[Show abstract][Hide abstract]ABSTRACT: We report the experimental details of how mechanical detwinning can be implemented in tandem with high sensitivity nuclear magnetic resonance measurements and use this setup to measure the in-plane anisotropy of the spin-lattice relaxation rate in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ with $x=0.048$. The anisotropy reaches a maximum of 30\% at $T_{N}$, and the recovery data reveal that the glassy behavior of the spin fluctuations present in the twinned state persist in the fully detwinned crystal. A theoretical model is presented to describe the spin-lattice relaxation rate in terms of anisotropic nematic spin fluctuations.
[Show abstract][Hide abstract]ABSTRACT: Measurements of the London penetration depth and tunneling conductance in single crystals of the recently discovered stoicheometric, iron - based superconductor, CaKFe$_4$As$_4$ (CaK1144) show nodeless, two effective gap superconductivity with a larger gap of about 6-9 meV and a smaller gap of about 1-4 meV. Having a critical temperature, $T_{c,onset}\approx$35.8 K, this material behaves similar to slightly overdoped Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (e.g. $x=$0.54, $T_c \approx$ 34 K)---a known multigap $s_{\pm}$ superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap $s_{\pm}$ superconductivity is an essential property of high temperature superconductivity in iron - based superconductors, independent of the degree of substitutional disorder.
[Show abstract][Hide abstract]ABSTRACT: Single crystalline, single phase CaKFe$_{4}$As$_{4}$ has been grown out of a high temperature, quaternary melt. Temperature dependent measurements of x-ray diffraction, anisotropic electrical resistivity, elastoresistivity, thermoelectric power, Hall effect, magnetization and specific heat, combined with field dependent measurements of electrical resistivity and field and pressure dependent measurements of magnetization indicate that CaKFe$_{4}$As$_{4}$ is an ordered, stoichiometric, Fe-based superconductor with a superconducting critical temperature, $T_c$ = 35.0 $\pm$ 0.2 K. Other than superconductivity, there is no indication of any other phase transition for 1.8 K $\leq$ $T$ $\leq$ 300 K. CaKFe$_{4}$As$_{4}$ has a pressure dependence of $T_c$ that is consistent with it being intrinsically near optimally- or slightly over-doped, a result consistent with its anisotropic electrical resistivity, elastoresistivity, anisotropic $H_{c2}(T)$, jump in specific heat at $T_c$ and temperature dependent thermoelectric power and Hall effect, all of which are very similar to those found for slightly over-doped (Ba$_{1-x}$K$_{x}$)Fe$_{2}$As$_{2}$ samples.
[Show abstract][Hide abstract]ABSTRACT: The mechanism of unconventional superconductivity in iron - based superconductors (IBS) is one of the most intriguing questions in current materials research. Among non-oxide IBS, has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from fully isotropic at optimal doping, \(x\approx\)0.4, to nodal at $x > $0.8. While this dramatic evolution was identified in several independent experiments, the details of the gap evolution are still missing due to lack of high-quality single-crystals covering the entire K - doping range of the superconducting dome. In this work, we conducted a systematic study of the London penetration depth, $\lambda(T)$, on sixteen different levels of potassium content varying from 0.2 (underdoped) to 1.0 (the end member) and for different concentrations of point-like defects introduced by 2.5 MeV electron irradiation. Fitting the low temperature variation with the power law, $\Delta \lambda \sim T^{n}$, we find that the exponent $n$ is the highest and $T_c$ suppression rate with disorder is the smallest at optimal doping, and they evolve with doping away from optimal consistent with increasing gap anisotropy, including an abrupt change around $x\simeq 0.8$ indicating the onset of nodal behavior. The effect of disorder on both $\lambda(T)$ and $T_c$ suppression rate was analyzed using a self-consistent $t$-matrix approach. We fitted both quantities simultaneously using an s$_{\pm}$ pairing model, where the order parameter on one of the hole pockets becomes highly anisotropic, acquiring nodes and eventually changing average sign somewhere around the Lifshitz transition. Our analysis suggests the ubiquitous and robust nature of s$_{\pm}$ pairing in IBS and argues against a previously suggested transition to a d-wave state near $x=1$ in this system.
[Show abstract][Hide abstract]ABSTRACT: A highly anisotropic superconducting gap is found in single crystals of FeSe by studying the London penetration depth, $\Delta \lambda$, measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with introduced point - like disorder, indicating the absence of symmetry - imposed nodes. Surprisingly, the superconducting transition temperature, $T_c$, \textit{increases} by 0.4 K from $T_{c0} \approx$ 8.8 K while the structural transition temperature, $T_s$, \textit{decreases} by 0.9 K from $T_{s0} \approx$91.2 K after electron irradiation. We discuss several explanations for the $T_c$ enhancement, and propose that local strengthening of the pair interaction by irradiation-induced Frenkel defects most likely explains the phenomenon.
[Show abstract][Hide abstract]ABSTRACT: The discovery of superconductivity in the rare earth-based half-Heusler semimetals RTBi (R=rare earth, T=Pt, Pd) has opened a new avenue for investigation of topological phases of matter. With charge carrier densities several orders of magnitude lower than conventional metals, these materials pose a challenge for investigating the nature of the superconducting state, which itself may have inherent topological properties. Here, we report experimental evidence for an unconventional superconducting state in YPtBi, which presents a striking linear temperature dependence of the London penetration depth consistent with line nodes in the superconducting order parameter $\Delta$. With strong antisymmetric spin-orbit coupling giving rise to spin-split Fermi surfaces -- as verified by angle-dependent Shubnikov-de Haas oscillations and angle-resolved photoemission spectroscopy -- we propose a $\mathbf{k\cdot p}$ model of $j=3/2$ fermions that naturally leads to exotic nodal Cooper pairing states with high total angular momentum. With the simplest gap model involving a dominant $J=3$ septet pairing between the $j=3/2$ fermions of the $p$-like $\Gamma_8$ band, the topological pairing states in this half-Heusler compound present a truly novel form of superconductivity that has strong potential for leading the development of a new series of topological superconductors.
[Show abstract][Hide abstract]ABSTRACT: We report a study of the Ca0.73La0.27FeAs2 single crystals. We unravel a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic phase transition at 54 K, below which spins order 45 away from the stripe direction. Furthermore, we demonstrate this material is substantially structurally untwinned at ambient pressure with the formation of spin rotation walls (S walls). Finally, in addition to the central-hole and corner-electron Fermi pockets usually appearing in Fe pnictide superconductors, angle-resolved photoemission measurements resolve a fermiology where an extra electron pocket of mainly As chain character exists at the Brillouin zone edge.
[Show abstract][Hide abstract]ABSTRACT: The thermal conductivity kappa of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured for heat currents parallel and perpendicular to the tetragonal c axis at temperatures down to 50 mK and in magnetic fields up to 15 T. Measurements were performed on samples with compositions ranging from optimal doping (x = 0.34; Tc = 39 K) down to dopings deep into the region where antiferromagnetic order coexists with superconductivity (x = 0.16; Tc = 7 K). In zero field, there is no residual linear term in kappa(T) as T goes to 0 at any doping, whether for in-plane or inter-plane transport. This shows that there are no nodes in the superconducting gap. However, as x decreases into the range of coexistence with antiferromagnetism, the residual linear term grows more and more rapidly with applied magnetic field. This shows that the superconducting energy gap develops minima at certain locations on the Fermi surface and these minima deepen with decreasing x. We propose that the minima in the gap structure arise when the Fermi surface of Ba1-xKxFe2As2 is reconstructed by the antiferromagnetic order.
[Show abstract][Hide abstract]ABSTRACT: The in-plane electrical resistivity, ρ, and thermal conductivity, κ, of the heavy-fermion superconductor CeIrIn5 were measured down to 40 mK in magnetic fields up to 11 T applied along the c axis. For all fields above Hc2=4 T of filamentary superconductivity, we find that the ratio of heat and charge conductivities in the T→0 limit obeys the Wiedemann-Franz law, κ/T=L0/ρ, where L0=2.45×10−8 WΩK−2 is the Sommerfeld value of the Lorenz number. The temperature-dependent parts of both the electrical and thermal resistivity, w≡T/L0κ, follow the functional dependence expected for the Fermi liquid theory of metals with ρ−ρ0=AT2, w−w0=BT2, with ρ0=w0 and B≈2A. The coefficient B does not show a significant field dependence even upon approaching Hc2=0.4 T of the bulk superconducting state. The weak response to the magnetic field is in stark contrast with the behavior found in the closely related CeCoIn5, in which the field-tuned quantum critical point coincides with Hc2. The value of the electron-electron mass enhancement, as judged by the A and B coefficients, is about one order of magnitude reduced in CeIrIn5 as compared to CeCoIn5 (in spite of the fact that the zero field γ0 in CeIrIn5 is twice as large as γ0 in CeCoIn5), which suggests that the material is significantly farther away from the magnetic quantum critical point at bulk Hc2 and at all of the studied fields. A suppressed Kadowaki-Woods ratio in CeIrIn5 compared to CeCoIn5 suggests a notably more localized nature of f electrons in the compound.
[Show abstract][Hide abstract]ABSTRACT: In the temperature-concentration phase diagram of most iron-based
superconductors, antiferromagnetic order is gradually suppressed to zero at a
critical point, and a dome of superconductivity forms around that point. The
nature of the magnetic phase and its fluctuations is of fundamental importance
for elucidating the pairing mechanism. In Ba{1-x}KxFe2As2 and Ba{1-x}NaxFe2As2,
it has recently become clear that the usual stripe-like magnetic phase, of
orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal
symmetry, near the critical point, between x = 0.24 and x = 0.28. Here we
report measurements of the electrical resistivity of Ba{1-x}KxFe2As2 under
applied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24 and 0.28. We
track the onset of the tetragonal magnetic phase using the sharp anomaly it
produces in the resistivity. In the temperature-concentration phase diagram of
Ba{1-x}KxFe2As2, we find that pressure greatly expands the tetragonal magnetic
phase, while the stripe-like phase shrinks. This raises the interesting
possibility that the fluctuations of the former phase might be involved in the
pairing mechanism responsible for the superconductivity.
[Show abstract][Hide abstract]ABSTRACT: The pressure dependence of the upper critical field, $H_\textrm{c2,c}$, of
single crystalline FeSe was studied using measurements of the inter-plane
resistivity, $\rho_{\textrm{c}}$ in magnetic fields parallel to tetragonal
$c$-axis. $H_\textrm{c2,c}(T)$ curves obtained under hydrostatic pressures up
to $1.56$ GPa, the range over which the superconducting transition temperature,
$T_\textrm{c}$, of FeSe exhibits a non-monotonic dependence with local maximum
at $p_1\approx$ 0.8 GPa and local minimum at $p_2\approx$ 1.2 GPa. The slope of
the upper critical field at $T_\textrm{c}$,
$\left(\textrm{d}H_\text{c2,c}/\textrm{d}T\right)_{T_\textrm{c}}$, also
exhibits a non-monotonic pressure dependence with distinct changes at $p_1$ and
$p_2$. For $p<p_1$ the slope can be described within multi-band orbital model.
For both $p_1<p <p_2$ and $p>p_2$ the slope is in good quantitative agreement
with a single band, orbital Helfand-Werthamer theory with Fermi velocities
determined from Shubnikov-de Haas measurements. This finding indicates that
Fermi surface changes are responsible for the local minimum of
$T_\textrm{c}(p)$ at $p_2\approx$ 1.2 GPa.
[Show abstract][Hide abstract]ABSTRACT: The in-plane resistivity anisotropy is studied in strain-detwinned single
crystals of FeSe. In contrast to other iron-based superconductors, FeSe does
not develop long-range magnetic order below the nematic/structural transition
at $T_{s}\approx$90~K. This allows for the disentanglement of the contributions
to the resistivity anisotropy due to nematic and magnetic orders. Comparing
direct transport and elastoresistivity measurements, we extract the intrinsic
resistivity anisotropy of strain-free samples. The anisotropy peaks slightly
below $T_{s}$ and decreases to nearly zero on cooling down to the
superconducting transition. This behavior is consistent with a scenario in
which the in-plane resistivity anisotropy in FeSe is dominated by inelastic
scattering by anisotropic spin fluctuations.
[Show abstract][Hide abstract]ABSTRACT: We report $^{75}$As nuclear magnetic resonance (NMR) measurements of
single-crystalline Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($x$ = 0.023, 0.028, 0.033,
and 0.059) annealed at 350~$^{\circ}$C for 7 days. From the observation of a
characteristic shape of $^{75}$As NMR spectra in the stripe-type
antiferromagnetic (AFM) state, as in the case of $x$ = 0 ($T_{\rm N}$ = 170 K),
clear evidence for the commensurate AFM phase transition with the concomitant
structural phase transition is observed in $x$ = 0.023 ($T_{\rm N}$ = 106 K)
and $x$ = 0.028 ($T_{\rm N}$ = 53 K). Through the temperature dependence of the
Knight shifts and the nuclear spin lattice relaxation rates (1/$T_1$), although
stripe-type AFM spin fluctuations are realized in the paramagnetic state as in
the case of other iron pnictide superconductors, we found a gradual decrease of
the AFM spin fluctuations below a crossover temperature $T^*$ which was nearly
independent of Co-substitution concentration, and is attributed to a
pseudogap-like behavior in the spin excitation spectra of these systems. The
$T^*$ feature finds correlation with features in the temperature-dependent
inter-plane resistivity, $\rho_c(T)$, but not with the in-plane resistivity
$\rho _a (T)$. The temperature evolution of anisotropic stripe-type AFM spin
fluctuations are tracked in the paramagnetic and pseudogap phases by the
1/$T_1$ data measured under magnetic fields parallel and perpendicular to the
$c$ axis. Based on our NMR data, we have added a pseudogap-like phase to the
magnetic and electronic phase diagram of Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$.
[Show abstract][Hide abstract]ABSTRACT: We present quantum oscillations observed in the heavy fermion compound YbPtBi
in magnetic fields far beyond its field-tuned, quantum critical point. Quantum
oscillations are observed in magnetic fields as low as 60 kOe at 60 mK and up
to temperatures as high as 3 K, which confirms the very high quality of the
samples as well as the small effective mass of conduction carriers far from the
quantum critical point. Although the electronic specific heat coefficient of
YbPtBi reaches ~ 7.4 J/mol K2 in zero field, which is one of the highest
effective mass value among heavy fermion systems, it is suppressed quickly by
applied magnetic field. The quantum oscillations were used to extract the
quasiparticle effective masses of the order of the bare electron mass, which is
consistent with the behavior observed in specific heat measurements. Such a
small effective masses at high fields can be understood by considering the
suppression of Kondo screening.
[Show abstract][Hide abstract]ABSTRACT: We report a comprehensive study of single crystals of the newly discovered
112 iron pnictide superconductors (FBS). In Ca$_{0.73}$La$_{0.27}$FeAs$_2$, we
unraveled a monoclinic to triclinic phase transition at 58 K, and a
paramagnetic to stripe antiferromagnetic (AFM) phase transition at 54 K, below
which a distinct magnetic structure appears with the spins ordering 45$^\circ$
away from the stripe direction. Both phase transitions can be suppressed upon
Co substitution on Fe sites and bulk superconductivity is stabilized up to 20
K. Furthermore, we demonstrate, as the structural and chemical consequences of
the As chains in the spacer layers, this magnetic FBS is naturally structurally
detwinned at ambient pressure with the formation of spin rotation walls
(S-walls). Finally, in addition to the central-hole and corner-electron Fermi
pockets usually appearing in FBS, angle-resolved photoemission (ARPES)
measurements resolve a novel Fermiology where an extra electron pocket exists
at the Brillouin zone edge with the As chain character. These unique features
open a new avenue to clarify the role of electronic nematicity and metallic
spacer layer in affecting the superconductivity.
[Show abstract][Hide abstract]ABSTRACT: The London penetration depth λ(T) was measured in single crystals of Ce_{1-x}R_{x}CoIn_{5}, R=La, Nd, and Yb down to T_{min}≈50 mK (T_{c}/T_{min}∼50) using a tunnel-diode resonator. In the cleanest samples Δλ(T) is best described by the power law Δλ(T)∝T^{n}, with n∼1, consistent with the existence of line nodes in the superconducting gap. Substitutions of Ce with La, Nd, and Yb lead to similar monotonic suppressions of T_{c}; however, the effects on Δλ(T) differ. While La and Nd substitution leads to an increase in the exponent n and saturation at n∼2, as expected for a dirty nodal superconductor, Yb substitution leads to n>3, suggesting a change from nodal to nodeless superconductivity. This superconducting gap structure change happens in the same doping range where changes of the Fermi-surface topology were reported, implying that the nodal structure and Fermi-surface topology are closely linked.
[Show abstract][Hide abstract]ABSTRACT: A single crystal of isovalently substituted Ba(Fe1−xRux)2As2 (x=0.24) is sequentially irradiated with 2.5 MeV electrons up to a maximum dose of 2.1×1019 e−/cm2. The electrical resistivity is measured in situ at T=22 K during the irradiation and ex situ as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature Tc decreases and the residual resistivity ρ0 increases. We find that electron irradiation leads to the fastest suppression of Tc compared to other types of artificially introduced disorder, probably due to the strong short-range potential of the pointlike irradiation defects. A more detailed analysis within a multiband scenario with variable scattering potential strength shows that the observed Tc versus ρ0 is fully compatible with s± pairing, in contrast to earlier claims that this model leads to a too rapid suppression of Tc with scattering.
[Show abstract][Hide abstract]ABSTRACT: The doping-evolution of the superconducting gap structure in iron-based superconductor (CaLax )10(Pt3As8)(Fe2As2)5 (x = 0.04, 0.06, 0.09, 0.11, and 0.18) was probed by high—resolution measurements of the London penetration depth, . The samples spanned compositions from underdoped to slightly overdoped with superconducting critical temperatures, Tc , from 12.7 K (x = 0.04) through (optimal) 23.3 K (x = 0.11) to 21.9 K (x = 0.18). The low-temperature variation (up to 0.3 Tc ) of was analysed using a power-law function, . For compositions close to the optimal doping, (x = 0.09, 0.11, and 0.18), characterized by , shows a tendency to saturation, indicative of a full gap on the Fermi surface. Fitting over the lowest temperature range () gives n = 2.6. This value is well outside the range expected for the line-nodal superconductor. The exponent n decreased to in the two most underdoped compositions x = 0.04 (Tc = 12.7 K) and 0.06 (Tc = 18.2 K), implying the development of a notable gap anisotropy revealed by the enhanced influence of pair-breaking scattering. This decrease is accompanied by a significant increase of the total variation of the penetration depth in a fixed temperature interval (e.g., ). Both the decrease of the exponent and the increase of the absolute value of in the underdoped regime are similar to the observations in other charge-doped iron-based superconductors, such as doped BaFe2As2 and NaFeAs, suggesting a universal behavior in iron-based superconductors.
Article · Sep 2014 · Superconductor Science and Technology
[Show abstract][Hide abstract]ABSTRACT: Temperature-dependent interplane resistivity ${$\rho${}}_{c}(T)$ was measured in an isovalent substituted iron-arsenide compound $\text{Ba}({\mathrm{Fe}}_{1$-${}x}{\mathrm{Ru}}_{x}){}_{2}{\mathrm{As}}_{2}$ over a substitution range from parent compound to slightly below optimal doping $x=0.29$. The feature of interest in the ${$\rho${}}_{c}(T)$, a broad resistivity crossover maximum found in the parent compound at ${T}_{\text{max}}$\approx${}200$ K, shifts to higher temperatures with Ru substitution, $$\sim${}340$ K for $x=0.161$ and goes out of the 400 K range for $x=0.29$. Nearly $T$-linear dependence of interplane resistivity is found at the highest substitution level $x=0.29$. This temperature-dependent ${$\rho${}}_{c}$ and its evolution with substitution bear close similarity to another type of isovalent substituted system ${\mathrm{BaFe}}_{2}({\mathrm{As}}_{1$-${}x}{\mathrm{P}}_{x}){}_{2}$. Similarly to the isovalent substitutions, the measurements of interplane resistivity in the parent ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ compound under pressures up to 20 kbar also revealed a rapid rise in ${T}_{\text{max}}$.