the ab initio density functional theory band structure and phonon calculations. We have found that the CDW instabilities for both cases arise from the q-dependent electron-phonon coupling with quasi-nesting feature of the Fermi surface. The band structure obtained by the band-unfolding technique reveals the sizable q-dependent electron-phonon coupling responsible for the CDW instability. The local split distortions of Pt atoms in the [As-Pt-As] layers play an essential role in driving the five-fold supercell CDW instability as well as the phonon softening instability in SrPt2As2. By contrast, the CDW and phonon softening instabilities in LaPt2Si2 occur without split distortions of Pt atoms. The phonon calculations suggest that the CDW and the superconductivity coexist in [X-Pt-X] layers (X = As or Si) for both cases.
Article
Full-text available
This review deals with the properties of superconductors with competing electron spectrum instabilities, namely, charge-density waves (CDWs) and spin-density waves (SDWs). The underlying reasons of the electron spectrum instability may be either Fermi surface nesting or the existence of Van Hove saddle points for lower dimensionalities. CDW superconductors include layered dichalcogenides, NbSe3, and compounds with the A15 and C15 structures among others. There is much evidence to show that high-Tc oxides may also belong to this group of materials. The SDW superconductors include URu2Si2 and related heavy-fermion compounds, Cr-Re alloys and organic superconductors. We review the experimental evidence for CDW and SDW instabilities in a wide range of different superconductors, and assess the competition between these instabilities of the Fermi surface and the superconducting gap. Issues concerning the superconducting order parameter symmetry are also touched upon. The accent is put on establishing a universal framework for further theoretical discussions and experimental investigations based on an extensive list of available and up-to-date references.
Article
Full-text available
We present new upper critical field Hc2(T) data in a broad temperature region 0.3 K ≤ T ≤ Tc for LuNi2B2C and YNi2B2C single crystals with well characterized low impurity scattering rates. The absolute values for all T, in particular, HC2(0), and the sizable positive curvature (PC) of HC2(T) at high and intermediate T are explained quantitatively within an effective two-band model. The failure of the isotropic single-band approach is discussed in detail. Supported by de Haas-van Alphen data, the superconductivity reveals direct insight into details of the electronic structure. The observed maximal PC near Tc gives strong evidence for clean limit type-II superconductors.
Article
Full-text available
We report on specific-heat, high-magnetic-field transport, and ac-susceptibility measurements on MgB2 single crystals. The upper critical field for magnetic fields perpendicular and parallel to the basal planes is presented in the entire temperature range. A very different temperature dependence has been observed in the two directions, which yields a temperature-dependent anisotropy with Γ∼5 at low temperatures and ∼2 near Tc. A peak effect is observed for μ0H∼2T parallel to the c axis, and the critical current density presents a sharp maximum for H parallel to the ab plane.
Article
Full-text available
Charge ordering and superconductivity are observed in the phase diagrams of a variety of materials such as NbSe3, layered transition-metal dichalcogenides and high-temperature copper oxide superconductors, low-dimensional organics, Ba1-xKxBiO3 and so forth. Although both conventional charge-density-wave (CDW) and superconducting transitions show an energy gap in the single-particle density of states at the Fermi level (EF), their physical properties are poles apart: insulating behaviour for the CDW and zero resistivity in superconductors. Consequently, these two ground states are believed to compete with each other. Here we provide evidence for maximized superconductivity at points in momentum (k) space that are directly connected by the CDW ordering vector. Temperature-dependent angle-resolved photoemission spectroscopy of 2H-NbSe2 across the CDW and superconducting transitions (TCDW33 K and Tc=7.2 K, respectively) shows CDW-induced spectral-weight depletion at the same Fermi-surface-crossing k points, which evolve into the largest superconducting gaps. These k points also show the highest electron–phonon coupling and lowest Fermi velocities. Our results demonstrate that charge order can boost superconductivity in an electron–phonon coupled system, in direct contrast to the prevailing view that it only competes with superconductivity.
Article
Full-text available
High-quality single crystals of Lu2Fe3Si5 and Sc2Fe3Si5 are grown by the floating-zone method, and their temperature dependences of specific heat are measured down to 0.35 K. Both compounds show anomalously small jump in specific heat at the superconducting transition temperature, Tc, and large residual specific heat coefficient even at 0.2 Tc, where BCS superconductor shows negligible electronic contribution. We also find drop in the specific heat coefficient below 0.2 Tc. These characteristics are very similar to MgB2, and temperature dependences of specific heat in both compounds are well reproduced by assuming two superconducting gaps. Upper critical field measurements confirm that both Lu2Fe3Si5 and Sc2Fe3Si5 are weakly one-dimensional superconductors.
Article
Full-text available
In spite of intrinsic limitations, neutron powder diffraction is, and will still be in the future, the primary and most straightforward technique for magnetic structure determination. In this paper some recent improvements in the analysis of magnetic neutron powder diffraction data are discussed. After an introduction to the subject, the main formulas governing the analysis of the Bragg magnetic scattering are summarized and shortly discussed. Next, we discuss the method of profile fitting without a structural model to get precise integrated intensities and refine the propagation vector(s) of the magnetic structure. The simulated annealing approach for magnetic structure determination is briefly discussed and, finally, some features of the program FullProf concerning the magnetic structure refinement are presented and discussed. The different themes are illustrated with simple examples.
Article
Full-text available
We have performed de Haas-van Alphen measurements of the Fermi surface of alpha-uranium single crystals at ambient pressure within the alpha-3 charge density wave (CDW) state from 0.020 K - 10 K and magnetic fields to 35 T using torque magnetometry. The angular dependence of the resulting frequencies is described. Effective masses were measured and the Dingle temperature was determined to be 0.74 K +/- 0.04 K. The observation of quantum oscillations within the alpha-3 CDW state gives new insight into the effect of the charge density waves on the Fermi surface. In addition we observed no signature of superconductivity in either transport or magnetization down to 0.020 K indicating the possibility of a pressure-induced quantum critical point that separates the superconducting dome from the normal CDW phase.
Article
Full-text available
The superconducting transition temperature (${T}_{c}$) is theoretically shown to rise as a result of the presence of the Peierls instability within a simple incomplete-nesting model in two dimensions. It is demonstrated that ${T}_{c}$ can exceed by an order of magnitude that in the absence of the Peierls transition under certain conditions. The implication of this is discussed for the high-${T}_{c}$ superconductors ${\mathrm{La}}_{2\mathrm{$-${}}\mathrm{x}}$${\mathrm{M}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$.
Article
Full-text available
The recent synthesis of the superconductor LaFeAsO(0.89)F(0.11) with transition temperature T(c) approximately 26 K (refs 1-4) has been quickly followed by reports of even higher transition temperatures in related compounds: 41 K in CeFeAsO(0.84)F(0.16) (ref. 5), 43 K in SmFeAsO(0.9)F(0.1) (ref. 6), and 52 K in NdFeAsO(0.89)F(0.11) and PrFeAsO(0.89)F(0.11) (refs 7, 8). These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LnOTMPn (Ln: La, Pr, Ce, Sm; TM: Mn, Fe, Co, Ni; Pn: P, As), because many features of these materials set them apart from other known superconductors. Here we report resistance measurements of LaFeAsO(0.89)F(0.11) at high magnetic fields, up to 45 T, that show a remarkable enhancement of the upper critical field B(c2) compared to values expected from the slopes dB(c2)/dT approximately 2 T K(-1) near T(c), particularly at low temperatures where the deduced B(c2)(0) approximately 63-65 T exceeds the paramagnetic limit. We argue that oxypnictides represent a new class of high-field superconductors with B(c2) values surpassing those of Nb(3)Sn, MgB(2) and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-T(c) copper oxides.
Article
We measured the thermal properties of polycrystalline samples of LaPt2Si2 and PrPt2Si2 using thermopower (S) along with thermal conductivity (κ) in the temperature range 10 K-300 K. Significant anomalies related to charge density waves (CDW) around 112 K and 88 K respectively have been observed in [Formula: see text] in both systems. Analysis of thermopower by a two band model suggests that the observations are consistent with a reduction of electron charge density. A change in slope accompanied by a drop in the value of thermal conductivity has been observed around T CDW in case of LaPt2Si2. Analysis of thermal conductivity of this material suggests that the CDW mainly affects electronic contribution to thermal transport. Only a slight change of slope has been detected in temperature dependent thermal conductivity in the case of PrPt2Si2 around T CDW, while its resistivity shows a clear anomaly which shows that electronic part of thermal conductivity is mainly influenced by the CDW in this case also. It is interesting to note that the lattice contribution to thermal conductivity remains unaffected by the CDWs in both materials.
Article
The thermoelectric (TE) efficiency is evaluated by the material thermoelectric figure of merit (ZT), which can be usually improved by enhancing the electrical transport properties and/or reducing the thermal conductivity. Seeking the material with low thermal conductivity is crucial for thermoelectrics, which enable us simplify complex thermoelectric parameters and focus on the optimization of electrical transport properties alone. Here, we summarized the relationship between anharmonicity and low thermal conductivity in thermoelectrics. Several strategies which yield anharmonicity are also suggested, including lone pair electron, resonant bonding and rattling model. At last, some intuitive methods were proposed and summarized to evaluate the anharmonicity.
Article
The superconducting gap structure of a charge density wave (CDW) superconductor LaPt2Si2 (Tc = 1.6~K) having a quasi two dimensional crystal structure has been investigated using muon spin rotation/relaxation (μSR) measurements carried out in transverse field (TF), zero field (ZF) and longitudinal field (LF) geometries. Rigorous analysis of TF-μSR spectra in the superconducting state corroborates that the temperature dependence of the effective penetration depth, λL, derived from muon spin depolarization, fits to an isotropic s+s−wave model suggesting that the Fermi surface contains two gaps of different magnitude rather than an isotropic gap expected for a conventional s−wave superconductor. On the other hand, ZF μSR data do not show any significant change in muon spin relaxation rate above and below the superconducting transition indicating the fact that time-reversal symmetry is preserved in the system.
Article
We present results of our comprehensive studies on single crystalline LaPt2Si2. Pronounced anomaly in electrical resistivity and heat capacity confirms the bulk nature of superconductivity (SC) and charge density wave (CDW) transition in the single crystals. While the charge density wave transition temperature is lower, the superconducting transition temperature is higher in single crystal compared to the polycrystalline sample. This result confirms the competing nature of CDW and SC. Another important finding is the anomalous temperature dependence of upper critical field H C2(T). We also report the anisotropy in the transport and magnetic measurements of the single crystal.
Article
Interplay between a charge density wave (CDW) and superconductivity in LaPt2(Si1-x Ge x )2 has been studied by electrical transport and magnetic measurements. LaPt2Si2 crystallizes in CaBe2Ge2 type structure which shows a first order structural phase transition from tetragonal to orthorhombic accompanied by a CDW transition at 112 K and superconducting transition at around 1.22 K as confirmed by temperature dependence of resistivity and magnetic measurements. For 2[Formula: see text] doping of germanium, while the CDW temperature T CDW decreases, the superconducting transition temperature T C shows an increase. T CDW increases for 5[Formula: see text] doping of germanium and the superconducting transition decreases. These findings demonstrate the competing nature of a CDW and superconductivity.
Article
X-ray scattering and electrical resistivity measurements were performed on GdNiC2 and TbNiC2. We found a set of satellite peaks characterized by q1=(0.5,η,0) below T1, at which the resistivity shows a sharp inflection, suggesting the charge density wave (CDW) formation. The value of η decreases with decreasing temperature below T1, and then a transition to a commensurate phase with q1C=(0.5,0.5,0) takes place. The diffuse scattering observed above T1 indicates the presence of soft phonon modes associated with CDW instabilities at q1 and q2=(0.5,0.5,0.5). The long-range order given by q2 is developed in addition to that given by q1C in TbNiC2, while the short-range correlation with q2 persists even at 6 K in GdNiC2. The amplitude of the q1C lattice modulation is anomalously reduced below an antiferromagnetic transition temperature TN in GdNiC2. In contrast, the q2 order vanishes below TN in TbNiC2. We demonstrate that RNiC2 (R = rare earth) compounds exhibit similarities with respect to their CDW phenomena, and discuss the effects of magnetic transitions on CDWs. We offer a possible displacement pattern of the modulated structure characterized by q1C and q2 in terms of frustration.
Article
The temperature dependence of electrical resistivity and magnetic susceptibility was studied for RPt2Si2 compounds with R = Y, La, Nd, and Lu. LaPt2Si2 and NdPt2Si2 show steps in the resistivity vs temperature curve at T* = 112 and 77 K, respectively. We performed X-ray diffraction and selected-area electron diffraction (SAED) measurements for LaPt2Si2 at low temperatures. It was found that LaPt2Si2 undergoes a structural transition from tetragonal to orthorhombic at T*. Moreover, (n/3, 0, 0) superlattice reflections with n = 1 and 2 were observed at 10 K in the SAED patterns. These results strongly suggest the formation of a charge density wave (CDW) in LaPt2Si2 below T*. On the other hand, the compounds R = Y, La, and Lu were found to show superconductivity below 1.5-1.9 K. The origin of a CDW and its coexistence with superconductivity in RPt2Si2 are discussed.
Article
The electrical and structural properties of NdNiC2 and PrNiC2 are examined in single-crystalline samples in order to investigate the interplay of charge-density wave (CDW) and magnetic order in ternary rare-earth nickel carbides, RNiC2, where R is a rare-earth element. PrNiC2 (no magnetic order down to 1.5 K) and NdNiC2 [antiferromagnetic (AF) transition at 17.2 K] show resistance anomalies at 89 and 121 K, respectively, which are revealed to be CDW transitions by observing X-ray satellite reflections. NdNiC2 also shows a gradual decrease in both resistivity and CDW amplitude below the Neel temperature. In the AF phase, the magnetoresistance sharply decreases where a ferromagnetic (FM) order occurs, which is realized above the spin-flop magnetic field. These results clearly suggest that the relationships between CDW and magnetic order in RNiC2 depend on whether the magnetic order is AF or FM.
Article
We report the observation of a strongly coupled first-order charge-density wave (CDW) transition in a high-quality single crystal of the intermetallic compound Lu5Ir4Si10. The first-order nature is ascertained by a very narrow and huge cusp (360 J/mol K) in the specific heat. The susceptibility and the resistivity also show sharp jumps at the transition TCDW=83 K. The periodic lattice distortion associated with the CDW is exemplified by the formation of x-ray superlattice reflections along the tetragonal c--> axis with q-->~(0,0,37) (T<83 K). Although our results are in accordance with a quasi-one-dimensional CDW scenario, the first-order transition suggests a strong interchain coupling. We propose Lu5Ir4Si10 as a paradigm of such strong-coupling CDW systems.
Article
Several U-based intermetallic compounds (UCoGe, UNiGe with the TiNiSi structure type and UNiAl with the ZrNiAl structure type) and their hydrides were studied from the point of view of compressibility and thermal expansion. Confronted with existing data for the compounds with the ZrNiAl structure type a common pattern emerges. The direction of the U–U bonds with participation of the 5f states is distinctly the “soft” crystallographic direction, exhibiting also the highest coefficient of linear thermal expansion. The finding leads to an apparent paradox: the closer the U atoms are together in a particular direction the better they can be additionally compressed together by applied hydrostatic pressure.
Article
Iron–pnictide superconductor (Ba, K)Fe2As2 has the ThCr2Si2-type crystal structure. On the other hand, recently found arsenide superconductor SrPt2As2 has the CaBe2Ge2-type structure, which is a different polymorphic form of the ThCr2Si2-type structure. LaPt2Si2 shows superconductivity with Tc = 2.2 K and the above-shown two different crystal structures have been reported previously. In this paper we have calculated the electronic structure of LaPt2Si2 from first-principles. From the total energy calculation, we show that the CaBe2Ge2-type structure is more stable in this compound. The density of states at the Fermi level (D(EF)) mainly consists of Pt1-d orbitals, while the Pt2-d orbitals contribute to D(EF) only half of Pt1-d orbitals. In contrast to iron–pnictide system, the total density of states at the Fermi level does not depend so much on the distortion of the PtX4 tetrahedra and/or the height of the X atom from the two-dimensional Pt plane. The calculated electronic specific heat coefficient is 7.0 mJ/mol K2 for LaPt2Si2, which is comparable with that of SrPt2As2, but larger than the experimental value 4 mJ/mol K2.
Article
The superconducting transition temperature is calculated as a function of the electron-phonon and electron-electron coupling constants within the framework of the strong-coupling theory. Using this theoretical result, we find empirical values of the coupling constants and the "band-structure" density of states for a number of metals and alloys. It is noted that the electron-phonon coupling constant depends primarily on the phonon frequencies rather than on the electronic properties of the metal. Finally, using these results, one can predict a maximum superconducting transition temperature.
Article
When metals are cooled, they often undergo a phase transition to a state exhibiting a new type of order. Metals such as iron and nickel become ferromagnetic below temperatures of several hundred degrees Celsius; electron spins order to produce a net magnetization in zero field. Other metals, such as lead and aluminum, become superconductors at cryogenic temperatures; electrons form Cooper pairs of opposite spin and momentum, leading to electrical conduction with zero resistance and to expulsion of magnetic fields. Low‐dimensional metals with moving lattice modulations display a host of unusual properties, including gigantic dielectric constants and the ability to ‘remember’ electrical pulse lengths.
Article
We have investigated the coupled structural and electronic phase transition in the rare-earth ternary silicide Lu2Ir3Si5 by means of electrical resistivity (rho) , Seebeck coefficient (S) , as well as thermal conductivity (kappa) measurements. Near the phase transition, pronounced anomalies in these transport properties with a significantly large hysteresis of about 40K were noticed. By comparing the transition characteristics with the earlier reported charge-density-wave (CDW) systems R5Ir4Si10 ( R=rare-earth elements), our present investigation infers the possibility for the CDW transition accompanying a structural transition in this compound. In addition, possible mechanisms for the observed thermal hysteresis have also been proposed.
Article
We report the investigations of specific heat, thermal conductivity, as well as thermoelectric power on the charge-density-wave ~CDW! compound Lu5Ir4Si10 as a function of temperature. All thermal measurements consistently exhibit anomalous features around the CDW transition temperature To;80 K. Although the ob- servations can be associated with the CDW formation, the measured anomalies are significantly large, in contrast to those in weak-coupled CDW materials. A quantitative analysis for the specific-heat data near the fluctuation region yields a critical exponent a;2, much larger than the predicted value a50.5 in the extended mean-field theory assuming three-dimensional fluctuations. We also obtained a ratio g*/g58.4, a factor of 6 larger than the BCS value 1.43 in the weak-coupling limit, indicating a strong coupling of this phase transition. Besides, the observed giant excess specific heat DCp /Cp;26% and thermal conductivity Dk/k;15% at To further support this strong-coupling scenario. These large enhancements in Cp and k are attributed to the results of substantially thermal excitation and heat carried by the soft phonons at the transition. In addition, a rapid change in the sign of thermoelectric power at To was observed, which provides a better understanding of the evolution of electronic band structure of the system below and above the CDW formation.
Article
We review our current understanding of the mechanical, electronic and structural properties of the element uranium, with the focus on the low-temperature regime in which the α-phase is stable. We start with a historical perspective, as the need for understanding the metallurgy of uranium so that its use as a nuclear fuel in the late 1940s could be optimized led to an increasing number of experiments aimed at exploring the solid-state properties. This was further accelerated when single crystals became available in the 1950s and when the superconductivity of the low-temperature α phase became the subject of controversy in the 1960s. The late 1960s and 1970s saw a considerable effort towards understanding the nature of the 5f electrons around the uranium nucleus. The increasing availability of sophisticated methods in computational physics first showed in the 1970s that the 5f electrons should be treated as itinerant, and the decade ended with the observation (by neutron diffraction) of a subtle periodic lattice distortion at 43 K in the α phase. The observation of this distortion, which is ascribed to a charge-density wave, leads to an explanation for many of the anomalies seen in over 30 years' work on the α phase. The review ends with a description of our current understanding and the problems still outstanding.
Article
A previously obtained solution of the linearized Gor'kov equations for the upper critical magnetic field Hc2 of a bulk type-II superconductor is extended to include the effects of Pauli spin paramagnetism and spin-orbit impurity scattering. To carry out the calculation, it is necessary to introduce an approximation which assumes that spin-orbit scattering is infrequent in comparison with spin-independent scattering. It is found that spin-orbit scattering counteracts the effects of the spin paramagnetism in limiting the critical field and improves agreement between theory and experiment.
Article
The one-dimensional case of free electrons interacting with lattice displacements is solved by a self-consistent method. It is found that for a certain range of the interaction parameter a single sinusoidal lattice displacement is strongly excited in the lowest level of the system. Its wave-length is such as to create an energy gap in the single-electron energy spectrum with all states below it filled, and all above it empty. This periodic lattice displacement plays the role of an 'inner field' and leads to periodic fluctuation in the electronic density in such a way that the two stabilize each other. In an infinite medium described by a periodic boundary condition they are not fixed absolutely in space, but only relative to each other. Excitation of electrons across the gap leads to a decrease in both the electronic density fluctuations and the width of the gap. The whole system, electrons plus lattice displacements, can move through the lattice without being disturbed provided the velocity v is sufficiently small. The inertia of this system is equal to that of all electrons augmented by a term due to the lattice displacements. Elastic scattering of individual electrons which normally leads to the residual resistance is impossible if v is sufficiently small. The linear specific heat of normal electrons is eliminated and replaced by an exponential term.
Article
It is observed that the ratio, A/γ2, has a common value of 1.0 × 10−5 μωcm (mole K/mJ)2 for all of the heavy Fermion compounds, where A is the coefficient of the quadratic term in the temperature dependence of the resistivity and γ is the coefficient of the linear term in the temperature dependence of the specific heat. This universal value, which is at least an order of magnitude above that of common d- band metals, is not accounted for by current microscopic theories and strongly indicates that a more unified understanding of the electron scattering mechanism is required for heavy Fermion materials.
Article
Temperature-dependent x-ray diffraction on SmNiC2 has shown that the orthorhombic lattice symmetry of this compound persist down to a temperature of at least 9 K, i.e., into the charge-density-wave (CDW) state below TCDW=148 K and in the ferromagnetically ordered state below TC=17.7 K. The modulated crystal structure has been determined for the incommensurate CDW state with qCDW=(0.5,0.516,0) at T=60 K. The observed atomic modulation displacements indicate that the CDW should be considered as a commensurate CDW centered on chains of Ni atoms along a. Frustrated interchain coupling is responsible for the incommensurability of the three-dimensionally ordered CDW state.
Article
We report the detailed studies of resistivity, magnetic susceptibility, heat capacity, thermal conductivity, and thermoelectric power on the charge-density-wave (CDW) material Lu5Rh4Si10 in the vicinity of its transition temperature T0∼147 K. Pronounced thermal hystereses loops between heating and cooling cycles have been observed in all measured quantities, making this system very unique as compared to other isostructural compounds such as Lu5Ir4Si10. The thermal hysteresis features in Lu5Rh4Si10 are attributed to the presence of metastable states arising from pinning of the CDW phase to impurities. In addition, the measured physical quantities are magnetic field independent up to 8 T, which excludes the possibility of a magnetic coupling to the static CDW structure in Lu5Rh4Si10.
Article
X-ray scattering measurements on LaAgSb2 have revealed the onset of charge-density-wave (CDW) modulations associated with anomalies in the resistivity and magnetic susceptibility of this compound. Below the transition temperature of T1,CDW=207K, a periodic charge and lattice modulation with τ1∼0.026(2π/a) develops along the a direction of the tetragonal structure (a<c). Further lowering of the temperature results in an additional charge-density-wave ordering below T2,CDW=186K along the c direction with τ2∼0.16(2π/c). This ordering is associated with a rather weak anomaly in the c-axis resistivity that was previously unnoticed. With decreasing temperature, the wave vector of this second CDW locks into a commensurate value of 1/6(2π/c) at 164(±1)K. The order parameters of τ1-CDW and τ2-CDW were deduced from the integrated intensities of the (1-τ107) and (107-τ2) satellite peaks, respectively, and are in good agreement with the BCS order parameter. The generalized susceptibility was also calculated, revealing enhanced nesting features for the observed modulation wave vectors.
Article
Measurements of low-temperature specific heats (2-37 K) are reported for the first time on some common paraelectrics (thallous halides, PbF2, KTaO3), ferroelectrics [BaTiO3, potassium dihydrogen phosphate or KDP, triglycine sulfate or TGS, LiNbO3, LiTaO3, Pb(Zr0.65 Ti0.35)O3 or PZT 65/35], and antiferroelectrics [Pb(Zr0.95 Ti0.05)O3 or PZT 95/5, Pb2Nb2O7]. All materials display maxima in CT-3, and excellent fits to experimental data are obtained with single Einstein frequencies. The Einstein frequencies vary from 19 cm-1 for TlCl to 99 cm-1 for BaTiO3. The frequencies in LiNbO3 (79 cm-1) and LiTaO3 (61 cm-1) agree reasonably well with earlier Raman data at 300 K on E-symmetry optic modes and with recent low-temperature pyroelectric data. The TlBr frequency (22 cm-1) agrees well with the lowest phonon anomaly determined from neutron data, and the KTaO3 frequency (26 cm-1) is in good agreement with the average soft-mode frequency in this temperature range. No evidence is seen for the suggested phase transition in KTaO3 at 10 K. The PZT materials, which are compositionally in a field inaccessible to powder Raman methods, have frequencies of 32 (65/35) and 38 cm-1 (95/5), due probably to low-lying TA phonons. An unusual T3/2 contribution to the specific heat of the ferroelectrics TGS, KDP, BaTiO3, and LiNbO3 was found at the lowest temperatures. Experimental data are in excellent agreement with C=AT3+BT3/2, and it is suggested that the T3/2 term is the domain-wall contribution.
Article
We report the results of our investigations on a polycrystalline sample of Lu2Ir3Si5 , which crystallizes in the U2Co3Si5 -type structure Ibam. These investigations comprise powder x-ray diffraction, magnetic susceptibility, electrical resistivity, and high-temperature 120– 300 K heat-capacity studies. Our results reveal that the sample undergoes a superconducting transition below 3.5 K. It also undergoes a first-order phase transition between 150–250 K as revealed by an upturn in the resistivity, a diasmagnetic drop in the magnetic susceptibility, and a large anomaly 20–30 J/mol K in the specific heat data. We observe a huge thermal hysteresis of almost 45 K between the cooling and warming data across this high-temperature transition in all our measurements. Low-temperature x-ray diffraction measurements at 87 K reveals that the compound undergoes a structural change at the high-temperature transition. Resistivity data taken in repeated cooling and warming cycles indicate that at the high-temperature transition, the system goes into a highly metastable state; successive heating and/or cooling curves are found to lie above the previous one, and the resistance keeps increasing with every thermal cycle. The room-temperature resistance of a thermally cycled piece of the sample decays exponentialy with time with a decay-time constant estimated to be about 10^4 s. The anomaly upturn in the resistivity and the large drop (~40%) in the susceptibility across the high-temperature transition suggest that the observed structural change is accompanied or induced by an electronic transition.
Article
T-linear term of specific heat and magnetic susceptibility at zero temperature are derived for the heavy electron systems, and relations among these quantities are discussed on the basis of the Fermi liquid theory. Further, a rigorous expression of the T2-term of resistivity at low temperatures is also obtained on the basis of Kubo formula. It is shown that the coefficient of the T2-term arising from the electron interaction is strongly enhanced though it vanishes because of the momentum conservation for a free electron system possessing no crystal lattice.
Article
In this review the author describes how the simple Ginzburg-Landau approach lies in the heart of the general theory of superconductors. The reader is introduced to the handling of the theory and to the numerous possibilities of applications. A general free energy functional for a superconductor is given and the different cases where it can be reduced to a Ginzburg-Landau form or to a simple generalization of this form is studied. It is emphasized that applications are not restricted to thermo-dynamical ones as the Ginzburg-Landau approach can be used in the calculation of dissipative phenomena. The possibility of an extension to the time-dependent phenomena is discussed in detail to present the difficult problems which arise in that case. The gapless regime of type II superconductors is given as an example.
Article
Anisotropic measurements of the magnetic and transport properties have been performed on high-quality single crystals of the RAgSb2 series of compounds with R=Y, La–Nd, Sm, Gd–Tm. For most of these compounds, strong magnetic anisotropies, created by crystalline electric field (CEF) splitting of the Hund's rule ground state, confine the moments to the basal plane. Additional anisotropy within the basal plane is also observed in DyAgSb2, leading to a series of metamagnetic transitions. All of the compounds order antiferromagnetically at low temperatures except for the nonmagnetic compounds LaAgSb2 and YAgSb2 and the ferromagnetic compound CeAgSb2. For R=Y, La–Nd, and Sm clear de Haas–van Alphen oscillations in fields as low as 30 kOe and at temperatures as high as 25 K can be measured. Throughout the series, the low-temperature transverse magnetoresistance is large [Δρ(H)/ρ(0)≈60 in SmAgSb2 at 55 kOe] and deviates strongly from quadratic behavior with field the dependence of Δρ(H)/ρ(0) ranging between H0.8 and H1.5.
Article
We report a study of the charge-density-wave (CDW) behavior in LaAgSb(2) by means of electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and nuclear magnetic resonance (NMR) measurements. Except for the Seebeck coefficient, apparent indications of CDW ordering at around 207 K were noticed in the physical quantities investigated. On the other hand, all measured physical properties are insensitive to the second CDW formation (∼184 K), as the transition character is considerably weaker than the high-temperature one. Further, analyses of the thermal conductivity and NMR Knight shift data revealed that the observed variations are essentially of electronic origin. The present findings are in good agreement with the previous results, indicating that the high-temperature CDW ordering is associated with a small gapping of the Fermi surface with a minor periodic displacement of the crystal lattice in LaAgSb(2).
Article
X-ray scattering and electrical resistivity measurements were performed on SmNiC2. Satellite peaks characterized by an incommensurate wave vector (0.5, eta, 0) appear below 148 K, at which the resistivity shows an anomaly. The temperature dependence of thermal diffuse scattering above 148 K suggests critical phonon softening. These results indicate the formation of a charge-density-wave. The satellite peaks abruptly disappear and the resistivity sharply decreases when a ferromagnetic transition takes place at 17.7 K.
Article
We calculate the dynamic susceptibility chi(q,..omega..) of the Hubbard model using the boson representation of Kotliar and Ruckenstein. The energy and momentum dependence of chi(q,..omega..) are the same as in the random-phase approximation, but the Gutzwiller mass enhancement (m/sup *//m) and spin Landau parameter are included. Combining this with the Kaiser-Doniach expression for the resistivity rho/sup s/⁻/sup d/ of a transition metal we obtain a T² term in rho/sup s/⁻/sup d/ which scales with (m/sup *//m)² in accordance with experiment and has the correct magnitude.
Article
By extension of the functional integral treatment of the Kondo lattice model of heavy-fermion metals to real times, a model transport equation is derived. Frequency- and temperature-dependent conductivity are calculated. Results are contrasted with transport properties of heavy-fermion metals.