non-Fermi Liquid Regime Research Articles

Determinação da temperatura Kondo a partir de curvas de susceptibilidade magnética em regime não líquido de Fermi

Em algumas ligas metálicas existem átomos magnéticos cercados por vários outros não magnéticos. Os primeiros são tão poucos que não interagem entre si. Os segundos são maioria e formam uma banda de condução. Nestas ligas é comum a ocorrência de um fenômeno conhecido como Efeito Kondo. Trata-se do acoplamento entre o spin do átomo magnético com os spins dos elétrons da banda de condução. Este fenômeno, em baixas temperaturas, é perceptível em propriedades como suscetibilidade magnética, calor específico eletrônico, resistividade elétrica, entre outros; e é caracterizado por um parâmetro denominado Temperatura Kondo. O valor deste parâmetro indica a faixa de temperatura em que o Efeito Kondo passa a ser significativo. Sua determinação é importante para a compreensão de compostos metálicos em baixa temperatura, como Ce0.5La0.5Ni9Ge4,URu2Si2 entre outros. Krishna-murthy e colaboradores, através de cálculos perturbativos do modelo de Anderson para ligas magnéticas diluídas, propuseram uma tabela de dados de suscetibilidade magnética a partir da qual é possível a determinação da temperatura Kondo para sistemas físicos no regime líquido de Fermi. Neste artigo foi reproduzido este procedimento utilizando regressão numérica para ajustar um tipo de equação não-linearizável aos dados desta tabela. Em seguida, determinou-se a temperatura Kondo de um modelo de Anderson de dois canais no regime não líquido de Fermi. Confirmou-se que o método numérico proposto por Krishna-murthy e colaboradores é válido para os dois regimes.

Non-Fermi liquid regimes with and without quantum criticality in Ce 1− x Yb x CoIn 5

One of the greatest challenges to Landau's Fermi liquid theory--the standard theory of metals--is presented by complex materials with strong electronic correlations. In these materials, non-Fermi liquid transport and thermodynamic properties are often explained by the presence of a continuous quantum phase transition that happens at a quantum critical point (QCP). A QCP can be revealed by applying pressure, magnetic field, or changing the chemical composition. In the heavy-fermion compound CeCoIn5, the QCP is assumed to play a decisive role in defining the microscopic structure of both normal and superconducting states. However, the question of whether a QCP must be present in the material's phase diagram to induce non-Fermi liquid behavior and trigger superconductivity remains open. Here, we show that the full suppression of the field-induced QCP in CeCoIn5 by doping with Yb has surprisingly little impact on both unconventional superconductivity and non-Fermi liquid behavior. This implies that the non-Fermi liquid metallic behavior could be a new state of matter in its own right rather than a consequence of the underlying quantum phase transition.

Variational local moment approach: From Kondo effect to Mott transition in correlated electron systems

The variational local moment approach (VLMA) solution of the single impurity Anderson model is presented. It generalizes the local moment approach of Logan et al. by invoking the variational principle to determine the lengths of local moments and orbital occupancies. We show that VLMA is a comprehensive, conserving and thermodynamically consistent approximation and treats both Fermi and non-Fermi liquid regimes as well as local moment phases on equal footing. We tested VLMA on selected problems. We solved the single- and multi-orbital impurity Anderson model in various regions of parameters, where different types of Kondo effects occur. The application of VLMA as an impurity solver of the dynamical mean-field theory, used to solve the multi-orbital Hubbard model, is also addressed.

Numerical Renormalization Group computation of temperature dependent specific heat for a two-channel Anderson model

The Numerical Renormalization Group (NRG) is applied to diagonalize a two-channel Anderson model describing a local magnetic impurity embedded in a fermionic bath. In spite of the difficulty in computing the specific heat using NRG, the interleaving discretization and multi-step iterative transformation virtually eliminate the numerical oscillations introduced by the logarithmic discretization of the conduction band. These allow to cover uniformly a large range of temperature, from the top of the band to a very small fraction of the bandwidth. This is relevant in describing, for instance, the presence of a low temperature Kondo resonance together with a high temperature Schottky peak, as well to cover Fermi and non-Fermi liquid regimes, like in the recent studied Ce1−xLaxNi9Ge4 family. We highlight the importance in describing the Schottky peak to define the number of degrees of freedom of the local levels, in order to correctly define the model to describe a given compound.

Thermal conductance of a two-channel Kondo model

A theory of thermal transport in a two-channel Kondo system, such as the one formed by a small quantum dot coupled to two leads and to a larger dot, is formulated. The interplay of the two screening constants allows an exploration of the Fermi liquid and non-Fermi liquid regimes. By using analytical, as well as numerical renormalization group methods, we study the temperature dependence of the thermal conductance and the Lorentz number. We find that in the low temperature limit, the Lorentz number attains its universal value, irrespective of the nature of the ground state.

Zeeman-Driven Lifshitz Transition: A Model for the Experimentally Observed Fermi-Surface Reconstruction inYbRh2Si2

The heavy-fermion metal YbRh2Si2 displays a field-driven quantum phase transition where signatures of a Fermi-surface reconstruction have been identified, often interpreted as breakdown of the Kondo effect. We argue that instead many properties of the material can be consistently described assuming a Zeeman-driven Lifshitz transition of narrow heavy-fermion bands. Using a suitable quasiparticle model, we find a smeared jump in the Hall constant and lines of maxima in susceptibility and specific heat, very similar to experimental data. An intermediate non-Fermi liquid regime emerges due to the small effective Fermi energy near the transition. Further experiments to discriminate the different scenarios are proposed.

Low-Temperature Electrical Resistivity of Slightly Ge-Doped YbRh2Si2

Quantum criticality in single crystalline YbRh 2 (Si 1- x Ge x ) 2 with x ≈0.01 is investigated by electrical resistivity measurements down to 15 mK at various magnetic fields applied parallel and perpendicular to the c -axis. Compared to undoped YbRh 2 Si 2 with T N =75 mK (from the onset of the resistive signature), the slightly Ge-doped sample exhibits a reduced Neel temperature of 53 mK. In contrast to the recent report on nominally YbRh 2 (Si 0.95 Ge 0.05 ) 2 , we do not observe evidence for a non-Fermi liquid regime at T →0 over an extended magnetic field range. The obtained phase diagrams rather resemble the case of undoped YbRh 2 Si 2 .

Non-fermi liquid transport in RCo2-based alloys

Results on the experimentally investigated dependence of the electrical resistivity of quasibinary alloys RxY1−xCo2, where R stands for magnetic rare earth elements, on temperature (2–300 K), composition (x = 0–1), and magnetic field (0–15 T) are presented. Non-Fermi liquid behavior of electrical resistivity in a magnetic field was observed in paramagnetic GdxY1−xCo2 alloys. The relative magneto-thermopower of these alloys exhibits diverging behavior with decreasing temperature, indicating anomalous temperature variation of the thermopower in the non-Fermi liquid regime.

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

Standard models for simple metals and insulators often fail for systems based on elements with unstable d- or f-electron shells, where strong electronic correlations can generate new and unexpected states of matter. Such a scenario can often be induced when a magnetic phase transition is tuned to absolute zero temperature by an external control parameter such as chemical composition, pressure or magnetic field. At the resulting quantum critical point (QCP), emergent phenomena, such as unconventional superconductivity and novel magnetic phases are frequently observed. The temperature and energy dependences of the physical properties are also found to deviate from expectations for a simple Fermi liquid. This “non-Fermi-liquid” (NFL) behavior is commonly manifested as weak power laws and logarithmic divergences in the physical properties at low temperatures and is often found in a V-shaped region near a QCP, which has become the “classic” QCP phase diagram. However, there is also a growing number of materials where the NFL behavior either occurs far away from the QCP, within an ordered phase, or may not be associated with any putative QCP. Thus, after nearly 20 years of research, it remains unknown whether NFL physics is universal, or if a multitude of unique subclasses exist. In this article, we review research that has primarily been carried out in our laboratory on systems that exhibit NFL behavior that does not conform to the “classic” QCP scenario.

Phenomenology of ESR in heavy-fermion systems: Fermi-liquid and non-Fermi-liquid regimes

We extend and apply a recent theory of the dynamical spin response of Anderson lattice systems to interpret ESR data on YbRh2Si2. Starting within a semiphenomenological Fermi liquid description at low temperatures T < Tx (a crossover temperature) and low magnetic fields B << Bx, we extend the description to the non-Fermi liquid regime by adopting a quasiparticle picture with effective mass and spin susceptibility varying logarithmically with energy/temperature, as observed in experiment. We find a sharp ESR resonance line slightly shifted from the local f-level resonance and broadened by quasiparticle scattering (taking unequal g-factors of conduction and f electrons) and by spin-lattice relaxation, both significantly reduced by the effect of ferromagnetic fluctuations. A detailed comparison of our theory with the data shows excellent agreement in the Fermi liquid regime. In the non-Fermi liquid regime we find a close relation of the T-dependence of the specific heat/spin susceptibility with the observed T-dependence of line shift and linewidth.

Energy scales and magnetoresistance at a quantum critical point

The magnetoresistance (MR) of CeCoIn 5 is notably different from that in many conventional metals. We show that a pronounced crossover from negative to positive MR at elevated temperatures and fixed magnetic fields is determined by the scaling behavior of quasiparticle effective mass. At a quantum critical point (QCP) this dependence generates kinks (crossover points from fast to slow growth) in thermodynamic characteristics (like specific heat, magnetization, etc.) at some temperatures when a strongly correlated electron system transits from the magnetic field induced Landau–Fermi liquid (LFL) regime to the non-Fermi liquid (NFL) one taking place at rising temperatures. We show that the above kink-like peculiarity separates two distinct energy scales in QCP vicinity – low temperature LFL scale and high temperature one related to NFL regime. Our comprehensive theoretical analysis of experimental data permits to reveal for the first time new MR and kinks scaling behavior as well as to identify the physical reasons for above energy scales.

Dynamical correlations in the spin-half two-channel Kondo model

Dynamical correlations of various local operators are studied in the spin-half two-channel Kondo (2CK) model in the presence of channel anisotropy or external magnetic field. A conformal field theory-based scaling approach is used to predict the analytic properties of various spectral functions in the vicinity of the two-channel Kondo fixed point. These analytical results compare well with highly accurate density matrix numerical renormalization group results. The universal cross-over functions interpolating between channel-anisotropy or magnetic field-induced Fermi liquid regimes and the two-channel Kondo, non-Fermi liquid regimes are determined numerically. The boundaries of the real 2CK scaling regime are found to be rather restricted, and to depend both on the type of the perturbation and on the specific operator whose correlation function is studied. In a small magnetic field, a universal resonance is observed in the local fermion's spectral function. The dominant superconducting instability appears in the composite superconducting channel.

Spectral properties of quasi-one-dimensional conductors with a finite transverse banddispersion

We determine the one-particle spectral function and the corresponding derivedquantities for the conducting chain lattice with finite inter-chain hopping and three-dimensional long-range Coulomb electron–electron interaction. The standardG0W0 approximation is used. It is shown that, due to the optical character of the anisotropicplasmon dispersion caused by the finite , a low energy quasi-particle δ-peak appears in the spectral function in addition to the hump present at energies of theorder of the plasmon energy. Particular attention is devoted to the continuous crossoverfrom the non-Fermi liquid regime to the Fermi liquid regime with increasing . It is shown that the spectral weight of the hump transfers to the quasi-particleas the optical gap in the plasmon dispersion increases together with , with the quasi-particle residuumZ behaving like in the limit . Our approach is appropriate for the wide range of energy scales given by the plasmonenergy and the width of the conduction band, and is complementary to the Luttingerliquid techniques that are limited to the low energy regime close to the Fermisurface.

On the Identification of Fermi-Liquid Behavior in Simple Transition Metal Compounds

The status of Fermi liquid behavior in weakly magnetic transition metal compounds is reviewed. Single-crystals of the weak itinerant ferromagnets ZrZn2, Ni3Al, and YNi3 display a non-Fermi liquid (NFL) temperature dependence of the resistivity consistent with an inhomogenous ferromagnetic state. But at high-magnetic fields they exhibit quantum oscillations of a Fermi liquid ground state. On the other hand, the helical magnet MnSi displays a pronounced change from a Fermi liquid to NFL resistivity under pressure, where a small regime of partial magnetic order in the NFL regime suggests unconventional textures driven by an unpinning of the helical order. But the stability of the chiral interactions under pressure and magnetic field points at a more complex origin of the NFL behavior.

Hall effect in the quasi two-dimensional strongly correlated metal [formula omitted] (M=Co, Rh)

We report on the evolution of the Hall effect in heavy fermion compounds CeMIn 5 (M=Co, Rh), ranging from antiferromagnetic metal to Fermi liquid regime through non-Fermi liquid regime by applying pressure. We found that anomalous transport properties of CeMIn 5 are strikingly similar to that of high- T c cuprates. We also found that the magnitude of Hall coefficient appears to diverge near an antiferromagnetic quantum critical point. These results strongly suggest that unusual transport properties of CeMIn 5 are dominated by anisotropic scattering due to strong spin fluctuations.

A quantum multi-critical point in CeCu 6−xAu x

CeCu 6− x Au x is a well-known heavy-fermion system in which the ground state is antiferromagnetically ordered for x>0.1 and temperatures below 1 K. Non-Fermi liquid behaviour occurs around this critical concentration. The parent compound, CeCu 6, exhibits a structural phase transition near 230 K, where it changes from the Pnma orthorhombic room-temperature structure to the P2 1/c monoclinic structure. The monoclinicity increases as temperature falls, with β reaching 91.44° at 10 K. In the work presented here, powder neutron diffraction is used to explore the monoclinicity at 8 K as a function of composition for 0.0< x<0.08, just short of the critical concentration. Extrapolation of the square of the monoclinic strain, ( ac cos β) 2, suggests that the distortion vanishes at x=0.13. A reanalysis of single-crystal diffraction data on the magnetically ordered side of the phase diagram, x>0.1, indicates that long-range order disappears at exactly the same critical concentration. At a minimum, the structural distortion and antiferromagnetism seem to be competing with each other, and this raises the intriguing possibility that lattice degrees of freedom are important in the non-Fermi liquid regime.

Scaling behavior of angular-dependent resistivity inCeCoIn5: Possible evidence ford-wave density waves

In-plane angular dependent resistivity ADR was measured in the non-Fermi liquid regime of CeCoIn$_5$ single crystals at temperatures $T \le 20$ K and in magnetic fields $H$ up to 14 T. Two scaling behaviors were identified in low field region where resistivity shows T-linear dependence, separated by a critical angle $\theta_{c}$ which is determined by the anisotropy of CeCoIn$_5$; i.e., ADR depends only on the perpendicular (parallel) field component below (above) $\theta_c$. These scaling behaviors and other salient features of ADR are consistent with d-wave density waves.

Probing the extended non-Fermi liquid regimes of MnSi and Fe

Recent studies show that the non-Fermi liquid (NFL) behavior of MnSi and Fe spans over an unexpectedly broad pressure range, between the critical pressure p c and around 2 p c . In order to determine the extension of their NFL regions, we analyze the evolution of the resistivity ρ ( T ) ∼ A ( p ) T n at higher pressures. We find that in MnSi the n = 3 2 exponent holds below 4.8 GPa ≈ 3 p c , but it increases above that pressure. At 7.2 GPa we observe the low-temperature Fermi liquid exponent n = 2 whereas for T > 1.5 K , n = 5 3 . Our measurements in Fe show that the NFL behavior ρ ∼ T 5 / 3 extends at least up to 30.5 GPa, above the entire superconducting (SC) region. In the studied pressure range, the onset of the SC transition reduces by a factor 10 down to T c onset ( 30.5 GPa ) = 0.23 K , while the A —coefficient diminishes monotonically by around 50%.

Low-lying electronic structure of triangular cobaltite

We report detailed Fermi surface topology and quasiparticle dynamics of the host cobaltate Na0.7CoO2. A direct mapping of the Fermi surface is carried out by angle-resolved photoemission spectroscopy (ARPES). Fermi surface at temperature 16K is a hole-pocket centered around the Gamma-point. The highly correlated nature of the electron liquid is evidenced from strong on-site Coulomb interaction seen through a valence satellite and via renormalization of the quasiparticle dispersion. The quasiparticle exhibits spectral coherence only below 150K which includes the non-Fermi liquid regime.

Investigation of the field-tuned quantum critical point in CeCoIn5

The main properties and the type of the field-tuned quantum critical point in the heavy-fermion metal CeCoIn$_5$ arisen upon applying magnetic fields $B$ are considered within the scenario based on the fermion condensation quantum phase transition. We analyze the behavior of the effective mass, resistivity, specific heat, charge and heat transport as functions of applied magnetic fields $B$ and show that in the Landau Fermi liquid regime these quantities demonstrate the critical behavior which is scaled by the critical behavior of the effective mass. We show that in the high-field non-Fermi liquid regime, the effective mass exhibits very specific behavior, $M^*\sim T^{-2/3}$, and the resistivity demonstrates the $T^{2/3}$ dependence. Finally, at elevated temperatures, it changes to $M^*\sim T^{-1/2}$, while the resistivity becomes linear in $T$. In zero magnetic field, the effective mass is controlled by temperature $T$, and the resistivity is also linear in $T$. The obtained results are in good agreement with recent experimental facts.