Heavy Fermion State Research Articles

Magnetic properties and effect of pressure on the electronic state of EuCo2Ge2

EuCo2Ge2 with the tetragonal structure is a Eu-divalent antiferromagnet with the Néel temperature TN=23 K. The magnetic easy-axis corresponds to the [100] direction (a-axis), while the [001] direction (c-axis) is a hard-axis. The magnetization for H∥[100] indicates a metamagnetic transition at 25kOe and saturates above 75kOe. On the other hand, the hard-axis magnetization increases approximately linearly and saturates above 110kOe. The magnetic phase diagram was constructed. A characteristic feature in EuCo2Ge2 is known as a valence transition under pressure, from Eu 2+δ to Eu 3−δ′(δ, δ′<1). We also clarified the valence transition by measuring the electrical resistivity under pressure. The valence transition occurs at 3GPa, with a hysteresis, and terminates at about 4.5GPa. Further increasing pressure, the electronic state is changed into a moderate heavy fermion state and approaches the nearly trivalent electronic state.

Specific heat and Hall effect of the ferromagnetic Kondo lattice UCu0.9Sb2

We have investigated the electrical resistivity ρ, specific heat Cp and Hall coefficient RH on a single crystal of a ferromagnetic Kondo lattice UCu0.9Sb2. The experimental , Cp(T) and data evidence a bulk magnetic phase transition at K, and additionally exhibit an unexpected bump located in the temperature range TC/10–TC/3. UCu0.9Sb2 has an enhanced electronic specific heat coefficient mJ molK−2, corresponding to Kondo temperature K. An analysis of the Hall effect data for j//(a, b)-plane and H// c-axis reveals that the low-temperature ordinary Hall coefficient R0 is positive, suggesting that p-type electrical conductivity is dominant. The density of the carriers at 2 K is about 0.6 holes f.u.−1, which may categorize the studied compound into class of low carrier density compounds. Combined γ and R0 data divulge an effective mass of charge carriers 27 me. This finding together with quite low Hall mobility cm2 Vs−1 and Kadowaki–Woods ratio cm (mol K2 mJ−1)2, manifest the development of heavy-fermion state in the ferromagnetic UCu0.9Sb2 compound at low temperatures.

Unique Pressure versus Temperature Phase Diagram for Antiferromagnets Eu2Ni3Ge5and EuRhSi3

We studied the magnetic properties of the antiferromagnets Eu2Ni3Ge5 and EuRhSi3 by measuring their electrical resistivity, specific heat, magnetic susceptibility, magnetization, and thermoelectric power, together with the electrical resistivities at high pressures of up to 15 GPa. These compounds have almost divalent Eu ions at ambient pressure and order antiferromagnetically with a successive change in the antiferromagnetic structure at TN = 19 K and \(T'_{\text{N}} = 17\) K in Eu2Ni3Ge5, and at TN = 49 K and \(T'_{\text{N}} = 45\) K in EuRhSi3. Magnetic field versus temperature (H–T) phase diagrams were constructed for both compounds from the magnetization measurements. The Neel temperature in Eu2Ni3Ge5 was found to increase up to 7 GPa but to decrease continuously with further increasing pressure, without the so-called valence transition. Under a high pressure of 15 GPa, Kondo-like behavior of the electrical resistivity was observed, suggesting the existence of the heavy-fermion state at low temperatu...

Heavy Weyl Fermion State in CeRu4Sn6

Weyl semimetals have highly mobile charged particles that may make them useful in electronic devices, but only a few of them have been identified thus far. Computational analysis reveals that the compound CeRu${}_{4}$Sn${}_{6}$ exhibits Weyl-like behavior and may be a new type of material known as a heavy Weyl fermion state.

Anisotropic B–T Phase Diagram of Non-Kramers System PrRh2Zn20

We have investigated the low temperature quadrupolar phenomena of the non-Kramers system PrRh2Zn20 under magnetic fields in the [100] and [110] directions. Our experiments reveal the B-T phase diagram of PrRh2Zn20 involving four electronic states regardless of the field direction, namely, a non-Fermi liquid (NFL) state, an antiferro-quadrupolar (AFQ) ordered state, a novel heavy-fermion (HF) state, and a field-induced singlet (FIS) state. In the wide range of the NFL state, the resistivity can be well scaled by a characteristic temperature, suggesting the realization of the quadrupole Kondo effect. The HF state exhibits a Fermi liquid behavior with a large A coefficient of the T^2 term in the resistivity, suggesting the formation of nontrivial heavy quasi-particles. The FIS state results from the considerable splitting of a non-Kramers doublet by a magnetic field. The phase diagram shows a large anisotropy with respect to the field direction. It is found that the anisotropy of the phase diagram can be explained in terms of that of the energy splitting of the non-Kramers doublet by a magnetic field. This indicates that the low temperature properties of PrRh2Zn20 are governed by the non-Kramers doublet, namely, quadrupole degrees of freedom. Since a similar phase diagram has been obtained for the related compound PrIr2Zn20, it is expected that the B-T phase diagram constructed in this work is universal throughout non-Kramers systems.

Pressure Evolution of Characteristic Electronic States in EuRh2Si2 and EuNi2Ge2

EuRh2Si2 and EuNi2Ge2 are known to reveal the valence transition at pressure of about 1 and 2 GPa, respectively, from a divalent state (Eu2+) to a nearly Eu-trivalent state (Eu3–δ′). We have succeeded in growing single crystals of EuRh2Si2 by the Bridgman method and EuNi2Ge2 by the In-flux method. In order to clarify the transition of electronic properties from Eu2+ to Eu3–δ′ in detail, we carried out electrical resistivity measurements under pressure. EuRh2Si2 indicates a remarkable first-order valence transition in the pressure range from 1 to 2 GPa with a sharp hysteresis in the electrical resistivity. At 2.12 GPa, the temperature dependence of the electrical resistivity exhibit a broad peak, implying a moderate heavy-fermion state such as EuPd2Si2 and EuIr2Si2. While, EuNi2Ge2 indicates a first-order valence transition in the pressure range from 2 to 3.22 GPa with a hysteresis in the electrical resistivity. At 3.60 GPa, the electrical resistivity of EuNi2Ge2 also shows a characteristic behavior for a moderate heavy-fermion compound. Pressure - temperature phase diagrams for both compounds are constructed.

Divalent, trivalent, and heavy fermion states in Eu compounds

Most of the Eu compounds are in the divalent (Eu) electronic state and order magnetically. On the other hand, the Eu-trivalent (Eu) compounds exist but are small in number. An energy difference between the Eu and Eu states is, however, not extremely large. Therefore, the valence transition occurs in some Eu compounds. We present the characteristic properties of the Eu compounds from several viewpoints: a canting magnetisation in the Eu-antiferromagnets, the Fermi surface property in the Eu-compounds of EuPd and EuCoSi, the heavy fermion state in EuNiP, the temperature-induced valence transition in EuPdSi, the pressure-induced valence transition in EuRhSi and EuRuP, and the heavy fermion state approaching to the quantum critical point with increasing pressure in EuNiGe .

Quadrupole-driven non-Fermi-liquid and magnetic-field-induced heavy fermion states in a non-Kramers doublet system

Orbital degrees of freedom in condensed matters could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focussed on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at TQ = 0.11 K. We observed non-Fermi liquid (NFL) behaviors emerging in the electrical resistivity and the 4f contribution to the specific heat, C_4f, in the paramagnetic state at T > TQ. Moreover, in magnetic fields below 6 T, all data set of the electrical resistivity and C_4f(T) are well scaled with characteristic temperatures T0's. This is the first observation of the NFL state in the nonmagnetic quadrupole-active system, whose origin is intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons. Below 0.13 K, the electrical resistivity and C_4f(T) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.

Possible Heavy Fermion State of the Caged Cubic Compound NdV2Al20

The magnetic susceptibility M/H, the magnetization M, and the specific heat C were measured for the caged cubic compound NdV2Al20 single crystal at temperatures down to 0.5 K in magnetic fields H along the three principal crystallographic axes [001], [101], and [111]. The electrical resistivity ρ was measured in the temperature range of 1.9–300 K. A ferromagnetic phase transition was observed at a Curie temperature TC of 1.81 K. The easy direction of magnetization was the [001] direction at low fields, and changed to the [101] direction above 2 T. On cooling below 2 K at zero field, the specific heat divided by temperature, C/T, resulted in a large electronic specific heat coefficient γ of about 2 J/(K2 mol). A large C/T of more than 1 J/(K2 mol) remained at 9 T below 2 K. The temperature dependence of ρ showed the −ln T dependence at around 10 K. These behaviors possibly originate from a heavy fermion state.

Intermediate valence to heavy fermion through a quantum phase transition inYb3(Rh1−xTx)4Ge13(T=Co,Ir)single crystals

Single crystals of ${\mathrm{Yb}}_{3}(\mathrm{Rh}{}_{1\ensuremath{-}x}{T}_{x}){}_{4}{\mathrm{Ge}}_{13}$ $(T\phantom{\rule{0.5em}{0ex}}=\phantom{\rule{0.5em}{0ex}}\mathrm{Co},\phantom{\rule{0.16em}{0ex}}\mathrm{Ir})$ have been grown using the self-flux method. Powder x-ray diffraction data on these compounds are consistent with the cubic structure with space group $Pm\overline{3}n$. Intermediate-valence behavior is observed in ${\mathrm{Yb}}_{3}(\mathrm{Rh}{}_{1\ensuremath{-}x}{T}_{x}){}_{4}{\mathrm{Ge}}_{13}$ upon $T$ = Co doping, while $T$ = Ir doping drives the system into a heavy-fermion state. Antiferromagnetic order is observed in the Ir-doped samples ${\mathrm{Yb}}_{3}(\mathrm{Rh}{}_{1\ensuremath{-}x}{T}_{x}){}_{4}{\mathrm{Ge}}_{13}$ for $0.5lx\ensuremath{\le}1$ with ${T}_{N}=0.96$ K for ${\mathrm{Yb}}_{3}\mathrm{Ir}{}_{4}{\mathrm{Ge}}_{13}$. With decreasing $x$, the magnetic order is suppressed towards a quantum critical point around ${x}_{c}$ = 0.5, accompanied by non-Fermi-liquid behavior evidenced by logarithmic divergence of the specific heat and linear temperature dependence of the resistivity. The Fermi-liquid behavior is recovered with the application of large magnetic fields.

Crossover from a heavy fermion to intermediate valence state in noncentrosymmetric Yb2Ni12(P,As)7.

We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb2Ni12Pn7 (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat and partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS). These demonstrate a crossover upon reducing the unit cell volume, from an intermediate valence state in Yb2Ni12As7 to a heavy-fermion paramagnetic state in Yb2Ni12P7, where the Yb is nearly trivalent. Application of pressure to Yb2Ni12P7 suppresses TFL, the temperature below which Fermi liquid behavior is recovered, suggesting the presence of a quantum critical point (QCP) under pressure. However, while there is little change in the Yb valence of Yb2Ni12P7 up to 30 GPa, there is a strong increase for Yb2Ni12As7 under pressure, before a near constant value is reached. These results indicate that any magnetic QCP in this system is well separated from strong valence fluctuations. The pressure dependence of the valence and lattice parameters of Yb2Ni12As7 are compared and at 1 GPa, there is an anomaly in the unit cell volume as well as a change in the slope of the Yb valence, indicating a correlation between structural and electronic changes.

Fermion production during and after axion inflation

We study derivatively coupled fermions in axion-driven inflation, specifically mϕ2ϕ2 and monodromy inflation, and calculate particle production during the inflationary epoch and the post-inflationary axion oscillations. During inflation, the rolling axion acts as an effective chemical potential for helicity which biases the gravitational production of one fermion helicity over the other. This mechanism allows for efficient gravitational production of heavy fermion states that would otherwise be highly suppressed. Following inflation, the axion oscillates and fermions with both helicities are produced as the effective frequency of the fermion field changes non-adiabatically. For certain values of the fermion mass and axion-fermion coupling strength, the two helicity states are produced asymmetrically, resulting in unequal number-densities of left- and right-helicity fermions.

Extended s-wave pairing symmetry on the triangular lattice heavy fermion system

We investigate the pairing symmetry of the Kondo-Heisenberg model on triangular lattice, which is believed to capture the core competition of Kondo screening and local magnetic exchange interaction in heavy electron compounds. On the dominant background of the heavy fermion state, the introduction of the Heisenberg antiferromagnetic interaction (J H ) leads to superconducting pairing instability. Depending on the strength of the interactions, it is found that the pairing symmetry favours an extended s-wave for small J H and high conduction electron density but a chiral $$d_{x^2 - y^2 } + id_{xy}$$ -wave for large J H and low conduction electron density, which provides a phase diagram of pairing symmetry from the calculations of the ground-state energy. The transition between these two pairing symmetries is found to be first-order. Furthermore, we also analyze the phase diagram from the pairing strengths and find that the phase diagram obtained is qualitatively consistent with that based on the ground-state energy. In addition, we propose an effective single-band BCS Hamiltonian, which is able to describe the low-energy thermodynamic behaviors of the heavy fermion superconducting states. These results further deepen the understanding of the antiferromagnetic interaction which results in a geometric frustration for the model studied. Our work may provide a possible scenario to understand the pairing symmetry of the heavy fermion superconductivity, which is one of active issues in very recent years.

Competing d-Wave and p-Wave Spin-Singlet Superconductivities in the Two-Dimensional Kondo Lattice.

The Kondo lattice model describes a quantum phase transition between the antiferromagnetic state and heavy-fermion states. Applying the dual-fermion approach, we explore possible superconductivities emerging due to the critical antiferromagnetic fluctuations. The d-wave pairing is found to be the leading instability only in the weak-coupling regime. As the coupling is increased, we observe a change of the pairing symmetry into a p-wave spin-singlet pairing. The competing superconductivities are ascribed to crossover between small and large Fermi surfaces, which occurs with the formation of heavy quasiparticles.

Crossover Phase Diagram and Electronic State in the Heavy-Fermion Metamagnets UIr2Zn20 and UCo2Zn20

Crossover phase diagrams in the magnetic field versus temperature (H–T) plane of the nonmagnetic heavy-fermion metamagnets UT2Zn20 (T:Ir, Co) are studied by measuring the magnetic and electronic transport properties. The crossover phase diagrams of UIr2Zn20 and UCo2Zn20 are composed of a low-magnetic-field region (LFR) and a high-magnetic-field region (HFR), which are characterized by magnetic properties and the Hall effect, respectively. The LFR is found to form a closed area in the H–T plane, which is a quite different feature from the conventional uranium heavy-fermion compounds and the first observation in uranium compounds. From the drastic anomaly in the Hall effect at a metamagnetic field of UIr2Zn20, it is found that the metamagnetic behavior in UIr2Zn20 corresponds to a crossover from the heavy-fermion state to the field-induced ferromagnetic or polarized paramagnetic state accompanied by the reconstruction or topology change of Fermi surfaces. In UCo2Zn20, on the other hand, no sign of abrupt ch...

Weak electronic correlations and absence of heavy-fermion state inKNi2Se2

We have studied the low-lying electronic structure of a new ThCr$_2$Si$_2$-type superconductor KNi$_2$Se$_2$ with angle-resolved photoemission spectroscopy. Three bands intersect the Fermi level, forming complicated Fermi surface topology, which is sharply different from its isostructural superconductor K$_x$Fe$_{2-y}$Se$_2$. The Fermi surface shows weak variation along the $k_z$ direction, indicating its quasi-two-dimensional nature. Further comparison with the density functional theory calculations demonstrates that there exist relatively weak correlations and substantial hybridization of the Ni 3$d$ and the Se 4$p$ orbitals in the low-lying electronic structure. Our results indicate that the large density of states at the Fermi energy leads to the reported mass enhancement based on the specific heat measurements. Moreover, no anomaly is observed in the spectra when entering the fluctuating charge density wave state reported earlier.

Point-Contact Spectroscopy of Heavy Fermion Compounds CeCu6 and CeAl3 in Magnetic Field

We have carried out point-contact spectroscopy measurements on CeCu6 and CeAl3 to study these heavy fermion states. Temperature and magnetic field dependences of the point-contact spectra were observed in detail. At the lowest temperature of 0.4K, differential resistance spectra of CeCu6 and CeAl3 as function of bias voltage exhibit each distinct minimum structure and additional small peak structure. Temperature evolutions of these spectra started at temperatures related to the Kondo effect, although the minimum and the peak structure emerged in reverse between both spectra of two compounds. In magnetic fields, both structures monotonically change with no distinct change without a change of curvature of magnetic field dependence of magnitude of differential resistance at zero bias voltage at critical magnetic field of meta-magnetization. This behavior in magnetic field is different from that of CeAl2, which shows antiferromagnetic ordering. This result may suggest that PCS is able to discriminate between heavy fermion state and antiferromagnetic state.

Elastic Softening in the Orthorhombic Compound YbPdGe

The Yb-based compound YbPdGe with the orthorhombic structure shows a ferromagnetic phase transition at 11.4K. YbPdGe has localized 4f-electrons of Yb3+ ions and the crystal electric field (CEF) effect at high temperatures. On the other hand, a heavy-fermion state is reported at low temperatures, indicating that YbPdGe is a heavy-fermion ferromagnet. To investigate the phase transition at 11.4K and the CEF effect, we carried out ultrasonic measurements on a single-crystalline sample. Characteristic elastic softening below 170K is observed at high temperatures in the transverse elastic modulus C66 in contrast to the longitudinal modulus C22 without the softening. In the orthorhombic compound YbIrGe, the transverse moduli exhibit a similar elastic softening originating from the CEF effect, suggesting that the softening of C66 in YbPdGe is also caused by the CEF effect. At the phase transition, C22 and C66 show abrupt elastic hardening and softening, respectively, due to the magnetostriction.

Single Crystal Growth and Electronic State of UPd2Cd20

We succeeded in growing single crystals of UPd2Cd20 by the Cd self-flux method. This compound crystallizes in the cubic CeCr2Al20-type structure. From the low-temperature electrical resistivity and Kadowaki-Woods relation, the electronic specific heat coefficient γ is estimated to be about 400mJ/mol ·K2. The magnetic susceptibility of UPd2Cd20 indicates a broad max-imum at 6.6K and a rapid decrease at 5.0K, which correspond to the formation of the heavy fermion state and the antiferromagnetic transition, respectively. These experimental results strongly suggest that UPd2Cd20 is a new heavy fermion antiferromagnet.

Common Anomalies of Transport Properties in PrTr2Zn20 (Tr=Ir, Rh) with Non-Kramers Doublet Ground State

We study the transport coefficients of PrIr2Zn20 and PrRh2Zn20 with non-Kramers doublet ground state. From the resistivity measurement, we commonly found a convex temperature dependence at high temperatures. At low temperatures, an anomaly at T* is discovered near the boundary of the antiferro-quadrupole (AFQ) ordering phase in both systems. The Fermi liquid behavior with a large electron mass is observed below T* Thus a non-trivial heavy Fermion state would be formed there. In contrast, from the quantum oscillation of the Seebeck coefficient of PrRh2Zn20, the electron mass is found to be small below the critical field for B || [110], which is consistent with negligibly mass Seebeck coefficient of PrIr2Zn20. Since these features are detected in both compounds, they are expected to be universal properties of non-Kramers systems.