Temperature and magnetic-field-enhanced hall slope of a dilute 2D hole system in the ballistic regime

We report the temperature (T) and perpendicular magnetic-field (B) dependence of the Hall resistivity rho(xy)(B) of dilute metallic 2D holes in GaAs over a broad range of temperature (0.02-1.25 K). The low B Hall coefficient, R(H), is found to be enhanced when T decreases. Strong magnetic fields further enhance the slope of rho(xy)(B) at all temperatures studied. Coulomb interaction corrections of a Fermi liquid (FL) in the ballistic regime can not explain the enhancement of rho(xy) which occurs in the same regime as the anomalous metallic longitudinal conductivity. In particular, although the metallic conductivity in 2D systems has been attributed to electron interactions in a FL, these same interactions should reduce, not enhance, the slope of rho(xy)(B) as T decreases and/or B increases.     

Transport properties of granular metals at low temperatures

We investigate transport in a granular metallic system at large tunneling conductance between the grains, g(T)>1. We show that at low temperatures, Tg(T)delta) behavior where conductivity is controlled by the scales of the order of the grain size. In three dimensions we predict the metal-insulator transition at the bare tunneling conductance g(C)(T)=(1/6pi)ln((E(C)/delta), where E(C) is the charging energy of a single grain. Corrections to the density of states of granular metals due to the electron-electron interaction are calculated. Our results compare favorably with the logarithmic dependence of resistivity in the high-T(c) cuprate superconductors indicating that these materials may have a granular structure.     

Direct observation of chiral susceptibility in the canonical spin glass AuFe

The extraordinary Hall resistivity rho(xy) and the magnetization M of a canonical spin glass AuFe (8 at.% Fe) were measured simultaneously as functions of temperature with the best care to the thermal and the magnetic field hysteresis. The data of rho(xy) show an anomaly at the spin glass transition temperature T(g) and have different zero field cooling (ZFC) and field cooling (FC) measurements below T(g). Moreover, the value of rho(xy)/M, which represents the chiral susceptibility of the system in the present case, also shows the difference between ZFC and FC measurements. The results are consistent with the predictions of the chirality scenario of canonical spin glasses by Kawamura.     

Suppression of superconductivity in granular metals

We investigate the suppression of the superconducting transition temperature due to Coulomb repulsion in granular metallic systems at large tunneling conductance between the grains, g(T)>1. We find the correction to the superconducting transition temperature for 3D granular samples and films. We demonstrate that, depending on the parameters of superconducting grains, the corresponding granular samples can be divided into two groups: (i). the granular samples that belong to the first group may have only insulating or superconducting states at zero temperature depending on the bare intergranular tunneling conductance g(T), while (ii). the granular samples that belong to the second group in addition have an intermediate metallic phase where superconductivity is suppressed while the effects of the Coulomb blockade are not yet strong.     

Low energy dynamics of two-dimensional lateral tunnel junctions

We report on the low temperature tunneling characteristics of two-dimensional lateral tunnel junctions (2DLTJs) consisting of two coplanar two-dimensional electron systems separated by an in-plane tunnel barrier. The tunneling conductance of the 2DLTJ exhibits a characteristic dip at small voltages—consistent with the phenomenon of zero-bias anomaly in low-dimensional tunnel junctions—and a broad conductance peak at the Coulombic energy scale. The conductance peak remains robust under magnetic fields well into the quantum Hall regime. We identify the broad conductance maxima as the signature of the pseudogap in the tunneling density of states below the characteristic Coulomb interaction energy of the 2DLTJ.     

Full current statistics of incoherent "cold electrons"

We evaluate the full current statistics (FCS) in the low-dimensional (1D and 2D) diffusive conductors in the incoherent regime eV>E(Th)=D/L(2), E(Th) being the Thouless energy. It is shown that the Coulomb interaction substantially enhances the probability of big current fluctuations for short conductors with E(Th)>1/tau(E), tau(E) being the energy relaxation time, leading to the exponential tails in the current distribution. The current fluctuations are most strong for low temperatures, provided E(Th) approximately [(eV)(2)/Dnu(2)(1)](1/3) for 1D and E(Th) approximately (eV/g)ln(g for 2D, where g is a dimensionless conductance and nu(1) is a 1D density of states. The FCS in the "hot electron" regime is also discussed.     

Dissipative quantum hall effect in graphene near the Dirac point

We report on the unusual nature of the nu=0 state in the integer quantum Hall effect (QHE) in graphene and show that electron transport in this regime is dominated by counterpropagating edge states. Such states, intrinsic to massless Dirac quasiparticles, manifest themselves in a large longitudinal resistivity rho(xx) > or approximately h/e(2), in striking contrast to rho(xx) behavior in the standard QHE. The nu=0 state in graphene is also predicted to exhibit pronounced fluctuations in rho(xy) and rho(xx) and a smeared zero Hall plateau in sigma(xy), in agreement with experiment. The existence of gapless edge states puts stringent constraints on possible theoretical models of the nu=0 state.     

Spin chirality fluctuations and anomalous Hall effect in itinerant ferromagnets

The anomalous Hall effect due to the spin chirality order and fluctuation is studied theoretically in a Kondo lattice model without the relativistic spin-orbit interaction. Even without the correlations of the localized spins, sigma(xy) can emerge depending on the lattice structure and the spin anisotropy. We reveal the condition for this chirality-fluctuation driven mechanism for sigma(xy). Our semiquantitative estimates for a pyrochlore oxide Nd2Mo2O7 give a finite sigma(xy) approximately equal 10 Omega(-1) cm(-1) together with a high resistivity rho(xx) approximately equal 10(-4)-10(-3) Omega cm, in agreement with experiments.     

Interaction effects in conductivity of Si inversion layers at intermediate temperatures

We compare the temperature dependence of resistivity rho(T) of Si-metal-oxide-semiconductor field-effect transistors with the recent theory by Zala et al. In this comparison, the effective mass m* and g* factor for mobile electrons have been determined from independent measurements. An anomalous increase of rho with temperature, which has been considered as a signature of the "metallic" state, can be described quantitatively by the interaction effects in the ballistic regime. The in-plane magnetoresistance rho(B(axially)) is only qualitatively consistent with the theory; the lack of quantitative agreement indicates that the magnetoresistance is more sensitive to sample-specific effects than rho(T).     

Unconventional anomalous Hall effect enhanced by a noncoplanar spin texture in the frustrated Kondo lattice Pr2Ir2O7

We have investigated the Hall effect in the geometrically frustrated Kondo lattice Pr2Ir2O7. In its spin-liquid-like paramagnetic regime, the Hall resistivity rho(xy) is found to increase logarithmically on cooling. Moreover, in this low temperature region, the field dependence of the Hall conductivity sigma(xy) shows a large enhancement up to 30 Omega(-1) cm(-1) as well as a nonmonotonic change with the magnetization. Our results are far different from the anomalous Hall effect due to the spin-orbit coupling observed in ordinary magnetic conductors. We discuss the possible spin-chirality effect in the Ir 5d conduction band due to the noncoplanar texture of Pr<111> Ising-like moments.     

Particle-hole symmetry and the Pfaffian state

We show that the particle-hole conjugate of the Pfaffian state-or "anti-Pfaffian" state-is in a different universality class from the Pfaffian state, with different topological order. The two states can be distinguished easily by their edge physics: their edges differ in both their thermal Hall conductance and their tunneling exponents. At the same time, the two states are exactly degenerate in energy for a nu=5/2 quantum Hall system in the idealized limit of zero Landau level mixing. Thus, both are good candidates for the observed sigma_{xy}=5/2(e;{2}/h) quantum Hall plateau.     

Enhancement of the quantum-liquid phase by increased resistivity in thick a-MoxSi1-x films

Effects of normal-state resistivity rho(n) on the vortex phase diagram at low temperature T have been studied based on dc and ac complex resistivities for thick amorphous MoxSi(1-x) films. It is commonly observed irrespective of rho(n) that, in the limit T=0, the vortex-glass-transition line B(g)(T) is independent of T and extrapolates to a field below the T=0 upper critical field B(c2)(0), indicative of the quantum-vortex-liquid (QVL) phase in the regime B(g)(0)相似文献   

Phase diagram of coulomb interactions across the metal-insulator transition in Si:B

Measurements of the single-particle density of states (DOS) near T=0 K in Si:B are used to construct an energy-density phase diagram of Coulomb interactions across the critical density n(c) of the metal-insulator transition. Insulators and metals are found to be distinguishable only below a phase boundary epsilon*(|n/n(c)-1|) determined by the Coulomb energy. Above epsilon* is a mixed state where metals and insulators equidistant from n(c) cannot be distinguished from their DOS structure. The data imply a diverging screening radius at n(c), which may signal an interaction-driven thermodynamic state change.     

Electronic correlation effects and the Coulomb gap at finite temperature

We have investigated the effect of the long-range Coulomb interaction on the one-particle excitation spectrum of n-type germanium, using tunneling spectroscopy on mechanically controllable break junctions. At low temperatures, the tunnel conductance shows a minimum at zero bias voltage due to the Coulomb gap. Above 1 K, the gap is filled by thermal excitations. This behavior is reflected in the variable-range hopping resistivity measured on the same samples: up to a few degrees Kelvin the Efros-Shklovskii lnR infinity T(-1/2) law is obeyed, whereas at higher temperatures deviations from this law occur. The type of crossover differs from that considered previously in the literature.     

Thermal conductivity of a granular metal

Using the Kubo formula approach, we study the effect of electron interaction on thermal transport in the vicinity of a metal-insulator transition, with a granular metal as our model. For small values of dimensionless intergrain tunneling conductance, g<1, we find that the thermal conductivity surprisingly shows a phononlike algebraic decrease, kappa(T) approximately g2T3/E2c even though the electrical conductivity obeys an Arrhenius law, sigma(T) approximately ge-Ec/T ; therefore the Wiedemann-Franz (WF) law is seriously violated. We explicitly show that this violation arises from nonmagnetic bosonic excitations of low energy that transport heat but not charge. At large values of intergrain tunneling, we find it plausible that the WF law weakly deviates from the free-electron theory due to potential fluctuations. Implications for experiment are discussed.     

Probing the quantum critical behavior of CeCoIn5 via Hall effect measurements

We present highly sensitive Hall effect measurements of the heavy fermion compound CeCoIn5 down to temperatures of 55 mK. A pronounced dip in the differential Hall coefficient | partial differential rho(xy)/ partial differential H| at low temperature and above the upper critical field of superconductivity, H(c2), is attributed to critical spin fluctuations associated with the departure from Landau Fermi liquid behavior. This identification is strongly supported by a systematic suppression of this feature at elevated pressures. The resulting crossover line in the field-temperature phase diagram favors a field induced quantum critical point at mu(0)H(qc) approximately 4.1 T below H(c2)(T=0) suggesting related, yet separate, critical fields.     

Cascade of quantum phase transitions in tunnel-coupled edge states

We report on the cascade of quantum phase transitions exhibited by tunnel-coupled edge states across a quantum Hall line junction. We identify a series of quantum critical points between successive strong and weak tunneling regimes in the zero-bias conductance. Scaling analysis shows that the conductance near the critical magnetic fields B(c) is a function of a single scaling argument /B-B(c)/T(-kappa), where the exponent kappa=0.42. This puzzling resemblance to a quantum Hall-insulator transition points to the importance of interedge correlation between the coupled edge states.     

Breakup densities of hot nuclei

Breakup densities of hot 197Au-like residues have been deduced from the systematic trends of Coulomb parameters required to fit intermediate-mass-fragment kinetic-energy spectra. The results indicate emission from nuclei near normal nuclear density below an excitation energy E(*)/A less, similar 2 MeV, followed by a gradual decrease to a near-constant value of rho/rho(0) approximately 0.3 for E(*)/A greater, similar 5 MeV. Temperatures derived from these data with a density-dependent Fermi-gas model yield a nuclear caloric curve that is generally consistent with those derived from isotope ratios.     

High-field quasiparticle tunneling in Bi2Sr2CaCu2O(8+delta): negative magnetoresistance in the superconducting state

We report on the c-axis resistivity rho(c)(H) in Bi(2)Sr(2)CaCu(2)O(8+delta) that peaks in quasistatic magnetic fields up to 60 T. By suppressing the Josephson part of the two-channel (Cooper pair/quasiparticle) conductivity sigma(c)(H), we find that the negative slope of rho(c)(H) above the peak is due to quasiparticle tunneling conductivity sigma(q)(H) across the CuO2 layers below H(c2). At high fields (a) sigma(q)(H) grows linearly with H, and (b) rho(c)(T) tends to saturate ( sigma(c) not equal0) as T-->0, consistent with the scattering at the nodes of the d-wave gap. A superlinear sigma(q)(H) marks the normal state above T(c).