Fermi Liquid State Research Articles

Generalizing the Cooper-pair instability to doped Mott insulators

Copper oxides become superconductors rapidly upon doping with electron holes, suggesting a fundamental pairing instability. The Cooper mechanism explains normal superconductivity as an instability of a fermi-liquid state, but high-temperature superconductors derive from a Mott-insulator normal state, not a fermi liquid. We show that precocity to pair condensation with doping is a natural property of competing antiferromagnetism and d-wave superconductivity on a singly-occupied lattice, thus generalizing the Cooper instability to doped Mott insulators, with significant implications for the high-temperature superconducting mechanism.

Quantum Criticality and Nodal Superconductivity in the FeAs-Based SuperconductorKFe2As2

The in-plane resistivity rho and thermal conductivity kappa of the FeAs-based superconductor KFe2As2 single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior rho(T) approximately T{1.5} at H{c{2}}=5 T, and the development of a Fermi liquid state with rho(T) approximately T{2} when further increasing the field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field H{c{2}}. In zero field, there is a large residual linear term kappa{0}/T, and the field dependence of kappa_{0}/T mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe2As2 have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.

Spontaneous inversion symmetry breaking in graphene bilayers

Electrons most often organize into Fermi-liquid states in which electron-electron interactions play an inessential role. A well known exception is the case of one-dimensional (1D) electron systems (1DES). In 1D the electron Fermi-surface consists of points, and divergences associated with low-energy particle-hole excitations abound when electron-electron interactions are described perturbatively. In higher space dimensions, the corresponding divergences occur only when Fermi lines or surfaces satisfy idealized nesting conditions. In this article we discuss electron-electron interactions in 2D graphene bilayer systems which behave in many ways as if they were one-dimensional, because they have Fermi points instead of Fermi lines and because their particle-hole energies have a quadratic dispersion which compensates for the difference between 1D and 2D phase space. We conclude, on the basis of a perturbative RG calculation similar to that commonly employed in 1D systems, that interactions in neutral graphene bilayers can drive the system into a strong-coupling broken symmetry state with layer-pseudospin ferromagnetism and an energy gap.

Non-fermi liquid behavior of the organic superconductor [formula omitted] probed by 13C-NMR

We assessed 13C-NMR measurements in an organic salt, κ ‐ ( BEDT ‐ TTF ) 4 Hg 2.89 Br 8 , which exhibits superconductivity at 4.3 K under ambient pressure. We observed that ( T 1 T ) − 1 of H // and ⊥ layer at ambient pressure increased as the temperature was decreased to 7 K, but decreased further at lower temperatures, suggesting that the decrease of ( T 1 T ) − 1 was not due to the superconductive fluctuations, but due to the magnetism of the conduction electrons. Application of pressure suppresses ( T 1 T ) − 1 , with ( T 1 T ) − 1 becoming constant above 2 GPa. These results suggest that applying pressure alters the electron system from a non-Fermi liquid (NFL) to a Fermi liquid (FL) state and that antiferromagnetic fluctuations contribute to the origin of NFL behavior. Whereas most organic conductors show Fermi liquid behavior, this salt is an organic superconductor that shows NFL behavior due to the antiferromagnetic fluctuations.

Spatial fluctuations in two-orbital Hubbard model

We study spin and orbital correlations in the two-orbital Hubbard model with the different bandwidths at quarter filling by means of the cellular dynamical mean field theory combined with the noncrossing approximation. We demonstrate that the ferromagnetic-antiferro-orbital insulating state is stabilized in the system with the same bandwidth and the antiferromagnetic-ferro-orbital insulating state appears when the difference of the bandwidth is quite large. In the intermediate region, we find that the competition between the two states occurs and this competition gives rise to the heavy Fermi-liquid metallic state.

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.

Observation of Zero-Sound at Atomic Wave-Vectors in a Monolayer of Liquid 3He

The elementary excitations of a strongly interacting two-dimensional Fermi liquid have been investigated by inelastic neutron scattering in an experimental model system: a monolayer of liquid 3He adsorbed on graphite preplated by a monolayer of solid 4He. We observed for the first time the particle-hole excitations characterizing the Fermi liquid state of two-dimensional liquid 3He, and we were also able to identify the highly interesting zero-sound collective mode above a particle-hole band. Contrarily to bulk 3He, at low wave-vectors this mode lies very close to the particle-hole band. At intermediate wave-vectors, the collective mode enters the particle-hole band, where it is strongly broadened by Landau damping. At high wave-vectors, where the Landau theory is not applicable, the zero-sound collective mode reappears beyond the particle-hole band as a well defined excitation, with a dispersion relation quite similar to that of superfluid 4He. This spectacular effect is observed for the first time in a Fermi liquid (including plasmon excitations in electronic systems).

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.

NMR and NQR study of pressure-induced superconductivity and the origin of critical-temperature enhancement in the spin-ladder cuprateSr2Ca12Cu24O41

Pressure-induced superconductivity was studied for a spin-ladder cuprate ${\text{Sr}}_{2}{\text{Ca}}_{12}{\text{Cu}}_{24}{\text{O}}_{41}$ using nuclear magnetic resonance under pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a transitional change from a spin-gapped state to a Fermi-liquid state at temperatures higher than ${T}_{c}$. The relaxation rate $1/{T}_{1}$ shows activated-type behavior at an onset pressure, whereas Korringa-like behavior becomes predominant at the optimal pressure, suggesting that an increase in the density of states at the Fermi energy leads to enhancement of ${T}_{c}$. Nuclear quadrupole resonance spectra suggest that pressure application causes transfer of holes from the chain to the ladder sites. The transfer of holes increases DOS below the optimal pressure. A dome-shaped ${T}_{c}$ versus pressure curve arises from naive balance between the transfer of holes and broadening of the band width.

Fermi-liquid state and enhanced electron correlations in the iron pnictideCaFe4As3

The recently discovered ${\text{CaFe}}_{4}{\text{As}}_{3}$ system displays low-temperature Fermi-liquid behavior, with enhanced electron-electron correlations. At high temperatures, the magnetic susceptibility shows Curie-Weiss behavior, with a large temperature-independent contribution. Antiferromagnetic ordering is observed below ${T}_{N}=(88.0\ifmmode\pm\else\textpm\fi{}1.0)\text{ }\text{K}$, possibly via a spin density wave transition. A remarkably sharp drop in resistivity occurs below ${T}_{2}=(26.4\ifmmode\pm\else\textpm\fi{}1.0)\text{ }\text{K}$, correlated with a similarly abrupt increase in the susceptibility but no visible feature in the specific heat. The electronic specific-heat coefficient $\ensuremath{\gamma}$ at low temperatures is close to $0.02\text{ }\text{J}\text{ }{\text{mol}}_{\text{Fe}}^{\ensuremath{-}1}\text{ }{\text{K}}^{\ensuremath{-}2}$, but a higher value for $\ensuremath{\gamma}\ensuremath{\sim}0.08\text{ }\text{J}\text{ }{\text{mol}}_{\text{Fe}}^{\ensuremath{-}1}\text{ }{\text{K}}^{\ensuremath{-}2}$ can be inferred from a linear $Cp/T$ vs ${T}^{2}$ just above ${T}_{2}$. The Kadowaki-Woods ratio $A/{\ensuremath{\gamma}}^{2}=55\ensuremath{\ast}{10}^{\ensuremath{-}5}\text{ }\ensuremath{\mu}\ensuremath{\Omega}\text{ }\text{cm}\text{ }{\text{mol}}^{2}\text{ }{\text{K}}^{2}\text{ }{\text{mJ}}^{\ensuremath{-}2}$ is nearly two orders of magnitude larger than that of heavy fermions.

Room for one more

In YbRh2Si2, the transitions to a heavy Fermi-liquid state and to a magnetic phase occur at a single quantum critical point. But under chemical pressure, these transitions separate, and a new phase of matter appears in between.

Spatial fluctuations of spin and orbital nature in the two-orbital Hubbard model

We investigate the quasiparticle dynamics in the two-orbital Hubbard model on the square lattice at quarter filling by means of the cellular dynamical mean field theory. We show that the Fermi-liquid state is stabilized up to the large Hubbard interactions in the symmetric case without the Hund's coupling, and find the heavy quasiparticles around the metal-insulator boundary. It is elucidated that the Hund's coupling enhances the antiferro-orbital correlations, which give rise to the pseudo gap behavior in the single-particle excitations. We also find the nonmonotonic temperature dependence in the quasiparticle dynamics for intermediate strength of the Hund's coupling, and clarify that it is caused by the competition between the Fermi-liquid formation and the antiferro-orbital fluctuations.

Renormalization group approach to anisotropic superconductivity

The superconducting instability of the Fermi-liquid state is investigated by considering anisotropic electron-boson couplings. Both electron-electron interactions and anisotropic electron-boson couplings are treated with a renormalization group method that takes into account retardation effects. Considering a noninteracting circular Fermi surface, we find analytical solutions for the flow equations and derive a set of generalized Eliashberg equations. Electron-boson couplings with different momentum dependences are studied and we find superconducting instabilities of the metallic state with competition between order parameters of different symmetries. Numerical solutions for some couplings are given to illustrate the frequency dependence of the vertices at different coupling regimes.

Erratum: Current Noise through a Kondo Quantum Dot in aSU(N)Fermi Liquid State [Phys. Rev. Lett.100, 036604 (2008)

Erratum: Current Noise through a Kondo Quantum Dot in a<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>SU</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mi>N</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math>Fermi Liquid State [Phys. Rev. Lett.<b>100</b>, 036604 (2008)

The peculiar physical properties and phase diagram of BaFe2- xCoxAs2 single crystals

In this paper, we systematically study the transport, susceptibility and heat capacity of single crystals of BaFe2−xCoxAs2. In the underdoped region, spin density wave (SDW) transition was observed in both resistivity and susceptibility. The magnetic susceptibility shows unusual T-linear dependence above the SDW transition up to 700 K. With Co doping, SDW ordering is gradually suppressed and superconductivity emerges with a dome-like shape. The electrical transport, specific heat and magnetic susceptibility indicate that the SDW and superconductivity coexist in the BaFe2−xCoxAs2 sample around x=0.17, similar to the case of (Ba,K)Fe2As2. The superconductivity completely disappears for samples with x>0.34. A crossover from the SDW state to the Fermi-liquid state is observed with an increase in Co doping. A detailed electronic phase diagram of the evolution from the SDW to the superconducting state is given.

Localization and Superconductivity in Doped Semiconductors

Motivated by the discovery of superconductivity in boron-doped (B-doped) diamond, we investigate the localization and superconductivity in heavily doped semiconductors. The competition between Anderson localization and s-wave superconductivity is investigated from the microscopic point of view. The effect of microscopic inhomogeneity and the thermal fluctuation in superconductivity are taken into account using the self-consistent 1-loop-order theory with respect to superconducting fluctuation. The crossover from superconductivity in the host band to that in the impurity band is described on the basis of the disordered three-dimensional attractive Hubbard model for binary alloys. We show that superconductor-insulator transition (SIT) accompanies the crossover. We point out an enhancement of Cooper pairing in the crossover regime. Further localization of the electron wave function gives rise to incoherent Cooper pairs and the pseudogap above T_c. A global phase diagram is drawn for host band superconductivity, impurity band superconductivity, Anderson localization, Fermi liquid state, and pseudogap state. A theoretical interpretation is proposed for superconductivity in the doped diamond, SiC, and Si.

Spatial fluctuations of spin and orbital in two-orbital Hubbard model: cluster dynamical mean field study

We investigate the single-particle dynamics and magnetic properties in the two-orbital Hubbard model on the square lattice at quarter filling by using a cluster extension of dynamical mean field theory. We find that the Fermi-liquid state is stabilized up to the large intra- and inter-orbital interactions in the symmetric case without Hund's coupling. It is clarified that the Hund's coupling enhances the antiferro-orbital fluctuations, which gives rise to the pseudo gap behavior in the single-particle excitations. We also find that the Hund's coupling with intermediate strength causes the competition between the Fermi-liquid formation and the antiferro-orbital fluctuations, resulting in the nonmonotonous temperature dependence in the single-particle dynamics.

Evidence for field-induced quantum critical route to a Fermi liquid in overdoped Tl2Ba2CuO6+x

We report an unanticipated transformation from a non-Fermi- to a Fermi-liquid state driven by magnetic field in a highly overdoped Tl2Ba2CuOe+x with Tc ∼ 15 K. From the c-axis resistivity measured up to 45 T, we show that the Fermi-liquid ρc = ρc(0) + AT2 features, accompanied by a field-linear magnetoresistance, appear above a temperature-dependent field HFL, which decreases linearly with decreasing temperature and points to a quantum critical point near the upper critical field HC2(0). The observed field-induced quantum criticality with a power-law diverging A(H) bears a striking resemblance to that of heavy-fermion superconductor CeCoIn5, suggesting a common underlying physics in these strongly correlated electron systems.

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.

Evolution of a Large Fermi Surface in the Kondo Lattice

The single-particle spectrum of the Kondo lattice model is derived with the use of the continuous-time quantum Monte Carlo method, combined with the dynamical mean-field theory. Crossover behavior is traced quantitatively either to a heavy Fermi-liquid state or to a magnetically ordered state from the local-moment state at high temperatures. The momentum distribution in the low-temperature limit acquires a discontinuity at the location that involves the local-spin degrees of freedom. Even without the charge degrees of freedom for local electrons, the excitation spectra exhibit hybridized bands similar to those in the Anderson lattice. Temperature dependence in the zero-energy component of the self-energy is crucial in forming the Fermi-liquid state with the large Fermi surface.