Fermi-liquid Ground State Research Articles

Low-temperature properties of single-crystalCrB2

We report the low-temperature properties of $^{11}$B-enriched single-crystal CrB$_{2}$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, $\rho_{\rm xx}$, Hall effect, $\rho_{\rm xy}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\,{\rm K}$ below $T_{\rm N}=88\,{\rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to0$ reported in the literature. Comparison of $C$ with $d\rho_{\rm xx}/dT$, where we infer lattice contributions from measurements of VB$_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\rm sf}\approx 257\,{\rm K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\rm N}$, and unusual excitations around $\approx T_{\rm N}/2$. The pronounced anisotropy of $\rho_{\rm xx}$ above $T_{\rm N}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the N$\acute{\rm{e}}$el temperatures, $f=-\Theta_{\rm CW}/T_{\rm N}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\,\,{\rm T}$, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB$_{2}$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration.

High-pressure synthesis of the BaIrO3perovskite: A Pauli paramagnetic metal with a Fermi liquid ground state

BaIrO${}_{3}$ adopts the 9R polytype structure if it is synthesized at ambient pressure. High-pressure and high-temperature treatments up to 10 GPa have led to other 5H and 6H polytype phases, which are quenchable to the ambient condition. However, the single-phase 3C perovskite, the end member of polytypes, has not been obtained so far. Here we report the high-pressure synthesis of BaIrO${}_{3}$ perovskite under 25 GPa and 1150 \ifmmode^\circ\else\textdegree\fi{}C. Rietveld refinement on the powder x-ray diffraction pattern revealed that this new compound crystallizes in a tetragonal $I$4/mcm structure rather than the expected cubic Pm-3$m$ structure. The new perovskite is a Pauli paramagnetic metal with a Fermi liquid ground state, i.e., \ensuremath{\rho} \ensuremath{\propto} ${T}^{2}$ below 6 K. The availability of BaIrO${}_{3}$ perovskite also allows completion of the structure-physical property evolution in the $A$IrO${}_{3}$ perovskites ($A$ = Ca, Sr, Ba). The Fermi energy located in bands originated from Ir: 5$d$ and O: 2p electrons are found to be very sensitive to the change of crystal structure.

Evidence for two energy gaps and Fermi liquid behavior in the SrPt2As2superconductor

We report a detailed calorimetric study on single crystals of the 5$d$-transition metal pnictide SrPt2As2 with a superconducting critical temperature $T_c$ $\sim$5K. The peculiar field dependence of the electronic specific heat coefficient $\gamma$ can be decomposed into two linear components. Moreover, the temperature evolution of the electronic specific heat below $T_c$ is best described by a two-gap model. These findings suggest that two energy gaps are associated with the superconductivity. In parallel, we show that the spin-lattice relaxation time $T_1$, through nuclear magnetic resonance measurement, obeys the so-called Korringa relation well. This, along with the $T^2$ dependence of resistivity at low temperatures, points to a Fermi liquid ground state in this material.

Magnetic order induced by Fe substitution of Al site in the heavy-fermion systemsα-YbAlB4andβ-YbAlB4

beta-YbAlB4 is a heavy fermion superconductor that exhibits a quantum criticality without tuning at zero field and under ambient pressure. We have succeeded in substituting Fe for Al in beta-YbAlB4 as well as the polymorphous compound alpha-YbAlB4, which in contrast has a heavy Fermi liquid ground state. Full structure determination by single crystal X-ray diffraction confirmed no change in crystal structure for both alpha- and beta-YbAlB4, in addition to volume contraction with Fe substitution. Our measurements of the magnetization and specific heat indicate that both alpha-YbAl0.93Fe0.07B4 and beta-YbAl0.94Fe0.06B4 exhibit a magnetic order, most likely of a canted antiferromagnetic type, at $7\sim 9$ K. The increase in the entropy as well as the decrease in the antiferromagnetic Weiss temperature with the Fe substitution in both systems indicates that the chemical pressure due to the Fe substitution suppresses the Kondo temperature and induces the magnetism.

Behavior near pressure induced quantum criticality in Sr3Ru2O7

The pressure dependence of the ac susceptibility is reported for the itinerant metamagnet Sr3Ru2O7 for the field applied in the ab-plane. At the critical pressure Pc for suppression of the first order jump in magnetization, the susceptibility shows a non-Fermi-liquid, quasi-linear temperature dependence down to the lowest temperature. For pressure P > Pc, the temperature dependence reveals a pressure-dependent cross-over, which we interpret as a recovery of Fermi liquid ground state.

Benchmark of a modified iterated perturbation theory approach on the fcc lattice at strong coupling

The Dynamical Mean-Field theory (DMFT) approach to the Hubbard model requires a method to solve the problem of a quantum impurity in a bath of non-interacting electrons. Iterated Perturbation Theory (IPT) has proven its effectiveness as a solver in many cases of interest. Based on general principles and on comparisons with an essentially exact Continuous-Time Quantum Monte Carlo (CTQMC) solver, here we show that the standard implementation of IPT fails away from half-filling when the interaction strength is much larger than the bandwidth. We propose a slight modification to the IPT algorithm that replaces one of the equations by the requirement that double occupancy calculated with IPT gives the correct value. We call this method IPT-$D$. We recover the Fermi liquid ground state away from half-filling. The Fermi liquid parameters, density of states, chemical potential, energy and specific heat on the FCC lattice are calculated with both IPT-$D$ and CTQMC as benchmark examples. We also calculated the resistivity and the optical conductivity within IPT-$D$. Particle-hole asymmetry persists even at coupling twice the bandwidth. Several algorithms that speed up the calculations are described in appendices.

Transport, Thermal, and Magnetic Properties of YbNi3X9(X= Al, Ga): A Newly Synthesized Yb-Based Kondo Lattice System

We have succeeded in synthesizing a new Yb-based Kondo lattice system, YbNi 3 X 9 ( X = Al, Ga). Our study reveals that YbNi 3 Al 9 shows typical features of a heavy-fermion antiferromagnet with a Neel temperature of T N = 3.4 K. All of the properties reflect a competition between the Kondo effect and the crystalline electric field (CEF) effect. The moderate heavy-fermion state leads to an enhanced Sommerfeld coefficient of 100 mJ/(mol·K 2 ), even if ordered antiferromagnetically. On the other hand, the isostructural gallide YbNi 3 Ga 9 is an intermediate-valence system with a Kondo temperature of T K = 570 K. A large hybridization scale can overcome the CEF splitting energy, and a moderately heavy Fermi-liquid ground state with high local moment degeneracy should form at low temperatures. Note that the quality of single-crystalline YbNi 3 X 9 is extremely high compared with those of other Yb-based Kondo lattice compounds. We conclude that YbNi 3 X 9 is a suitable system for investigating the electronic s...

Ground State of the Parallel Double Quantum Dot System

We resolve the controversy regarding the ground state of the parallel double quantum dot system near half filling. The numerical renormalization group predicts an underscreened Kondo state with residual spin-1/2 magnetic moment, ln2 residual impurity entropy, and unitary conductance, while the Bethe ansatz solution predicts a fully screened impurity, regular Fermi-liquid ground state, and zero conductance. We calculate the impurity entropy of the system as a function of the temperature using the hybridization-expansion continuous-time quantum Monte Carlo technique, which is a numerically exact stochastic method, and find excellent agreement with the numerical renormalization group results. We show that the origin of the unconventional behavior in this model is the odd-symmetry "dark state" on the dots.

Anisotropic heavy-Fermi-liquid formation in valence-fluctuatingα-YbAlB4

$\alpha$-YbAlB$_4$ is the locally isostructural polymorph of $\beta$-YbAlB$_4$, the first example of an Yb-based heavy fermion superconductor which exhibits pronounced non-Fermi-liquid behavior above $T_{\rm c}$. Interestingly, both $\alpha$-YbAlB$_4$ and $\beta$-YbAlB$_4$ have strongly intermediate valence. Our single crystal study of the specific heat, magnetization and resistivity has confirmed the Fermi liquid ground state of $\alpha$-YbAlB$_4$ ~in contrast with the quantum criticality observed in $\beta$-YbAlB$_4$. Both systems exhibit Kondo lattice behavior with the characteristic temperature scale $T^* \sim$ 8 K in addition to a valence fluctuation scale $\sim 200$ K. Below $T^*$, $\alpha$-YbAlB$_4$ a heavy Fermi liquid state with an electronic specific heat coefficient $\gamma\sim$ 130 mJ/mol K$^2$ and a large Wilson ratio more than 7, which indicates ferromagnetic correlation between Yb moments. A large anisotropy in the resistivity suggests that the hybridization between 4$f$ and conduction electrons is much stronger in the $ab$-plane than along the c-axis. The strongly anisotropic hybridization as well as the large Wilson ratio is the key to understand the unusual Kondo lattice behavior and heavy fermion formation in mixed valent compounds.

Behavior of the Quantum Critical Point and the Fermi-Liquid Domain in the Heavy Fermion Superconductor CeCoIn5 Studied by Resistivity

We report detailed very low temperature resistivity measurements on the heavy fermion compounds Ce_{1-x}La_{x}CoIn5 (x=0 and x=0.01), with current applied in two crystallographic directions [100] (basal plane) and [001] (perpendicular to the basal plane) under magnetic field applied in the [001] or [011] direction. We found a Fermi liquid (\rho \propto T^{2}) ground state, in all cases, for fields above the superconducting upper critical field. We discuss the possible location of a field induced quantum critical point with respect to Hc2(0), and compare our measurements with the previous reports in order to give a clear picture of the experimental status on this long debated issue.

Kondo Lattice Behavior in the Valence Fluctuating Systems α- and β-YbAlB4

We have measured specific heat and magnetization of the Yb-based heavy Fermion systems α-YbAlB 4 and β-YbAlB 4 . α-YbAlB 4 is a locally isostructural polymorph of the first example of an Yb-based heavy Fermion superconductor β-YbAlB 4 , which exhibits pronounced non-Fermi-liquid behavior above the superconducting transition temperature T c = 80 mK. The results reveal a Fermi liquid ground state for α-YbAlB 4 in contrast to the quantum criticality found for β-YbAlB 4 . Interestingly, both systems exhibit Kondo lattice behavior with its characteristic temperature scale T * ∼8 K, despite of their strongly intermediate valence with the temperature scale of ∼ 200 K. We discuss the origin of this unusual behavior, making a comparison with the typical mixed valent compound YbAl 3 . Large Wilson ratios indicate the existence of ferromagnetic correlation as a possible origin for the Kondo lattice behavior.

Competing magnetic interactions in CeNi9−xCoxGe4

CeNi9Ge4 exhibits outstanding heavy fermion features with remarkable non-Fermi-liquid behavior which is mainly driven by single-ion effects. The substitution of Ni by Cu causes a reduction of both, the RKKY coupling and Kondo interaction, coming along with a dramatic change of the crystal field (CF) splitting. Thereby a quasi-quartet ground state observed in CeNi9Ge4 reduces to a two-fold degenerate one in CeNi8CuGe4. This leads to a modification of the effective spin degeneracy of the Kondo lattice ground state and to the appearance of antiferromagnetic (AFM) order. To obtain a better understanding of consequences resulting from a reduction of the effective spin degeneracy, we stepwise replaced Ni by Co. Thereby an increase of the Kondo and RKKY interactions through the reduction of the effective d-electron count is expected. Accordingly, a paramagnetic Fermi liquid ground state should arise. Our experimental studies, however, reveal AFM order already for small Co concentrations, which becomes even more pronounced with increasing Co content x. Thereby the modification of the effective spin degeneracy seems to play a crucial role in this system.

Local moment behaviors of the valence fluctuating systems β-YbAlB4 and α-YbAlB4

β-YbAlB4 is the first example of an Yb-based heavy Fermion superconductor with Tc = 80 mK and exhibits pronounced non-Fermi-liquid behavior above Tc. On the other hand, recent hard x-ray photoemission spectroscopy measurements have revealed strong intermediate valence of both β-YbAlB4 and its locally isostructural polymorph α-YbAlB4 . Here we present the results of the specific heat and magnetization measurements of β-YbAlB4 and α-YbAlB4 . The results indicate a Fermi liquid ground state for α-YbAlB4 in contrast to the quantum criticality found in β-YbAlB4 . Interestingly, both systems exhibit Kondo lattice behavior with large Wilson ratio below T* ~ 8K, through the renormalization of a high valence fluctuation scale ~ 200 K.

Universal short-distance structure of the single-particle spectral function of dilute Fermi gases

We show that the universal $1/k^4$ tail in the momentum distribution of dilute Fermi gases implies that the spectral function $A(\kk,\omega)$ must have weight below the chemical potential for large momentum $k \gg k_F$, with observable consequences in RF spectroscopy experiments. We find that this incoherent spectral weight is centered about $\omega \simeq - \epsilon(\kk)$ in a range of energies of order $v_F k$. This "bending back" in the dispersion, while natural for superfluids, is quite surprising for normal gases. This universal structure is present in the hard-sphere gas as well as the Fermi liquid ground state of the highly imbalanced, attractive gas near unitarity. We argue that, even in the BCS superfluid, this bending back at large $k$ is dominated by interaction effects which do not reflect the pairing gap.

Kondo effects in a triangular triple quantum dot: Numerical renormalization group study in the whole region of the electron filling

We study the low-energy properties of a triangular triple quantum dot connected to two noninteracting leads in a wide parameter range, using the numerical renormalization group (NRG). Various kinds of Kondo effects take place in this system depending on the electron filling ${N}_{\text{tot}}$, or the level position ${ϵ}_{d}$ of the triple dot. The SU(4) Kondo behavior is seen in the half-filled case ${N}_{\text{tot}}\ensuremath{\simeq}3.0$ at the dip of the series conductance, and it causes a charge redistribution between the even and odd orbitals in the triangle. We show generally that the quasiparticle excitations from a local Fermi-liquid ground state acquire a channel symmetry at zero points of the two-terminal conductance, in the case the system has time-reversal and inversion symmetries. It causes the SU(4) behavior at low energies, while the orbital degeneracy in the triangle determines the high-energy behavior. At four-electron filling ${N}_{\text{tot}}\ensuremath{\simeq}4.0$, a local $S=1$ moment emerges at high temperatures due to a Nagaoka ferromagnetic mechanism. It is fully screened by the electrons from the two conducting channels via a two-stage Kondo effect, which is caused by a difference in the charge distribution in the even and odd orbitals.

Quantum critical behavior in the organic conductor κ-(DHDA-TTP) 2SbF 6

Temperature–pressure phase diagram of the 2-D organic conductor, κ-(DHDA-TTP) 2SbF 6 (DHDA-TTP = 2-(1,3-dithiolan-2-ylidene)-5-(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene) is presented. This phase diagram exhibits a pressure boundary between insulator and Fermi liquid (FL). This boundary is considered as a critical pressure of quantum fluctuation between probable charge ordering and FL ground states. In the vicinity of the specific pressure, divergence of the prefactor A in ρ = ρ 0 + AT 2, is observed, where ρ is the in-plane resistivity and T is the temperature. Following the similar phase diagram to κ-(MeDH-TTP) 2AsF 6 by Yasuzuka et al., this is the second example in the two-dimensional 1/4-filled organic conductors, showing the temperature–pressure phase diagram of quantum critical behavior. It is remarkable that this kind of divergence of A at certain critical point is rather general phenomena at least in these 1/4-filled 2-D organic conductors, in contrast to the other 1/2-filled κ-type organic conductors, which show Kanoda's phase diagram with Mott insulator and superconductor.

Complex electronic states in double-layered ruthenates(Sr1−xCax)3Ru2O7

The magnetic ground state of ${({\text{Sr}}_{1\ensuremath{-}x}{\text{Ca}}_{x})}_{3}{\text{Ru}}_{2}{\text{O}}_{7}$ $(0\ensuremath{\le}x\ensuremath{\le}1)$ is complex, ranging from an itinerant metamagnetic state $(0\ensuremath{\le}x<0.08)$ to an unusual heavy-mass nearly ferromagnetic (FM) state $(0.08<x<0.4)$, and finally to an antiferromagnetic (AFM) state $(0.4\ensuremath{\le}x\ensuremath{\le}1)$. In this report we elucidate the electronic properties for these magnetic states, and show that the electronic and magnetic properties are strongly coupled in this system. The electronic ground state evolves from an AFM quasi-two-dimensional metal for $x=1.0$ to an Anderson localized state for $0.4\ensuremath{\le}x<1.0$ (the AFM region). When the magnetic state undergoes a transition from the AFM to the nearly FM state, the electronic ground state switches to a weakly localized state induced by magnetic scattering for $0.25\ensuremath{\le}x<0.4$, and then to a magnetic metallic state with the in-plane resistivity ${\ensuremath{\rho}}_{ab}\ensuremath{\propto}{T}^{\ensuremath{\alpha}}$ $(\ensuremath{\alpha}>2)$ for $0.08<x<0.25$. The system eventually transforms into a Fermi-liquid ground state when the magnetic ground state enters the itinerant metamagnetic state for $x<0.08$. When $x$ approaches the critical composition $(x\ensuremath{\sim}0.08)$, the Fermi-liquid temperature is suppressed to zero Kelvin, and non-Fermi-liquid behavior is observed. These results demonstrate the strong interplay between the charge and spin degrees of freedom in the double-layered ruthenates.

Holographic responses of fermion matter

We consider the D 4 – D 8 – D 8 ¯ brane system which serves as ultraviolet completion of the Nambu–Jona-Lasinio model, where the only degrees of freedom carrying baryon charge are fermions. By turning on chemical potential for this charge one may expect the formation of the Fermi liquid ground state. At strong coupling we use the dual holographic description to investigate the responses of the system to small perturbations. In the chirally symmetric phase we find that the density-dependent part of the heat capacity vanishes linearly with temperature. We also observe a zero sound excitation in the collisionless regime, whose speed is equal to that of normal sound in the hydrodynamic regime. Both the linear dependence of the heat capacity and the existence of zero sound are properties of the Fermi liquid ground state. We also compute the two-point function of the currents at vanishing frequency but do not find any singularities at finite values of the momentum.

Electronic structure of CeRhX (X = Sn, In)

Electronic structure of the compounds CeRhIn and CeRhSn have been studied by the X-ray photoemission spectroscopy (XPS) and ab initio band structure calculations. CeRhSn shows the non-Fermi liquid characteristics at low temperatures, while CeRhIn exhibits a Fermi-liquid ground state. At ambient temperature the XPS data reveal an intermediate valence state of Ce ions in both systems. The Ce core-level XPS spectra are very similar and indicate the strong coupling of the Ce 4f and the conduction band states (Δ ≈ 100 meV). The valence band spectra we interpret with the help of ab initio calculations as well as using the results for the reference compounds LaRhIn and LaRhSn. The comparative analysis of the theoretical band structures and charge density plots reveal the changes in chemical bonding and the hybridization between the Ce 4f and the other valence states introduced by the replacement of In by Sn atoms. The more covalent character of the chemical bonding in the stannides is in line with the smaller thermal expansion. Finally, for CeRhIn we found a typical temperature dependence of the crystal lattice, while CeRhSn shows distinct anomaly at about 120 K, presumably related to the change in planar Ce–Rh bonds.

Signature of Quantum Criticality in Photoemission Spectroscopy

A quantum phase transition in a heavy-fermion compound may destroy the Fermi-liquid ground state. However, the conditions for this breakdown have remained obscure. We report the first direct investigation of heavy quasiparticle formation and breakdown in the canonical system CeCu(6-x)Au(x) by ultraviolet photoemission spectroscopy at elevated temperatures without the complications of lattice coherence. Surprisingly, the single-ion Kondo energy scale T(K) exhibits an abrupt step near the quantum critical Au concentration of x(c) = 0.1. We show theoretically that this step is expected from a highly nonlinear renormalization of the local spin coupling at each Ce site, induced by spin fluctuations on neighboring sites. It provides a general high-temperature indicator for heavy-fermion quasiparticle breakdown at a quantum phase transition.