Kadowaki Woods Research Articles

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.

Enhanced density of states in Li(Fe1−xCox)As single crystals near x = 0.06 as implied by transport properties

Single crystals of Li(Fe1−xCox)As (x=0.00, 0.02, 0.04, 0.06, 0.08 and 0.1) were grown by the Sn-flux method and their resistivity behaviors were investigated. As the Co concentration x increased, the superconducting transition temperature Tc as well as the residual resistivity ratio systematically decreased, indicating a monotonic doping of electron carriers. Upon the analysis of the Fermi-liquid behavior of the resistivity and by using the Kadowaki–Woods relation, the T2 coefficient of the resistivity A and the Sommerfeld coefficient γN were found to increase abruptly near x=0.06. This is the point in the phase diagram at which the density of states at the Fermi energy (proportional to γN) is expected to be enhanced due to the collapse of the small hole-pockets. This result suggests that the Li(Fe1−xCox)As series at least up to x=0.08 remain as compensated metals and their transport properties are determined by multi-band effects.

Dense Kondo Effect in Caged Compound CeRu2Zn20

We prepared single crystals of the caged compound CeRu2Zn20 by the flux method, and measured the magnetization, magnetic susceptibility, specific heat, and electrical resistivity. The lattice constant measured at room temperature suggests that the Ce ion is trivalent in this compound. The temperature dependence of magnetic susceptibility obeys the Curie–Weiss law with the effective Bohr magneton of Ce3+ at high temperatures. However, at low temperatures, CeRu2Zn20 indicates no phase transition down to 2 K and shows typical behaviors of a heavy Fermion system; the enhanced magnetic susceptibility χ0, the enhanced electronic specific-heat coefficient γ0, and the electrical resistivity with the temperature dependence of \(AT^{2}\), although these parameters somewhat depend on samples. The Wilson ratio \(R_{\text{W}}\) of χ0/γ0 is approximately 2, which does not depend on samples. A/γ0 is plotted on the line of \(N = 2\) in the new Kadowaki–Woods plots. The Kondo temperature \(T_{\text{K}}\) is roughly estima...

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.

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.

Magnetic and Superconducting Properties in Single Crystalline Fe1+δTe1-xSex (x<0.50) System

The spin-fluctuation effect in the Se-substituted single crystalline Fe 1+δ Te 1- x Se x ( x = 0, 0.05, 0.12, 0.20, 0.30, 0.33, 0.45, and 0.48; 0≤δ≤0.12) and the polycrystalline Fe 1.11 Se has been studied by the measurements of the X-ray diffraction, the magnetic susceptibility under high magnetic fields and the electrical resistivity under magnetic fields up to 14 T. The samples with x = 0.05, 0.12, 0.20, 0.30, 0.33, 0.45, and 0.48 show superconducting transition temperatures in the ranger of 10–14 K. We obtained their intrinsic susceptibilities by the Honda–Owen method. A nearly linear-in- T behavior in magnetic susceptibility of Se-rich superconducting samples was observed, indicating the antiferromagnetic spin fluctuations have a strong link with the superconductivity in this series. The upper critical field µ 0 H c2 orb for T →0 was estimated to exceed the Pauli paramagnetic limit. The Kadowaki–Woods and Wilson ratios indicate that electrons are strongly correlated in this system. Furthermore, the s...

Evidence of a heavy fermion state in the disordered Ce-alloy system without translation symmetry

We have measured the specific heat ${C}_{p}$ and the resistivity $\ensuremath{\rho}$ on sputtered amorphous ${\text{Ce}}_{x}{\text{Y}}_{80\ensuremath{-}x}{\text{Mn}}_{20}$ alloys. The low-temperature specific heat has been found to follow ${C}_{p}=\ensuremath{\gamma}T+\ensuremath{\beta}{T}^{3}$ for all the samples. The $T$-linear coefficient $\ensuremath{\gamma}$ increases linearly with increasing Ce concentration and becomes very large $(\ensuremath{\gamma}>100\text{ }\text{mJ}\text{ }{\text{mol}}^{\ensuremath{-}1}\text{ }{\text{K}}^{\ensuremath{-}2})$ in the Ce-rich region. The ${\ensuremath{\gamma}}_{\text{Ce}}$ per Ce atom is about $200\text{ }\text{mJ}\text{ }{\text{Ce-mol}}^{\ensuremath{-}1}\text{ }{\text{K}}^{\ensuremath{-}2}$ and almost independent of the Ce concentration. The large ${\ensuremath{\gamma}}_{\text{Ce}}$ has been interpreted in terms of the electronic specific heat owing to the $4f$-electron mixed with conduction electrons. In the resistivity $\ensuremath{\rho}$, the $\ensuremath{-}\text{log}\text{ }T$ dependence has been observed at low temperature for $x\ensuremath{\ge}51$, which suggests the formation of the dense Kondo state. The $\ensuremath{\rho}$ vs $T$ for $x\ensuremath{\ge}62$ exhibits a maximum and decreases as $\ensuremath{\rho}\ensuremath{\propto}A{T}^{2}$ with a large coefficient $A$ down to the low-temperature limit. These results suggest the formation of a heavy fermion state as the ground state for $x\ensuremath{\ge}62$. We made the Kadowaki-Woods (KW) plot for $x\ensuremath{\ge}62$ and found the $A/{\ensuremath{\gamma}}^{2}$ value to be much smaller than that of the conventional KW relation probably due to the $f$-orbital degeneracy.

High-pressure transport and microcalorimetry studies on high qualityYbCu2Si2single crystals

We report electrical transport and calorimetry studies on high quality single crystals of ${\text{YbCu}}_{2}{\text{Si}}_{2}$ under pressure in diamond and Bridgman anvil pressure cells with liquid pressure-transmitting media. At ambient pressure, in the best samples we find residual resistivities of less than $0.5\text{ }\ensuremath{\mu}\ensuremath{\Omega}\text{ }\text{cm}$. We have also determined the anisotropy of the temperature dependence of the resistivity and shown that this can account for sample-dependent differences of the resistivity. We confirm the previously reported transition to a magnetically ordered ground state with pressure and we have precisely determined the $(p,T)$ phase diagram using pure samples, good hydrostatic pressure conditions, and calorimetry measurements. Above 8.8 GPa we see a clear signature of the transition in the specific heat. We discuss the low-temperature resistivity and specific-heat behaviors in respect to the universal Kadowaki Woods ratio, to other ytterbium-based strongly correlated systems, and to the cerium-based counterpart, ${\text{CeCu}}_{2}{\text{Si}}_{2}$.

A unified explanation of the Kadowaki–Woods ratio in strongly correlated metals

Discoveries of ratios whose values are constant within broad classes of materials have led to many deep physical insights. The Kadowaki–Woods ratio (KWR; refs 1, 2) compares the temperature dependence of a metal’s resistivity to that of its heat capacity, thereby probing the relationship between the electron–electron scattering rate and the renormalization of the electron mass. However, the KWR takes very different values in different materials3,4. Here we introduce a ratio, closely related to the KWR, that includes the effects of carrier density and spatial dimensionality and takes the same (predicted) value in organic charge-transfer salts, transition-metal oxides, heavy fermions and transition metals—despite the numerator and denominator varying by ten orders of magnitude. Hence, in these materials, the same emergent physics is responsible for the mass enhancement and the quadratic temperature dependence of the resistivity, and no exotic explanations of their KWRs are required.

Heavy-Mass Behavior of Ordered Perovskites ACu3Ru4O12 (A = Na, Ca, La)

We synthesized A Cu 3 Ru 4 O 12 ( A = Na, Na 0.5 Ca 0.5 , Ca, Ca 0.5 La 0.5 , La) and measured their DC magnetization, AC susceptibility, specific heat, and resistivity, in order to investigate the effects of the hetero-valent substitution. A broad peak in the DC magnetization around 200 K was observed only in CaCu 3 Ru 4 O 12 , suggesting the Kondo effect due to localized Cu 2+ ions. However, the electronic specific heat coefficients γ exhibit large values not only for CaCu 3 Ru 4 O 12 but also for all the other samples. Moreover, the Wilson ratio and the Kadowaki–Woods ratio of our samples are all similar to the values of other heavy-fermion compounds. These results question the Kondo effect as the dominant origin of the mass enhancement, and rather indicate the importance of correlations among itinerant Ru electrons.

Kondo universality and energy scales in plutonium

The macroscopic properties of α and δ-plutonium were analyzed within the Fermi-liquid approach аs well as the properties of a few model rare earth-based systems. The following major parameters in Pu were estimated within the single-site approximation: the characteristic Kondo energy, the f-electron shell occupation number, the effective degeneracy of the ground f-multiplet, the crystal field splitting energy. The ground state in δ-plutonium is a quantum superposition of two atomic states Pu 3+ and Pu 2+. According to its macroscopic physical properties δ-Pu at low temperature is very close to the intermediate valence regime. The temperature dependence of the static magnetic susceptibility in plutonium was calculated. Since the universal Wilson criterion and the Kadowaki–Woods universal relation are valid in δ-Pu it can be considered as a Kondo system, while the position of α-plutonium in the general classification of solids remains a puzzle. The plutonium homology issue is discussed.

Investigation of Yb2Pt6Al15 single crystals: heavy fermion system with a large local moment degeneracy

We grew single crystals of Yb2Pt6Al15 and investigated the magnetic properties of this compound by means of susceptibility χ(T), specific heat C(T), resistivity ρ (T) and thermoelectric power S(T) measurements. While all properties follow in general the behavior typical for Kondo-lattice systems, χ(T) and C(T)/T present broad maxima in the T range 17–35 K, which matches nicely the prediction of the Coqblin–Schrieffer model for J= 7/2. A large degeneracy of the local moment is also supported by a reduced Kadowaki–Woods ratio. Thus, the analysis of all investigated properties evidences Yb2Pt6Al15 to be a paramagnetic Kondo-lattice system with the whole J= 7/2 multiplet involved in the formation of the Kondo state, a Kondo temperature of the order of 60 K, and a heavy Fermi-liquid ground state with a Sommerfeld coefficient γ 0 = 0.33 J (mol-Yb)−1 K−2 corresponding to a mass enhancement of the order of 30.

Anomalous Anisotropic Magnetoresistance in Heavy-Fermion PrFe4P12

We have investigated the anisotropy of the magnetoresistance in the Pr-based HF compound PrFe4P12. The large anisotropy of effective mass and its strong field dependence have been confirmed by resistivity measurements. Particularly for H||[111], where the effective mass is most strongly enhanced, the non-Fermi liquid behavior has been observed. Also, we have found the angular dependence of the magnetoresistance sharply enhanced at H||[111], which is evidently correlated with both the non-Fermi liquid behavior and the high-field ordered state (B-phase).

Myriad of correlated electron effects found in the [formula omitted] family

The dilute, rare earth bearing family of RT 2 Zn 20 (R = rare earth, T = transition metal) intermetallic compounds can be tuned to a nearly ferromagnetic Fermi liquid that is closer to the Stoner limit than Pd (for R = Y, Lu and T = Fe). The submersion of moment bearing rare earths (Gd–Tm) into this highly polarizable matrix gives rise to exceptionally high temperature ferromagnetism (e.g. T C = 86 K for GdFe 2 Zn 20 ) for a compound with only one moment bearing ion out of 23 in the formula unit. In addition to this d-shell correlated electron behavior, 4f hybridization gives rise to six, new, Yb-based heavy fermion compounds: YbT 2 Zn 20 (T = Fe, Ru, Os, Co, Rh, Ir). These half dozen compounds manifest a range of Yb ion degeneracy values ( N ) for T ∼ T K , ranging from N = 4 – 8 . Not only do these compounds offer a clear testing ground for the effects of low temperature degeneracy on the correlated electron state, they also present an exceptionally clear experimental confirmation of the generalized Kadowaki–Woods formalism.