non-Fermi Liquid System Research Articles

Zero sound and plasmon modes for non-Fermi liquids

We derive the quantum Boltzmann equation (QBE) by using generalized Landau-interaction parameters, obtained through the nonequilibrium Green's function technique. This is a generalization of the usual QBE formalism to non-Fermi liquid (NFL) systems, which do not have well-defined quasiparticles. We apply this framework to a controlled low-energy effective field theory for the Ising-nematic quantum critical point, in order to find the collective excitations of the critical Fermi surface in the collisionless regime. We also compute the nature of the dispersion after the addition of weak Coulomb interactions. The zero angular momentum longitudinal vibrations of the Fermi surface show a linear-in-wavenumber dispersion, which corresponds to the zero sound of Landau's Fermi liquid theory. The Coulomb interaction modifies it to a plasmon mode in the long-wavelength limit, which disperses as the square-root of the wavenumber. Remarkably, our results show that the zero sound and plasmon modes exhibit the same behaviour as in a Fermi liquid, although an NFL is fundamentally different from the former.

Mixed-valent metallic pyrochlore iridate: A possible route to non-Fermi liquids

Non-Fermi liquid behavior in some fermionic systems has attracted significant interest in last few decades. Certain pyrochlore iridates with stronger spin-orbit coupling strength have recently been added to the list. Here, we provide evidence of such a non-Fermi liquid ground state in another mixed-valent metallic pyrochlore iridate ${\mathrm{Pb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$, through the combined investigation of electronic, magnetic, and thermodynamic properties as a function of temperature ($T$) and applied magnetic field ($H$). Resistivity measurement showed a linear temperature dependence down to 15 K below which it shows $\ensuremath{\rho}\ensuremath{\sim}{T}^{3/2}$ dependence while magnetic susceptibility diverges as $\ensuremath{\chi}(T$) $\ensuremath{\sim}{T}^{\ensuremath{-}\ensuremath{\alpha}}$ ($\ensuremath{\alpha}<1$) below 10 K. While a strong negative ${\mathrm{\ensuremath{\Theta}}}_{\text{CW}}$ has been observed from Curie-Weiss fitting, the absence of any long-range order down to 80 mK only indicates the presence of strong inherent geometric frustration in the system. Heat capacity data showed ${C}_{p}\ensuremath{\sim}Tln({T}_{0}/T)+\ensuremath{\beta}{T}^{3}$ dependence below 15 K down to 1.8 K. More importantly spin-orbit coupling strength by x-ray absorption spectroscopy was found to be weaker in ${\mathrm{Pb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ compared to other pyrochlore iridates. In the absence of any large moment rare earth magnetic ion, ${\mathrm{Pb}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ presents a rare example of an iridate system showing non-Fermi liquid behavior due to disordered distribution of ${\mathrm{Ir}}^{4+}$ and ${\mathrm{Ir}}^{5+}$ having markedly different strengths of spin-orbit coupling which might offer a prescription for achieving new non-Fermi liquid systems.

Coexistence of non-Fermi liquid behavior and biquadratic exchange coupling in La-substituted CeGe: Nonlinear susceptibility and DFT+DMFT study

Studies connected with the investigations of non-Fermi liquid (NFL) systems continue to attract interest in condensed matter physics community. Understanding the anomalous physical properties exhibited by such systems and its related electronic structures is one of the central research topics in this area. In this context, Ce-based and Ce-site diluted (with non-magnetic ions) compounds provide a fertile playground. Here, we present a detailed study of non-linear DC susceptibility and combined density functional theory plus dynamical mean field theory (DFT+DMFT) on Ce0.24La0.76Ge. Theoretical investigation of 4f partial density of states, local susceptibility and self-energy demonstrates the presence of NFL behavior which is associated with fluctuating local moments. Non-linear DC susceptibility studies on this compound reveal that the transition from NFL state to the new phase is due to development of the bi-quadratic exchange coupling and it obeys the non-linear susceptibility scaling. Under the application of magnetic fields, local moments interact spatially through conduction electrons resulting in magnetic fluctuations. Our studies point to the fact that the origin of the observed bi-quadratic exchange coupling is due to the spatial magnetic fluctuations.

Wiedemann-Franz law in a non-Fermi liquid and Majorana central charge: Thermoelectric transport in a two-channel Kondo system

Quantum dot devices allow one to access the critical point of the two-channel Kondo model. The effective critical theory involves a free Majorana fermion quasiparticle localized on the dot. As a consequence, this critical point shows both the phenomenon of non-Fermi liquid physics and fractionalization. Although a violation of the Wiedemann-Franz law is considered a defining feature of non-Fermi liquid systems, we show by exact calculations that it holds at the critical point, providing a counterexample to this lore. Furthermore, we show that the fractionalized Majorana character of the critical point can be unambiguously detected from the heat conductivity, opening the door to a direct experimental measurement of the elusive Majorana central charge $c=\tfrac{1}{2}$.

Emergence of superconductivity from fully incoherent normal state in an iron-based superconductor (Ba0.6K0.4)Fe2As2

In unconventional superconductors, it is generally believed that understanding the physical properties of the normal state is a pre-requisite for understanding the superconductivity mechanism. In conventional superconductors like niobium or lead, the normal state is a Fermi liquid with a well-defined Fermi surface and well-defined quasipartcles along the Fermi surface. Superconductivity is realized in this case by the Fermi surface instability in the superconducting state and the formation and condensation of the electron pairs (Cooper pairing). The high temperature cuprate superconductors, on the other hand, represent another extreme case that superconductivity can be realized in the underdoped region where there is neither well-defined Fermi surface due to the pseudogap formation nor quasiparticles near the antinodal regions in the normal state. Here we report a novel scenario that superconductivity is realized in a system with well-defined Fermi surface but without quasiparticles along the Fermi surface in the normal state. High resolution laser-based angle-resolved photoemission measurements have been performed on an optimally-doped iron-based superconductor (Ba0.6K0.4)Fe2As2. We find that, while sharp superconducting coherence peaks emerge in the superconducting state on the hole-like Fermi surface sheets, no quasiparticle peak is present in the normal state. Its electronic behaviours deviate strongly from a Fermi liquid system. The superconducting gap of such a system exhibits an unusual temperature dependence that it is nearly a constant in the superconducting state and abruptly closes at Tc. These observations have provided a new platform to study unconventional superconductivity in a non-Fermi liquid system.

Zero-Field Ambient-Pressure Quantum Criticality in the Stoichiometric Non-Fermi Liquid System CeRhBi

The strange electronic state of a class of materials which violates the predictions of conventional Fermi-liquid theory of metals remains enigmatic. Proximity to a quantum critical point is a possible origin of this non-Fermi liquid (NFL) behavior, which is usually accomplished by tuning the ground state with non-thermal control parameters such as chemical composition, magnetic field or pressure. We present the spin dynamics study of a stoichiometric NFL system CeRhBi, using low-energy inelastic neutron scattering (INS) and muon spin relaxation (muSR) measurements. It shows evidence for an energy-temperature (E/T) scaling in the INS dynamic response and a time-field scaling of the muSR asymmetry function indicating a quantum critical behavior in this compound. The E/T scaling reveals a local character of quantum criticality consistent with the power-law divergence of the magnetic susceptibility, logarithmic divergence of the magnetic heat capacity and T-linear resistivity at low temperature. The NFL behavior and local criticality occur over a very wide dynamical range at zero field and ambient pressure without any tuning in this stoichiometric heavy fermion compound is striking, making CeRhBi an exemplary model system amenable to in-depth studies for quantum criticality.

Hydrodynamic flows of non-Fermi liquids: Magnetotransport and bilayer drag

We consider a hydrodynamic description of transport for generic two dimensional electron systems that lack Galilean invariance and do not fall into the category of Fermi liquids. We study magnetoresistance and show that it is governed only by the electronic viscosity provided that the wavelength of the underlying disorder potential is large compared to the microscopic equilibration length. We also derive the Coulomb drag transresistance for double-layer non-Fermi liquid systems in the hydrodynamic regime. As an example, we consider frictional drag between two quantum Hall states with half-filled lowest Landau levels, each described by a Fermi surface of composite fermions coupled to a $U(1)$ gauge field. We contrast our results to prior calculations of drag of Chern-Simons composite particles and place our findings in the context of available experimental data.

Evolution of a magnetic order in the quasi-kagome lattice system CeRh1–xPdxSn (x ≤ 0.75)

The equiatomic compound CeRhSn with a quasi-kagome lattice of Ce atoms displays a non-Fermi liquid behavior at low temperatures. We have studied how the ground state changes with the substitution of Pd for Rh in CeRh1–xPdxSn (0 ≤ x ≤ 0.75) by measuring the specific heat C, magnetic susceptibility χ, and magnetization M. With increasing x, the lattice parameters a and c increase by 1.6%, the paramagnetic Curie temperature in χ(T) changes from –112 K to 16 K, and M(B = 5 T) at T = 1.8 K increases from 0.05μB/Ce to 1.4μB/Ce. A long-range magnetic order manifests in sharp peaks in C at 0.7, 1.0 and 1.5 K for x = 0.4, 0.5 and 0.65, respectively. An antiferromagnetic order for x = 0.75 is indicated by the observations of peaks in both χ(T) and C(T) at around 3.0 K and metamagnetic behavior in M(B) at B = 2 T. Our results point out that the doping of 4d electrons in the non-Fermi liquid system CeRhSn destroys the coherence of the quasi-kagome lattice and weakens the hybridization between the 4f state and conduction bands, leading to the evolution of antiferromagnetic order.

Localization-driven metal–insulator transition in epitaxial hole-doped Nd1−xSrxNiO3 ultrathin films

Advances in thin film growth technologies make it possible to obtain ultra-thin perovskite oxide films and open the window for controlling novel electronic phases for use in functional nanoscale electronics, such as switches and sensors. Here, we study the thickness-dependent transport characteristics of high-quality ultrathin Nd0.9Sr0.1NiO3 (Sr-NNO) films, which were grown on LaAlO3 (0 0 1) single-crystal substrates by using pulsed laser deposition method. Thick Sr-NNO films (25 unit cells) exhibit metallic behavior with the electrical resistivity following the T n (n < 2) law corresponding to a non-Fermi liquid system, while a temperature driven metal–insulator transition (MIT) is observed with films of less than 15 unit cells. The transition temperature increases with reducing film thickness, until the insulating characteristic is observed even at room temperature. The emergence of the insulator ground state can be attributed to weak localization driven MIT expected by considering Mott–Ioffe–Regel limit. Furthermore, the magneto-transport study of Sr-NNO ultrathin films also confirms that the observed MIT is due to the disorder-induced localization rather than the electron–electron interactions.

Study of the indirect interaction in a non-Fermi liquid within the AdS/CFT correspondence framework

The indirect spin–spin interaction between impurities in a non-Fermi quantum liquid system is theoretically investigated in this paper. The poles of the Green’s functions are shown to be responsible for the observed excitation spectra. Specifically, the anti-de Sitter/conformal field theory (AdS/CFT) correspondence is used to gain access to the analytical expressions of the Green’s functions for our particular problem.

Some thermodynamic non-Fermi liquid properties of correlated electron systems. (...)Cu&lt;sub&gt;&lt;i&gt;m&lt;/i&gt;&lt;/sub&gt;O&lt;sub&gt;&lt;i&gt;n-x&lt;/i&gt;&lt;sub&gt;

A mean-field Hamiltonian model has been used to investigate some thermodynamic properties of the normal states of non-Fermi liquid (NFL) systems,(...)CumOn-x . This Hamiltonian is like that of the Bardeen-Cooper-Schrieffer model [Phys. Rev. 108 (1957) 1175] but differs from the latter in (i) being multiband, (ii) the gap in energy being a function of the hopping integral and (iii) band energies of electrons being dependent upon spin orientation. The Hamiltonian is, therefore, similar to the Paring t-model [Physica 258 (9166) 30] but differs from it in not incorporating hybridization term and hybrid pair superconductivity. The analysis of the model yields magnetic energy spectrum for Cu (3d) bands` and non-magnetic energy spectrum for the O (2p) bands. Inverse temperature dependences of electronic specific heat Cv , entropy function (S) and pair susceptibility (&#935&#8593&#8595) are computed and exhibited. The specific heat dependence upon inverse temperature shows a linear form at very high temperature. It displays inverse-square-law temperature dependence, approximately, for lower temperatures. In the very low temperature range, the actual curve of the theoretical specific heat with temperature is rather like that of the Cp versus T-2 curve obtained for Bi4Cu4O16+y and Ti2Ba2Cn-1CunO2n+4 down to millikelvin temperature. This is in contradistinction to the linear temperature dependence (Cv = &#947T) of Fermi liquid systems. The specific entropy dependence on temperature shows correct physical response of systems to order (disorder) with varying temperatures. The pair susceptibility is linear at very high temperature and constant (X=X0) at moderate/low temperatures. The latter is as in Fermi liquid systems, but the former is an NFL manifestation. Journal of the Nigerian Association of Mathematical Physics Vol. 10 2006: pp. 149-156

Possible non-Fermi liquid behaviour in [formula omitted

We report on magnetic susceptibility χ , electrical resistivity ρ , magnetoresistance MR, thermoelectric power S, and specific heat C p of a new intermetallic compound Yb ( Cu 0.151 Si 0.849 ) 1.883 . We found several features characteristic of non-Fermi liquid systems: at low temperatures χ ∝ T 0.3 , ρ ∝ T and C p / T ∝ - T 1 / 2 .

Spin freezing and non-glassy dynamics in Ce 0.2La 0.8RhIn 5

Zero-field muon spin relaxation ( μ SR ) in non-Fermi liquid Ce 0.2La 0.8RhIn 5 reveals disordered spin freezing with a sharp transition at 0.16 K. The frozen moment at T = 0 is estimated as ∼ 0.13 μ B / Ce , an order of magnitude smaller than found from thermodynamic data at higher fields and temperatures. Longitudinal-field μ SR exhibits no dynamic relaxation at any temperature, showing that the spin freezing is not accompanied by the slow spin fluctuations found in spin glasses above the glass temperature and in other disordered non-Fermi liquid systems.

Highly correlated electron behavior in NpCu 5−x(Pd,Ni) x—a contrast to UCu 5−x(Pd,Ni) x

We report the low-temperature specific heats of NpCu 5− x Pd x ( x = 0 , 0.5, 1.0, and 1.5), the “plus one f-electron” analogs to the well-studied non-Fermi liquid (nFl) system UCu 5− x Pd x . The Np materials were shown to form in the same AuBe 5 structure type as the U pseudobinary alloys. A number of differences between the Np and U materials are found. The magnetic transition at 15 K in the U system is evidently suppressed to lower temperatures in the Np analog, and the low-temperature C/ T values are strongly enhanced in the NpCu 5− x Pd x ( x = 1.0 and 1.5) alloys vs. their U counterparts. Finally, NpCu 4Ni appears to show non-Fermi liquid behavior similar to that of the NpCu 5− x Pd x system, just as UCu 4Ni is quite similar to UCu 4Pd in its nFl behavior.

Spin fluctuations in the skutterudite compound LaFe4Sb12

We report transport, magnetic and thermodynamic properties of the skutterudite compound LaFe4Sb12. The basic features are a large magnetic susceptibility χ(T), and large electronic coefficient γ of the heat capacity. In particular, a T1.35, T1.7, and T-2/3 temperature dependence of the magnetic susceptibility χ(T), resistivity ρ(T), and Gruneisen parameter Γ(T), respectively, is found at low temperature. An overall understanding of these physical properties is achieved, assuming that LaFe4Sb12 is a non-Fermi liquid system close to a ferromagnetic quantum critical point, with a spin fluctuation temperature Tsf=50±15 K.

Magnetoresistance of UCu 4Pd

We present magnetoresistance ( MR) measurements of the non-Fermi liquid system UCu 4Pd. At low temperatures the MR is small and negative. In the temperature range from 2 to 30 K we observe a linear dependence of the MR on B 2 , i.e., Δ ρ / ρ 0 = α B 2 . The temperature dependence of α scales with χ , indicating spin disorder scattering as the source for the MR. We discuss the implications of the temperature dependence of the magnetoresistive behavior.

Superconductivity inCeCoIn5−xSnx: Veil over an Ordered State or Novel Quantum Critical Point?

Measurements of specific heat and electrical resistivity in magnetic fields up to 9 T along [001] and temperatures down to 50 mK of Sn-substituted CeCoIn5 are reported. The maximal -ln(T) divergence of the specific heat at the upper critical field Hc2 down to the lowest temperature characteristic of non-Fermi-liquid systems at the quantum critical point (QCP), the universal scaling of the Sommerfeld coefficient, and agreement of the data with spin-fluctuation theory provide strong evidence for quantum criticality at Hc2 for all x< or =0.12 in CeCoIn5-xSnx. These results indicate the "accidental" coincidence of the QCP located near Hc2 in pure CeCoIn5, in actuality, constitute a novel quantum critical point associated with unconventional superconductivity.

Lattice disorder and magnetism in f-electron intermetallics

Real materials can have real differences compared to ideal systems. For instance, non-Fermi liquid (NFL) behavior was initially thought to be due to chemical disorder, since the first such materials were all substituted. Although several nominally well-ordered NFLs have been discovered and extensively studied, the effect of disorder on the magnetic properties of f-electron intermetallic systems remains poorly understood. Disorder in NFL systems is reviewed from an experimental, local structure point of view, including a discussion of results on the nominally ordered U 3 Ni 3 Sn 4 and CeCoIn 5 systems, and the chemically disordered UCu 4 Pd and CeRhRuSi 2 systems.

Disorder, inhomogeneity and spin dynamics in f-electron non-Fermi liquid systems

Muon spin rotation and relaxation(μSR) experiments have yielded evidence that structural disorder is an important factor in manyf-electron-based non-Fermi-liquid (NFL) systems. Disorder-driven mechanisms for NFLbehaviour are suggested by the observed broad and strongly temperature-dependentμSR (and NMR) linewidths in several NFL compounds and alloys. Local disorder-driventheories (Kondo disorder, Griffiths–McCoy singularity) are, however, not capable ofdescribing the time-field scaling seen in muon spin relaxation experiments, which suggestcooperative and critical spin fluctuations rather than a distribution of local fluctuationrates. A strong empirical correlation is established between electronic disorder and slowspin fluctuations in NFL materials.

Magnetic, electrical resistivity, heat-capacity, and thermopower anomalies inCeCuAs2

The results of magnetic susceptibility, electrical resistivity ($\rho$), heat-capacity (C) and thermopower (S) measurements on CeCuAs2, forming in ZrCuSi2-type tetragonal structure, are reported. Our investigations reveal that Ce is trivalent and there is no clear evidence for long range magnetic ordering down to 45 mK. The $\rho$ behavior is notable in the sense that (i) the temperature (T)-coefficient of $\rho$ is negative in the entire range of measurement (45 mK to 300 K) with large values of $\rho$, while S behavior is typical of metallic Kondo lattices, and (ii) $\rho$ is proportional to T-0.6 at low temperatures, without any influence on the exponent by the application of a magnetic field, which does not seem to classify this compound into hither-to-known non-Fermi liquid (NFL) systems. In contrast to the logarithmic increase known for NFL systems, C/T measured down to 0.5 K exhibits a fall below 2 K. The observed properties of this compound are unusual among Ce systems.