Magnetothermal Transport in Spin-Ladder Systems

We study a theoretical model for the magnetothermal conductivity of a spin-1/2 ladder with low exchange coupling ($J\ll\Theta_D$) subject to a strong magnetic field $B$. Our theory for the thermal transport accounts for the contribution of spinons coupled to lattice phonon modes in the one-dimensional lattice. We employ a mapping of the ladder Hamiltonian onto an XXZ spin-chain in a weaker effective field B_{eff}=B-B_{0}$, where$B_{0}=(B_{c1}+B_{c2})/2$corresponds to half-filling of the spinon band. This provides a low-energy theory for the spinon excitations and their coupling to the phonons. The coupling of acoustic longitudinal phonons to spinons gives rise to hybridization of spinons and phonons, and provides an enhanced$B$-dependant scattering of phonons on spinons. Using a memory matrix approach, we show that the interplay between several scattering mechanisms, namely: umklapp, disorder and phonon-spinon collisions, dominates the relaxation of heat current. This yields magnetothermal effects that are qualitatively consistent with the thermal conductivity measurements in the spin-1/2 ladder compound${\rm Br_4(C_5H_{12}N)_2}$ (BPCB).Comment: 14 pages, 4 figure

Large thermomagnetic effects in weakly disordered Heisenberg chains

The interplay of different scattering mechanisms can lead to novel effects in transport. We show theoretically that the interplay of weak impurity and Umklapp scattering in spin-1/2 chains leads to a pronounced dip in the magnetic field dependence of the thermal conductivity $\kappa$ at a magnetic field $B \sim T$. In sufficiently clean samples, the reduction of the magnetic contribution to heat transport can easily become larger than 50% and the effect is predicted to exist even in samples with a large exchange coupling, J >> B, where the field-induced magnetization is small. Qualitatively, our theory might explain dips at $B \sim T$ observed in recent heat transport measurements on copper pyrazine dinitrate, but a fully quantitative description is not possible within our model.Comment: 5 pages, 2 figure

Heat transport of clean spin-ladders coupled to phonons: Umklapp scattering and drag

We study the low-temperature heat transport in clean two-leg spin ladder compounds coupled to three-dimensional phonons. We argue that the very large heat conductivities observed in such systems can be traced back to the existence of approximate symmetries and corresponding weakly violated conservation laws of the effective (gapful) low--energy model, namely pseudo-momenta. Depending on the ratios of spin gaps and Debye energy and on the temperature, the magnetic contribution to the heat conductivity can be positive or negative, and exhibit an activated or anti-activated behavior. In most regimes, the magnetic heat conductivity is dominated by the spin-phonon drag: the excitations of the two subsystems have almost the same drift velocity, and this allows for an estimate of the ratio of the magnetic and phononic contributions to the heat conductivity.Comment: revised version, 8 pages, 3 figures, added appendi

The Quantized Hall Insulator: A New Insulator in Two-Dimensions

Quite generally, an insulator is theoretically defined by a vanishing conductivity tensor at the absolute zero of temperature. In classical insulators, such as band insulators, vanishing conductivities lead to diverging resistivities. In other insulators, in particular when a high magnetic field (B) is added, it is possible that while the magneto-resistance diverges, the Hall resistance remains finite, which is known as a Hall insulator. In this letter we demonstrate experimentally the existence of another, more exotic, insulator. This insulator, which terminates the quantum Hall effect series in a two-dimensional electron system, is characterized by a Hall resistance which is approximately quantized in the quantum unit of resistance h/e^2. This insulator is termed a quantized Hall insulator. In addition we show that for the same sample, the insulating state preceding the QHE series, at low-B, is of the HI kind.Comment: 4 page

Magnetothermal transport in the spin-1/2 chains of copper pyrazine dinitrate

We present experiments on the thermal transport in the spin-1/2 chain compound copper pyrazine dinitrate Cu(C_4 H_4 N_2)(NO_3)_2. The heat conductivity shows a surprisingly strong dependence on the applied magnetic field B, characterized at low temperatures by two main features. The first one appearing at low B is a characteristic dip located at mu_B B ~ k_B T, that may arise from Umklapp scattering. The second one is a plateau-like feature in the quantum critical regime, mu_B |B-B_c| < k_B T, where B_c is the saturation field at T=0. The latter feature clearly points towards a momentum and field independent mean free path of the spin excitations, contrary to theoretical expectations.Comment: 4 pages, 4 figure

Transport Through Quantum Melts

We discuss superconductor to insulator and quantum Hall transitions which are first order in the clean limit. Disorder creates a nearly percolating network of the minority phase. Electrical transport is dominated by tunneling or activation through the saddle point junctions, whose typical resistance is calculated as a function of magnetic field. In the Boltzmann regime, this approach yields resistivity laws which agree with recent experiments in both classes of systems. We discuss the origin of dissipation at zero temperature.Comment: 4 pages, 1 figur

Effective Drag Between Strongly Inhomogeneous Layers: Exact Results and Applications

We generalize Dykhne's calculation of the effective resistance of a 2D two-component medium to the case of frictional drag between the two parallel two-component layers. The resulting exact expression for the effective transresistance, $\rho^D_{eff}$, is analyzed in the limits when the resistances and transresistances of the constituting components are strongly different - situation generic for the vicinity of the {\em classical} (percolative) metal-insulator transition (MIT). On the basis of this analysis we conclude that the evolution of $\rho^D_{eff}$ across the MIT is determined by the type of correlation between the components, constituting the 2D layers. Depending on this correlation, in the case of two electron layers, $\rho^D_{eff}$ changes either monotonically or exhibits a sharp maximum. For electron-hole layers $\rho^D_{eff}$ is negative and $|\rho^D_{eff}|$ exhibits a sharp minimum at the MIT.Comment: 7 pages, 3 figure

Reflection Symmetry and Quantized Hall Resistivity near Quantum Hall Transition

We present a direct numerical evidence for reflection symmetry of longitudinal resistivity $\rho_{xx}$ and quantized Hall resistivity $\rho_{xy}$ near the transition between $\nu=1$ quantum Hall state and insulator, in accord with the recent experiments. Our results show that a universal scaling behavior of conductances, $\sigma_{xx}$ and $\sigma_{xy}$, in the transition regime decide the reflection symmetry of $\rho_{xx}$ and quantization of $\rho_{xy}$, independent of particle-hole symmetry. We also find that in insulating phase away from the transition region $\rho_{xy}$ deviates from the quantization and diverges with $\rho_{xx}$.Comment: 3 pages, 4 figures; figure 4 is replace

Coulomb Drag at the Onset of Anderson Insulators

It is shown that the Coulomb drag between two identical layers in the Anderson insulting state indicates a striking difference between the Mott and Efros-Shklovskii (ES) insulators. In the former, the trans-resistance $\rho_t$ is monotonically increasing with the localization length $\xi$; in the latter, the presence of a Coulomb gap leads to an opposite result: $\rho_t$ is enhanced with a decreasing $\xi$, with the same exponential factor as the single layer resistivity. This distinction reflects the relatively pronounced role of excited density fluctuations in the ES state, implied by the enhancement in the rate of hopping processes at low frequencies. The magnitude of drag is estimated for typical experimental parameters in the different cases. It is concluded that a measurement of drag can be used to distinguish between interacting and non-interacting insulating state.Comment: 15 pages, revte