Article

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

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## 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.

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... 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 . ...
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... 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]. ...
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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. ...
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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.
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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/â»/sup d/ of a transition metal we obtain a TÂ² term in rho/sup s/â»/sup d/ which scales with (m/sup *//m)Â² 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.