Magnetic-field-driven quantum critical behavior in graphite and bismuth

We study magnetotransport properties of graphite and rhombohedral bismuth samples and found that in both materials applied magnetic field induces the metal-insulator- (MIT) and reentrant insulator-metal-type (IMT) transformations. The corresponding transition boundaries plotted on the magnetic field-temperature (B − T) plane nearly coincide for these semimetals and can be best described by power laws T ∼ (B − B_c)^κ, where B_c is a critical field at T = 0 and κ = 0.45 ± 0.05. We show that insulator-metal-insulator (I-M-I) transformations take place in the Landau level quantization regime and illustrate how the IMT in quasi-3D graphite transforms into a cascade of I-M-I transitions, related to the quantum Hall effect in quasi-2D graphite samples. We discuss the possible coupling of superconducting and excitonic correlations with the observed phenomena, as well as signatures of quantum phase transitions associated with the M-I and I-M transformations.     

Localization Exponent for the Second Landau Level in the Quantum Hall Effect

At temperature above 1 K, we measured the temperature dependence of the longitudinal and Hall resistivity ρxx,ρxy in the regime of the quantum Hall plateau-to-plateau transitions. The localization exponent v is extracted with an approach based on the variable range hopping theory. We find the quantity v ≈ 2.3 at the second Landau level, which is proven to be accurately universal.     

Magnetic field induced metal-insulator transitions in p-SiGe

Low density modulation doped p-SiGe, where the holes lie in a strained SiGe quantum well, frequently exhibits anomalous insulating behaviour between filling factors ν=2 and 1. There is also anomalous metallic behavior with a metal-insulator transition between the two. It is shown that in these samples exchange effects are sufficiently large to induce the paramagnetic-ferromagnetic phase transition predicted by Giuliani and Quinn in 1985, also that the metallic and insulating behavior is associated with the coincidence of two Landau levels of opposite spin. A model calculation shows that while a ferromagnetic polarization may occur at integer filling factors screening suppresses it for non-integer filling factors. It is argued the Landau levels then stick-together and allow a spin-density instability to form. Because of the strong spin-orbit coupling in p-SiGe the transport properties of this state differ from those of other systems where a similar quantum Hall ferromagnet probably forms.     

Induced currents, frozen charges and the quantum Hall effect breakdown

Puzzling results obtained from torque magnetometry in the quantum Hall effect regime are presented, and a theory is proposed for their explanation. Magnetic moment saturation, which is usually attributed to the quantum Hall effect breakdown, is shown to be related to the charge redistribution across the sample.     

Metal-insulator transition induced by the magnetic field in n-type GaSb

The metal-insulator (MI) transition induced by a magnetic field was evidenced for the first time in compensated n-type GaSb layers grown by molecular beam epitaxy. The free electron densities were in the low 10¹⁶ cm⁻³ range or even slightly lower, so that the zero-field 3D electron gas was degenerate and, at the B_MI magnetic field of the MI transition, it populates only the spin-split 0⁽⁺⁾ Landau level (extreme quantum limit). On the metallic side of the MI transition a T^1/3 dependence of the conductivity was assumed to fit the low-T data and to estimate the B_MI value, which resulted of 9.1 T in the purest sample. The MI transition manifests in a strong increase of the diagonal resistivity with the magnetic field, but not of the Hall coefficient, suggesting that the apparent electron density is practically constant, whereas the mobility varies strongly. The evidence of a maximum in the temperature dependence of the Hall coefficient has been explained through a two channels transport mechanism involving localized and extended states.     

Form of Scaling Function in Quantum Hall Plateau Transitions

We have measured the temperature dependence of the resistance Rxx and Rxy of a two-dimensional electron system in the regime of the quantum Hall plateau transition. We observe for our sample a considerable large critical exponent κ～ 0.66 - 0.77, which may be due to the dominant electron-phonon scattering. Further we find a simple exponential form of Rxx = Rc exp（-s） in agreement with the theoretically proposed universal scaling function.     

Universal scaling results for the plateau-insulator transition in the quantum Hall regime

The plateau-insulator (PI) transition in the quantum Hall regime, in remarkable contrast to the plateau-plateau (PP) transition, exhibits very special features that enable one for the first time to disentwine the quantum critical aspects of the electron gas (scaling functions, critical indices) from the sample dependent effects of macroscopic inhomogeneities (contact misalignments, density gradients). In this communication we report new experimental data taken from the PI transition of a low-mobility InGaAs/InP heterostructure and propose universal scaling functions for the transport coefficients. Our new findings elucidate fundamental theoretical aspects of quantum criticality that have so far remained inaccessible.     

Absence of metal-insulator transition and coherent interlayer transport in oriented graphite in parallel magnetic fields

Measurements of the magnetoresistivity of graphite with a high degree of control of the angle between the sample and magnetic field indicate that the metal-insulator transition, shown to be induced by a magnetic field applied perpendicular to the layers, does not appear in parallel field orientation. Furthermore, we show that interlayer transport is coherent in less ordered samples and high magnetic fields, whereas appears to be incoherent in less disordered samples. Our results demonstrate the two-dimensionality of the electron system in ideal graphite samples.     

Electronic behavior in mats of single-walled carbon nanotubes under pressure

Single-walled carbon nanotubes (SWNTs) have many interesting properties; they may be metallic or semiconducting depending on their diameter and helicity of the graphene sheet. Hydrostatic or quasi-hydrostatic high pressures can probe many electronic features. Resistance-temperature measurements in SWNTs from normal condition and under 0.4 GPa of quasi-hydrostatic pressures reveal a semiconducting-like behavior. From 0.5 to about 2.0 GPa, the resistance changes to a Kondo-like feature due to magnetic impurities used to catalyse the nanotube formation. Above 2.0 GPa, they become metallic and at about 2.4 GPa, the resistance decreases dramatically around 3 K suggesting a superconducting transition.     

Exact results for systems of electrons in the fractional quantum Hall regime

There has been a great deal of interest over the last two decades on the fractional quantum Hall effect, a novel quantum many-body liquid state of strongly correlated two-dimensional electronic systems in a strong perpendicular magnetic field. The most pronounced fractional quantum Hall states occur at odd denominator filling factors of the lowest Landau level and are described by the Laughlin wave function. It is well known that exact closed-form solutions for many-body wave functions, including the Laughlin wave function, are generally very rare and hard to obtain. In this work we present some exact results corresponding to small systems of electrons in the fractional quantum Hall regime at odd denominator filling factors. Use of Jacobi coordinates is the key tool that facilitates the exact calculation of various quantities. Expressions involving integrals over many variables are considerably simplified with the help of Jacobi coordinates allowing us to calculate exactly various quantities corresponding to systems with several electrons.     

Exact results for systems of electrons in the fractional quantum Hall regime II

In a previous work [O. Ciftja, Physica B 404 (2009) 227] we reported the exact calculation of energies for the fractional quantum Hall Laughlin state at filling factor for systems with up to N=4 electrons in a disk geometry. The purpose of this brief extension of the earlier work is to report similar exact results for the other Laughlin state at filling factor . We use the same method of orthogonal Jacobi variables adopted in the earlier work.     

Forward scattering approximation and bosonization in integer quantum Hall systems

In this work, we present a model and a method to study integer quantum Hall (IQH) systems. Making use of the Landau levels structure we divide these two-dimensional systems into a set of interacting one-dimensional gases, one for each guiding center. We show that the so-called strong field approximation, used by Kallin and Halperin and by MacDonald, is equivalent, in first order, to a forward scattering approximation and analyze the IQH systems within this approximation. Using an appropriate variation of the Landau level bosonization method we obtain the dispersion relations for the collective excitations and the single-particle spectral functions. For the bulk states, these results evidence a behavior typical of non-normal strongly correlated systems, including the spin-charge splitting of the single-particle spectral function. We discuss the origin of this behavior in the light of the Tomonaga-Luttinger model and the bosonization of two-dimensional electron gases.     

Magneto-oscillations of the Hall coefficient in n-type GaSb at the extreme quantum limit

Magnetoquantum oscillations of the Hall coefficient R_H were observed in Te-doped GaSb layers grown by molecular beam epitaxy. The free electron densities were in the low 10¹⁶ cm⁻³ range or even slightly lower, thus achieving, for the first time in GaSb, the extreme quantum limit, where all the electrons occupy the spin-split 0⁽⁺⁾ Landau level (LL). Similarly to other known cases, the amplitude of the last maximum of R_H could be explained as enhanced by the metal-to-insulator transition of the spin-down electron system in the n=0 LL. The occurrence of the last negative oscillation of R_H below its classical value, called Hall dip, could be frustrated, in samples with sufficiently low carrier densities, by an incipient carrier freeze-out at donor impurities induced by the magnetic field.     

A capacitive method for hall effect measurements

A new, simple and reliable method for Hall effect measurements is introduced. The method employs a capacitive probe technique and requires neither special shaping of samples nor probing Hall contacts. With this method, Hall effect measurements onp-Ge have been first extended to high electric fields up to 3kV/cm at 4.2 K. The wide applicability of this method is discussed.     

Structural,magnetic, and transport properties in La(2+4x)/3Sr(1−4x)/3Mn1−xCuxO3 (0⩽x⩽0.20) system

A series of the double-doping samples La_(2+4x)/3Sr_(1−4x)/3Mn_1−xCu_xO₃(0?x?0.2)$(0?x?0.2)$ with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been prepared. The structural, magnetic, transport properties and magnetoresistance of the series samples have been investigated. It is found that no apparent crystal structure change is introduced by Cu doping up to x=0.20$x=0.20$. But the Curie temperature _TC$T_{\mathrm{C}}$ and magnetization M   are strongly affected by Cu substitution. A remarkable magnetotransport behavior, characterized by double bumps, is observed, and an obvious low-temperature upturn is found in the range of 0.07?x?0.12$0.07?x?0.12$. As a result, the temperature range of colossal magnetoresistance (CMR) is greatly broadened. Moreover, it is found that the room temperature magnetoresistance (MR) of double-doping samples is obviously larger that the undoped La_2/3Sr_1/3Mn_1−xCu_xO₃ at 300 K, which can give a guide for the adequate selection of the room temperature CMR materials.     

Preexponential factor in variable-range hopping conduction in CuInTe2

We study the temperature dependence of the electrical resistivity in a single crystal of p-type uncompensated CuInTe₂ on the insulating side of the metal-insulator transition down to 0.4 K. We observe a crossover from Mott to Efros-Shklovskii variable-range hopping conduction. In Efros-Shklovskii-type conduction, the resistivity is best described by explicitly including a preexponential temperature dependence according to the general expression ρ=ρ₀T^αexp(T_ES/T)^1/2, with α≠0. A theory based on the resistor network model was developed to derive an explicit relation between α and the decay of the wavefunction of the localized states. A consistent correspondence between the asymptotic extension of the wavefunction and the conduction regime is proposed. The results indicate a new mechanism for a local resistivity maximum in insulators, not involving magnetic effects.     

New picture of high-temperature superconductivity

The case for high-temperature superconductivity originating in SrO or BaO planes, or in interstitial regions, is made, including (i) four successfully predicted superconductors; (ii) evidence that the superconductivity of the major cuprates is associated with holes in these layers; (iii) data showing that Pr on one side of a cuprate-plane kills the superconductivity, but Pr on the other side does not; and (iv) evidence that doped Sr₂YRuO₆ has an onset of superconductivity at ∼45 K despite having no cuprate-planes.     

Prediction of giant antiferromagnetic coupling in exotic fluorides of AgII

We show, using Density Functional Theory (DFT) calculations, that compressed AgF₂ should turn above 17 GPa into a layered narrow‐gap material with a huge intralayer antiferromagnetic (AFM) coupling constant, reminiscent of those seen for parent copper (II) oxides (e.g., La₂CuO₄). Compressed AgF₂ is thus the first candidate for the non‐oxocuprate two‐dimensional antiferromagnet. Calculations indicate that AgF₂ could subsequently be metallised above 38 GPa, likely giving rise to superconductivity (SC).

     

Anomalous paramagnetic behavior in the paramagnetic phase of La2/3−xYxCa1/3MnO3 studied through EPR and its relation to colossal magnetoresistance

Electron paramagnetic resonance on La_2/3−xY_xCa_1/3MnO₃ in the paramagnetic (PM) regime is presented for 0≤x≤0.133. The resonance linewidth (ΔH_pp) decreases with cooling, reaches the minimum at T_min, and then anomalously increases with further cooling toward T_c. Our analysis on ΔH_pp(T) below T_min shows that the anomalous PM behavior below T_min is due to the appearance of a ferromagnetic (FM) phase within the PM matrix caused by the applied magnetic fields. The correlation between the anomalous PM and the colossal magnetoresistance is discussed. We argue that both are caused by the phase segregation in which the compound is phase-separated into a mixture of FM and PM regions.