Kadowaki-Woods Ratio Research Articles

Superconductivity in a new layered nickel selenide CsNi2Se2

The physical properties of CsNi2Se2 were characterized by electrical resistivity, magnetization and specific heat measurements. We found that the stoichiometric CsNi2Se2 compound undergoes a superconducting transition at Tc = 2.7 K. A large Sommerfeld coefficient (∼77.90 mJ/mol K−2) was obtained from the normal state electronic specific heat. However, the Kadowaki–Woods ratio of CsNi2Se2 was estimated to be about 0.041 × 10cm(mol K2/mJ)2, indicating the absence of strong electron–electron correlations. In the superconducting state, we found that the zero-field electronic specific heat data, Ces(T) (0.5 K T < 2.7 K), can be fitted well with a two-gap BCS model, indicating the multi-gap feature of CsNi2Se2. The comparison with the density functional theory (DFT) calculations suggested that the large in these nickel selenide superconductors may be related to the large density of states (DOS) at the Fermi surface.

Heat transport study of field-tuned quantum criticality inCeIrIn5

The in-plane electrical resistivity, rho, and thermal conductivity, kappa, of the heavy-fermion superconductor CeIrIn5 were measured down to 40mK in magnetic fields up to 11 T applied along the c axis. For all fields above Hc2=4T of filamentary superconductivity, we find that the ratio of heat and charge conductivities in the T to 0 limit obeys the Wiedemann-Franz law, kappa/T=L0/rho, where L0 = 2.45*10^-8 WOhmK^-2 is the Sommerfeld value of the Lorenz number. The temperature-dependent parts of both the electrical and thermal resistivity,w = T/L0 kappa, follow the functional dependence expected for the Fermi liquid theory of metals with rho - rho_0 = AT^2, w - w_0 = BT^2, with rho0 = 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 gamma_0 in CeIrIn5 is twice as large as gamma_0 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.

‘Hard’ crystalline lattice in the Weyl semimetal NbAs

We report the effect of hydrostatic pressure on the magnetotransport properties of the Weyl semimetal NbAs. Subtle changes can be seen in the profiles with pressure up to 2.31 GPa. The Fermi surfaces undergo an anisotropic evolution under pressure: the extremal areas slightly increase in the - plane, but decrease in the -() plane. The topological features of the two pockets observed at atmospheric pressure, however, remain unchanged at 2.31 GPa. No superconductivity can be seen down to 0.3 K for all the pressures measured. By fitting the temperature dependence of specific heat to the Debye model, we obtain a small Sommerfeld coefficient mJ (mol·K2)−1 and a large Debye temperature, K, confirming a ‘hard’ crystalline lattice that is stable under pressure. We also studied the Kadowaki–Woods ratio of this low-carrier-density massless system, cm mol2 K2 J−2. After accounting for the small carrier density in NbAs, this indicates a suppressed transport scattering rate relative to other metals.

Ginzburg-Landau Analysis on the Superconductivity in TlNi2Se2

Based on a two-band isotropic Ginzburg-Landau theory, we study the magnetic properties of the recently observed superconducting crystal TlNi2Se2. Our exact solution of upper critical field reproduces the experimental data in a broad temperature range. It directly underlies the multi-gap superconductivity in this crystal. We also show that the effective mass of the electron for one band is about 20me while that of the other band is only 0.6me. This semi-heavy-fermion feature can qualitatively explain the experimental value of the Kadowaki-Woods ratio in TlNi2Se2.

Fermi liquid behaviour in weakly disordered metals close to a quantum critical point

We calculate analytically the low-temperature quasi-particle scattering rate, the conductivity, and the specific heat in weakly disordered metals close to a quantum critical point, via the use of a proper fluctuation potential between the quasi-particles. We obtain typical Fermi liquid results proportional to T2 and T, respectively, with prefactors which diverge as power laws of the control parameter a upon approaching the critical point. The Kadowaki-Woods ratio is shown to be independent of a (possibly times a logarithmic dependence on a) only for the case of three-dimensional ferromagnetic fluctuations. Our results are consistent with experiments on the eight materials CeCoIn5, Sr3Ru2O7, YbRh2Si2, La2−xCexCuO4, Tl2Ba2CuO6+x, CeAuSb2, YbAlB4, and CeRuSi2.

Breakdown of the universality of the Kadowaki-Woods Ratio in multi-band metals

We calculate the Kadowaki-Woods ratio (KWR) in Fermi liquids with arbitrary band structures. We find that, contrary to the single band case, the ratio is not generally independent of the effects of electronic correlations (universal). This is very surprising given the experimental findings of a near universal KWR in many multiband metals. We identify a limit where the universality of the ratio, which has been observed experimentally in many strongly correlated electron systems, is recovered. We discuss the KWR in Dirac semimetals in two and three dimensions. In the two-dimensional case we also generalize the KWR to account for the logarithmic factor in the self-energy. In both cases we find that the KWR is independent of correlations, but strongly dependent on the doping of the system: for massless fermions the KWR is proportional to the inverse square of the carrier density, whereas the KWR for systems with massive quasiparticles is proportional to the inverse of the carrier density.

Cooperative magnetic behaviour in the new valence fluctuating compound Ce2Rh3Ge

In this study we report the physical properties of the new ternary compound Ce2Rh3Ge that crystallizes in the rhombohedral, triple hexagonal MgCu2-type of structure. The electronic ground state properties of Ce2Rh3Ge were characterized by magnetic susceptibility, specific heat, electrical resistivity and thermal transport measurements. The results indicate the presence of short range magnetic interaction, probably of ferromagnetic origin below TC = 4 K. The shape of χ−1(T) deviates from the Curie–Weiss behavior with a broad minimum at about = 450 K reminiscent of valence fluctuating cerium systems. At T = 10 K, the magnetic part of the resistivity ρ4 f (T) exhibits a shallow minimum followed by increase of resistivity ρ(T) −lnT, which hints at a substantial Kondo screening effect. Ce2Rh3Ge belongs to a small group of strongly correlated cerium compounds in which the two competing effects of Kondo and RKKY interactions produce long-range magnetic order from strongly hybridized and intermediate-valent 4 f spins. At sufficiently low temperatures Ce2Rh3Ge scales well with the Kadowaki–Woods ratio A/γ2 and the value of the Wilson ratio χ(T → 0)/γ found for this compound classifies it as a mixed-valence compound. The presence of valence fluctuation and magnetic order it is rare for these attributes to be found simultaneously in same compound, in same temperature range. In our opinion a novelty of presented results of Ce2Rh3Ge is that this compound adds a new member to a small but growing class of systems bearing a strongly mixed- or intermediate-valent 4 f magnetic moment, but in which the lattice of spins nevertheless end up finding it possible to order magnetically.

Investigation of chiral phases near helical ordering temperature in MnSi

Chiral modulation crossovers and phase transitions of MnSi in the vicinity of the long-range magnetic ordering temperature are investigated. Notable anomalies in specific heat as a function of magnetic fields that directly correspond to the speculated chiral modulations in the precursor region are observed. The magneto-thermal (H–T) phase diagram reveals the existence of repulsive, attractive, and confined skyrmion phases. Resistivity analysis suggests strong electron–magnon coupling as well as the possibility of strong electron correlations in MnSi supported by the Kadowaki–Woods ratio (), which is close to that of moderate heavy Fermion metals.

Synthesis and physical properties of Cr–P-based intermetallic compounds: Cr3P, Cr3PC, and Cr3PN

We report the synthesis, crystal structures and electrical/thermal transport properties of polycrystalline Cr3PX (X=C, N, and vacancy) compounds. As a result, Cr3PC and Cr3PN have an orthorhombic crystal structure with space group Cmcm, while Cr3P shows a tetragonal structure with space group I−4. All samples show good metallic behavior between 2 and 350K in the measurement of electrical transport. At the same time, the Fermi-liquid behavior is observed at low temperatures for Cr3PC (N) (2–50K) and Cr3P (2–22K). Based on the fitting results of low-temperature resistivity and specific heat, large Kadowaki–Woods ratio of Cr3PC (N) and Cr3P were obtained, suggesting a considerably strong electron–electron correlation for all our samples. By analyzing the thermal conductivity data, it is found that the electron thermal conductivity is predominantly stronger than the phonon one at low temperatures for Cr3P. As to Cr3PC and Cr3PN, however, both contributions are comparable in the whole temperature range investigated. Besides, phonon-drag effect was observed in the Seebeck coefficient α(T) curves for all the samples, which may explain the crossover of sign at 70K in α(T) of Cr3PN.

Good Thermoelectric Performance in Strongly Correlated System SnCCo3 with Antiperovskite Structure

We report the magnetic, electrical, and thermoelectric properties of SnCCo3, where good thermoelectric performance [figure of merit ZT ∼ 0.035(2), 258 K] and strong electron correlation (Kadowaki-Woods ratio RKW ∼ 4a0) are observed. The thermoelectric properties of ACCo3 (A = Al, Ga, Ge) and SnCM3 (M = Mn, Fe) were also investigated for comparison. As a result, the ZT value of SnCCo3 is the largest among all of those samples, which is mainly attributed to the large Seebeck coefficient caused by the strong electron correlation and low carrier density. Moreover, the ZT value can be effectively enhanced by proper chemical doping in SnCCo3.

In-plane superfluid density and microwave conductivity of the organic superconductorκ-(BEDT-TTF)2Cu[N(CN)2]Br: Evidence ford-wave pairing and resilient quasiparticles

We report the in-plane microwave surface impedance of a high-quality single crystal of $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu[N(CN)${}_{2}$]Br. In the superconducting state, we find three independent signatures of $d$-wave pairing: (i) a strong, linear temperature dependence of superfluid density; (ii) deep in the superconducting state the quasiparticle scattering rate $\ensuremath{\Gamma}\ensuremath{\sim}{T}^{3}$; and (iii) no BCS coherence peak is observed in the quasiparticle conductivity. Above ${T}_{c}$, the Kadowaki-Woods ratio and the temperature dependence of the in-plane conductivity show that the normal state is a Fermi liquid below $\ensuremath{\simeq}23$ K, yet resilient quasiparticles dominate the transport up to $\ensuremath{\simeq}50$ K.

Mn1-xFexCoGe: A strongly correlated metal in the proximity of a noncollinear ferromagnetic state

An unusually large Kadowaki–Woods ratio of A/γ2 ∼ 43 μΩ·cm·mol2·K2·J−2 has been observed for intermetallic Mn1-xFexCoGe compounds in the proximity of x = 0.2 where the magnetic state of itinerant electrons system changes. The ratio is approximately four times larger than observed for heavy fermion systems. The manifestation of the strong electron correlations can be realized from the anisotropic origin of the effect through the substantial reduction of interlayer transport of heavy quasiparticles with comparable mean-free path and interlayer spacing in the proximity of a noncollinear ferromagnetic state associated with a large density of states at the Fermi level.

Valence fluctuation in CeMo2Si2C

We report on the valence fluctuation of Ce in CeMo2Si2C as studied by means of magnetic susceptibility χ(T), specific heat C(T), electrical resistivity ρ(T) and X-ray absorption spectroscopy. Powder X-ray diffraction revealed that CeMo2Si2C crystallizes in CeCr2Si2C-type layered tetragonal crystal structure (space group P4/mmm). The unit cell volume of CeMo2Si2C deviates from the expected lanthanide contraction, indicating non-trivalent state of Ce ions in this compound. The observed weak temperature dependence of the magnetic susceptibility and its low value indicate that Ce ions are in valence fluctuating state. The formal LIII Ce valence in CeMo2Si2C〈ν∼〉=3.14 as determined from X-ray absorption spectroscopy measurement is well below the value 〈ν∼〉≃3.4 in tetravalent Ce compound CeO2. The temperature dependence of specific heat does not show any anomaly down to 1.8 K which rules out any magnetic ordering in the system. The Sommerfeld coefficient obtained from the specific heat data is γ = 23.4 mJ/mol K2. The electrical resistivity follows the T2 behavior in the low temperature range below 35 K confirming a Fermi liquid behavior. Accordingly both the Kadowaki Woods ratio A/γ2 and the Sommerfeld Wilson ratio χ(0)/γ are in the range expected for Fermi-liquid systems. In order to get some information on the electronic states, we calculated the band structure within the density functional theory, eventhough this approach is not able to treat 4f electrons accurately. The non-f electron states crossing the Fermi level have mostly Mo 4d character. They provide the states with which the 4f sates are strongly hybridized, leading to the intermediate valent state.

Observation of superconductivity in the intermetallic compound β-IrSn4

Low-temperature dc-magnetization, ac electrical resistivity and specific heat measurements were performed on single crystals of the intermetallic compound β-IrSn4. The compound crystallizes in the tetragonal MoSn4-type structure (space group I41/acd) and exhibits superconductivity below Tc = 0.9 ± 0.05 K. Further, the magnitude of the ratios ΔCp/(γnkBTc) = 1.29, 2Δ/(kBTc) = 3.55 and of the electron–phonon coupling imply that superconductivity in β-IrSn4 can be ascribed to a s-wave weak coupling regime. We determined crucial thermodynamic characteristics of the superconducting state. It turned out that depending on the assumption of either a spherical or non-spherical Fermi surface, the superconductivity can be ascribed to either a type-I and type-II/1 or type-II in clean limit, respectively. However, the behavior of the upper critical field and the anisotropic crystalline structure of the studied compound provide strong support to the type-II superconductivity. In the normal state the resistivity exhibits a prominent quadratic temperature dependence, which together with a large Kadowaki–Woods ratio and with the enhanced effective mass indicate that the electrons in β-IrSn4 are strongly correlated.

The magnetic, electrical transport and thermal transport properties of Fe-based antipervoskite compounds ZnCxFe3

The effects of carbon concentration on the crystal structure, magnetic, and electrical/thermal transport properties of ZnCxFe3 (1.0 ≤ x ≤ 1.5) have been investigated systematically. Both the Curie temperature and the saturated magnetization decrease firstly and then reach saturation with increasing x. The investigations of heat capacity and resistivity indicate that ZnC1.2Fe3 displays a strongly correlated Fermi liquid behavior considering its Kadowaki-Woods ratio (∼0.64 a0). Around the ferromagnetic-paramagnetic phase transition (∼358 K), a reversible room-temperature magnetocaloric effect is observed. The relative cooling power (RCP) is ∼164 J/kg (∼385 J/kg) with the magnetic field change ΔH = 20 kOe (45 kOe). Considering the considerable large RCP, inexpensive and innoxious raw materials, ZnC1.2Fe3 is suggested to be a promising candidate for room-temperature magnetic refrigeration. Furthermore, the studies of thermal transport properties indicate that ZnC1.2Fe3 can also be a potential thermoelectric material with the dimensionless figure of merit (ZT = α2T/ρk) reaching its maximum of 0.0112 around 170 K.

Thermal and electrical transport in the spin density wave antiferromagnet CaFe4As3

We present here measurements of the thermopower, thermal conductivity, and electrical resistivity of the newly reported compound CaFe${}_{4}$As${}_{3}$. Evidence is presented from specific heat and electrical resistivity measurements that a substantial fraction of the Fermi surface survives the onset of spin density wave (SDW) order at the N\'eel temperature ${T}_{\mathrm{N}}=88$ K and its subsequent commensurate lock-in transition at ${T}_{2}=26.4$ K. The specific heat below ${T}_{2}$ consists of a normal metallic component from the ungapped parts of the Fermi surface and a Bardeen-Cooper-Schrieffer (BCS) component that represents the SDW gapping of the Fermi surface. A large Kadowaki-Woods ratio is found at low temperatures, showing that the ground state of CaFe${}_{4}$As${}_{3}$ is a strongly interacting Fermi liquid. The thermal conductivity $\ensuremath{\kappa}$ of CaFe${}_{4}$As${}_{3}$ is an order of magnitude smaller than those of conventional metals at all temperatures, due to a strong phonon scattering. The thermoelectric power $S$ displays a sign change from positive to negative indicating that a partial gap forms at the Fermi level with the onset of commensurate spin density wave order at ${T}_{2}=26.4$ K. The small value of the thermopower $S$ and the enhancements of the resistivity due to gap formation and strong quasiparticle interactions offset the low value of the thermal conductivity $\ensuremath{\kappa}$, yielding only a modest value for the thermoelectric figure of merit $\mathcal{Z}\ensuremath{\leqslant}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ in CaFe${}_{4}$As${}_{3}$. The results of ab initio electronic structure calculations are reported, confirming that the sign change in the thermopower at ${T}_{2}$ is reflected by a sign change in the slope of the density of states at the Fermi level. Values for the quasiparticle renormalization $Z$ are derived from measurements of the specific heat and thermopower, indicating that as $T\ensuremath{\rightarrow}0$, ${\mathrm{CaFe}}_{4}{\mathrm{As}}_{3}$ is among the most strongly correlated of the known Fe-based pnictide and chalcogenide systems.

AlN x Mn 3 : A possible high-temperature soft magnetic material and strongly correlated system

Structural, magnetic, electrical, and thermal transport properties of antiperovskite compounds AlNxMn3 (x=1,1.1,1.2) have been investigated systematically. With increasing x, the lattice constant increases monotonously while the Curie temperature TC decreases. Both the high TC and small coercive fields consistently indicate AlNxMn3 may be a promising high-temperature soft magnetic material. The resistivity displays T2-dependence below 30 K and the Kadowaki–Woods ratio is about 107.7 μΩ cm/K2, indicating a possible strongly correlated Fermi-liquid behavior in AlNMn3. Further analysis suggests that the electron-type carriers are dominant and the thermal conductivity mainly originates from the lattice contribution.

Magnetic transition and spin fluctuations in the unconventional antiferromagnetic compoundYb3Pt4

Muon spin rotation and relaxation measurements have been carried out on the unconventional antiferromagnetYb3Pt4. Oscillations areobserved below TN = 2.22(1) K, consistent with the antiferromagnetic (AFM) Néel temperature observed in bulkexperiments. In agreement with neutron diffraction experiments the oscillation frequencyωμ(T)/2π followsan S = 1/2 mean-field temperature dependence, yielding a quasistatic local field of 1.71(2) kOe atT = 0. A crude estimate gives an ordered moment of ∼ 0.66 μB atT = 0, comparableto 0.81 μB from neutron diffraction. As from above the dynamic relaxation rateλd exhibits no critical slowing down, consistent with a mean-field transition. In the AFM phase aT-linearfit to λd(T), appropriate to a Fermi liquid, yields highly enhanced values ofλd/T and the Korringaconstant Kμ2T/λd, with Kμ the estimated muon Knight shift. A strong suppression ofλd by applied field is observed in the AFM phase. These properties are consistentwith the observed large Sommerfeld–Wilson and Kadowaki–Woods ratios inYb3Pt4 (although the data do not discriminate between Fermi-liquidand non-Fermi-liquid states), and suggest strong enhancement ofq≈0 spin correlations between large-Fermi-volume band quasiparticles in the AFM phase ofYb3Pt4.

Coexistence of Superconductivity and Ferromagnetic Spin-Fluctuation in Ca3Ir4Sn13 Single Crystal

Temperature dependences of the electrical resistivity, magnetic susceptibility and specific heat have been measured on the single crystal Ca 3 Ir 4 Sn 13 . This compound shows a bulk superconducting transition at ≈7.0 K. We observe peak-like anomalies, which are attributed to the ferromagnetic spin-fluctuations, in susceptibility and resistivity. The resistivity shows a non Fermi-liquid behavior as ρ=ρ 0 + A * T n ( n =1.2 for H =0) in normal state at low temperatures, and develops into a Fermi-liquid behavior by the suppression of the ferromagnetic spin fluctuation on increasing magnetic fields ( n =2 for H =14 T). The superconductivity disappears for magnetic field H ≥7 T as well as the diminution of the ferromagnetic spin fluctuations. We find that the superconductivity coexists with the ferromagnetic spin fluctuations and seems to cooperate in this material. The large values of Δ C /γ n T c =2.78, Kadowaki–Woods ratio A /γ n 2 =1.14×10 -5 µΩ·cm (K·mol/mJ) 2 and Wilson ratio R w =π 2 k B 2 χ p /3µ B 2 γ≈1.7, indicate the electrons are highly correlated in this system.

Low temperature properties of pnictide CrAs single crystal

High quality single crystal CrAs was grown by Sn flux method. The results of magnetic susceptibility and electrical resistivity are reported in a temperature range of 2 to 800 K. At low temperatures, a T2 dependence of resistivity is observed showing a Fermi-liquid behavior. The Kadowaki-Woods ratio is found to be 1×10−5 μΩ cm mol2 K2 mJ−2, which fits well to the universal value for many correlated electron systems. At about 270 K, a clear magnetic transition is observed with sharp changes of resistivity and susceptibility. Above 270 K, a linear-temperature dependence of the magnetic susceptibility is observed up to 700 K, which resembles the T-dependent magnetic susceptibility of parents of iron-pnictides superconductors.