Evolution from a non-Fermi liquid Kondo lattice to intermediate valence behaviour in CeRhSn(1-x)In(x)

We present investigations of the magnetic and electric transport properties, specific heat, and electronic structure of the intermetallic and strongly correlated system of CeRhSn(1-x)In(x) compounds. The main goal of this paper is to determine the hybridization energy between the f electron and conduction electron states, V(cf), and its influence on the ground state properties of the system. The complementary experimental data are discussed on the basis of the Anderson model for a periodic Kondo lattice. CeRhSn is known as a non-Fermi liquid, while CeRhIn is a valence fluctuating system. We discuss the ground state properties of CeRhSn(1-x)In(x) and compare the results with those obtained for the doped Ce-based Kondo insulators.     

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.     

Avoided antiferromagnetic order and quantum critical point in CeCoIn5

We measured the specific heat and resistivity of heavy fermion CeCoIn5 between the superconducting critical field H(c2)=5 T and 9 T, with the field in the [001] direction, and at temperatures down to 50 mK. At 5 T the data show a non-Fermi liquid (NFL) behavior down to the lowest temperatures. At the field above 8 T the data exhibit a crossover from the Fermi liquid to a non-Fermi liquid behavior. We analyzed the scaling properties of the specific heat and compared both the resistivity and the specific heat with the predictions of a spin-fluctuation theory. Our analysis leads us to suggest that the NFL behavior is due to incipient antiferromagnetism (AFM) in CeCoIn5 with the quantum critical point in the vicinity of H(c2). Below H(c2) the AFM phase which competes with the paramagnetic ground state is superseded by the superconducting transition.     

Field-induced quantum critical point in CeCoIn5

The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a function of temperature, down to 25 mK and in magnetic fields of up to 16 T applied perpendicular to the basal plane. With increasing field, we observe a suppression of the non-Fermi liquid behavior, rho approximately T, and the development of a Fermi liquid state, with its characteristic rho=rho(0)+AT2 dependence. The field dependence of the T2 coefficient shows critical behavior with an exponent of 1.37. This is evidence for a field-induced quantum critical point (QCP), occurring at a critical field which coincides, within experimental accuracy, with the superconducting critical field H(c2). We discuss the relation of this field-tuned QCP to a change in the magnetic state, seen as a change in magnetoresistance from positive to negative, at a crossover line that has a common border with the superconducting region below approximately 1 K.     

63Cu nuclear relaxation in the quantum critical point system CeCu5.9Au0.1

63Cu NQR measurements of the 63Cu T1 are reported for the quantum critical point system CeCu5.9Au0.1 over temperatures ranging from 0.1 up to 4.2 K. Below approximately 1 K the magnetization recovery exhibits a stable, nonexponential decay function which we believe signals the onset of 2D quantum critical fluctuations, as has been noted in the literature. We find T1(-1) is proportional to T0.75 for the region T < 1 K. The observed temperature dependence is in agreement with a phenomenological model of non-Fermi liquid behavior based on the uniform susceptibility but is inconsistent with calculations based on susceptibility peaks identified via neutron scattering experiments.     

Magnetic field and pressure dependence of small angle neutron scattering in MnSi

We report small angle neutron scattering of spontaneous and magnetic field aligned components of the helical spin polarization in MnSi for temperatures T down to 0.35 K, at pressures p up to 21 kbar, and magnetic field B up to 0.7 T. The parameter range of our study spans the first order transition between helical order and partial magnetic order at p{c}=14.6 kbar, which coincides with the onset of an extended regime of non-Fermi liquid resistivity. Our study suggests that MnSi above p{c} is not dominated by the remnants of the first order transition at p{c}, but that an unidentified mechanism favors stabilization of a new ground state other than helical order.     

Quantum critical transport near the Mott transition

We perform a systematic study of incoherent transport in the high temperature crossover region of the half filled one-band Hubbard model. We demonstrate that the family of resistivity curves displays characteristic quantum critical scaling of the form ρ(T, δU) = ρ(c)(T)f(T/T?(δU)), with T?(δU) ~ |δU|(zν), and ρ(c)(T) ~ T. The corresponding β function displays a "strong coupling" form β ~ ln(ρ(c)/ρ), reflecting the peculiar mirror symmetry of the scaling curves. This behavior, which is surprisingly similar to some experimental findings, indicates that Mott quantum criticality may be acting as the fundamental mechanism behind the unusual transport phenomena in many systems near the metal-insulator transition.     

Strongly correlated Fermi-systems: Non-Fermi liquid behavior, quasiparticle effective mass and their interplay

Basing on the density functional theory of fermion condensation, we analyze the non-Fermi liquid behavior of strongly correlated Fermi-systems such as heavy-fermion metals. When deriving equations for the effective mass of quasiparticles, we consider solids with a lattice and homogeneous systems. We show that the low-temperature thermodynamic and transport properties are formed by quasiparticles, while the dependence of the effective mass on temperature, number density, magnetic fields, etc., gives rise to the non-Fermi liquid behavior. Our theoretical study of the heat capacity, magnetization, energy scales, the longitudinal magnetoresistance and magnetic entropy are in good agreement with the remarkable recent facts collected on the heavy-fermion metal YbRh₂Si₂.     

Heavy fermion state and pressure-induced resistive behaviour in CeRu2Si2

Pressure-induced variation in the resistive behaviour of CeRu₂Si₂, a Kondo-lattice system, has been studied in the temperature interval 1.5K – 300K. At low temperatures, resistivity exhibits (i) a linear temperature dependence at low applied pressures (P ?2.26 kbar) and (ii) a quadratic temperature dependence

(T) ～ AT² at higher pressures (6.5 kbar ? P ? 16.8 kbar). The coefficient A decrease, which in turn implies that the spin fluctuation temperature T_sf increases, with increasing applied pressure. The temperature coefficient γ of specific heat C = γ T at low temperature (0.3K ? T ? 1.6K) is quite large (385 mJ/mole-K²) confirming the heavy fermi liquid nature of the material.     

Energy scales and the non-Fermi liquid behavior in YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript>

Multiple energy scales are detected in measurements of the thermodynamic and transport properties in heavy fermion metals. We demonstrate that the experimental data on the energy scales can be well described by the scaling behavior of the effective mass at the fermion condensation quantum phase transition, and show that the dependence of the effective mass on temperature and applied magnetic fields gives rise to the non-Fermi liquid behavior. Our analysis is placed in the context of recent salient experimental results. Our calculations of the non-Fermi liquid behavior, of the scales and thermodynamic and transport properties are in good agreement with the heat capacity, magnetization, longitudinal magnetoresistance and magnetic entropy obtained in remarkable measurements on the heavy fermion metal YbRh₂Si₂.     

Intermediate valent metallic SmS and SmSe: A new puzzle

We have discovered that the R-T characteristic of so-called metallic SmS decreases with decreasing T at low temperatures for pressures exceeding?20 kbar. This feature is reminiscent of a metal and in sharp contrast to the previously reported negative temperature coefficient at 10 and 20 kbar. The same fundamental change of the R-T dependence occurs in SmSe at ?100 kbar.     

Exponential Increase in the Resistivity upon Cooling and Superconductivity in Indium-Doped Pb<Subscript>0.45</Subscript>Sn<Subscript>0.55</Subscript>Te

We have analyzed the temperature and magnetic-field dependences of resistivity ρ(T, H) of semiconducting compound Pb_0.45Sn_0.55Te doped with 5 at % In under a hydrostatic compression at P < 12 kbar. It is found that the temperature dependence ρ(T) at all pressures at T < 100 K is exponential with the activation energy decreasing upon an increase in pressure; this is accompanied with a superconducting transition on the ρ(T) and ρ(H) dependences at P > 4.8 kbar at T > 1 K (T _c = 1.72 K at a level of 0.5ρ _N at P = 6.8 kbar). We consider the model describing the low-temperature “dielectrization” of the semiconducting solid solution and the formation of the superconducting state upon an increase in the hydrostatic compression P > 4 kbar.     

Roles of zeros of the Green function in Fermi arc and non-Fermi liquid in the two-dimensional Hubbard model

We clarify effects of zeros of the Green function on a Fermi arc and on a non-Fermi liquid behavior in the two-dimensional Hubbard model by means of the cellular dynamical mean-field theory (CDMFT). We study in detail the state with a hole-pocket Fermi surface and zeros of the Green function, which was found for a slightly doped Mott insulator in an earlier CDMFT calculation [T.D. Stanescu, G. Kotliar, Phys. Rev. B 74 (2006) 125110; T.D. Stanescu, M. Civelli, K. Haule, G. Kotliar, Ann. Phys. (N.Y.) 321 (2006) 1682]. As thermal or other extrinsic scatterings of electrons broaden the zeros, regions around the zero surface gain an imaginary part of the self-energy, which strongly suppresses the spectral intensity, especially on the closer side of the hole pocket to the zero surface. Then the rest emerges as a Fermi arc. Quasiparticle weight becomes ill defined on the closer side of the Fermi pocket while it is well defined on the opposite side, which means that a differentiation of electrons occurs in the momentum space, indicating an emergence of a non-Fermi liquid phase.     

Thermal conductivity in the vicinity of the quantum critical end point in Sr3Ru2O7

Thermal conductivity of Sr3Ru2O7 was measured down to 40 mK and at magnetic fields through the quantum critical end point at Hc=7.85 T. A peak in the electrical resistivity as a function of the field was mimicked by the thermal resistivity. In the limit as T-->0 K, we find that the Wiedemann-Franz law is satisfied to within 5% at all fields, implying that there is no breakdown of the electron despite the destruction of the Fermi liquid state at quantum criticality. A significant change in disorder [from rho0(H=0 T)=2.1 to 0.5 microOmega cm] does not influence our conclusions. At finite temperatures, the temperature dependence of the Lorenz number is consistent with ferromagnetic fluctuations causing the non-Fermi liquid behavior as one would expect at a metamagnetic quantum critical end point.     

杂质引起的非费密液体行为

研究含杂质散射的电子系统的非费密液体行为.采用单杂质模型,通过格林函数的计算,证明无论有无束缚态,n(=3,2,1)维电子系统都可出现非费密液体行为. 关键词： 杂质 非费密液体行为     

Controlling electron-phonon interactions in graphene at ultrahigh carrier densities

We report on the temperature dependent electron transport in graphene at different carrier densities n. Employing an electrolytic gate, we demonstrate that n can be adjusted up to 4 × 10(14) cm(-2) for both electrons and holes. The measured sample resistivity ρ increases linearly with temperature T in the high temperature limit, indicating that a quasiclassical phonon distribution is responsible for the electron scattering. As T decreases, the resistivity decreases more rapidly following ρ(T) ～ T(4). This low temperature behavior can be described by a Bloch-Grüneisen model taking into account the quantum distribution of the two-dimensional acoustic phonons in graphene. We map out the density dependence of the characteristic temperature Θ(BG) defining the crossover between the two distinct regimes, and show that, for all n, ρ(T) scales as a universal function of the normalized temperature T/Θ(BG).     

Energy scales and magnetoresistance at a quantum critical point

The magnetoresistance (MR) of CeCoIn₅ 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.     

Specific heat jump in non-Fermi superconductors

We derive the jump in the specific heat at T=T _c for a superconductor in a non-Fermi liquid model. We took into consideration the two possible limits in this problem: the spin-charge separation model for a Fermi liquid and the usual non-Fermi liquid model which satisfies the homogeneity relation for the spectral function , ). We also derive the order parameter behavior for these two cases in the vecinity of the critical temperature. Received: 25 January 1998 / Revised: 25 March 1998 / Accepted: 25 March 1998     

Localization-induced Griffiths phase of disordered Anderson lattices

We demonstrate that local density of states fluctuations in disordered Anderson lattice models universally lead to the emergence of non-Fermi liquid (NFL) behavior. The NFL regime appears at moderate disorder ( W = Wc) and is characterized by power-law anomalies, e.g., C/T approximately 1/T((1-alpha)), where the exponent alpha varies continuously with disorder, as in other Griffiths phases. This Griffiths phase is not associated with the proximity to any magnetic ordering, but reflects the approach to a disorder-driven metal-insulator transition (MIT). Remarkably, the MIT takes place only at much larger disorder W(MIT) approximately 12Wc, resulting in an extraordinarily robust NFL metallic phase.     

Low temperature electron spin resonance of the Kondo ion in a heavy fermion metal: YbRh2Si2

We report an electron spin resonance (ESR) study on single crystals of the heavy fermion metal YbRh2Si2 which shows pronounced non-Fermi liquid behavior related to a close antiferromagnetic quantum critical point. It is shown that the observed ESR spectra can be ascribed to a bulk Yb3+ resonance. This is the first observation of ESR of the Kondo ion itself in a dense Kondo lattice system. The ESR signal occurs below the Kondo temperature (T(K)) which thus indicates the existence of large unscreened Yb3+ moments below T(K). We observe the spin dynamics as well as the static magnetic properties of the Yb3+ spins to be consistent with the results of nuclear magnetic resonance and magnetic susceptibility.