arXiv subjects

Ora Entin-Wohlman

Publications and source records attributed to Ora Entin-Wohlman.

49 records · Page 3Linked to original sources

arXiv2004

The spin-wave spectrum of the Jahn-Teller system LaTiO3

We present an analytical calculation of the spin-wave spectrum of the Jahn-Teller system LaTiO3. The calculation includes all superexchange couplings between nearest-neighbor Ti ions allowed by the space-group symmetries: The isotropic Heisenberg couplings and the antisymmetric (Dzyaloshinskii-Moriya) and symmetric anisotropies. The calculated spin-wave dispersion has four branches, two nearly degenerate branches with small zone-center gaps and two practically indistinguishable high-energy branches having large zone-center gaps. The two lower-energy modes are found to be in satisfying agreement with neutron-scattering experiments. In particular, the experimentally detected approximate isotropy in the Brillouin zone and the small zone-center gap are well reproduced by the calculations. The higher-energy branches have not been detected yet by neutron scattering but their zone-center gaps are in satisfying agreement with recent Raman data.

cond-mat.str-el↗

arXiv2004

A Landauer transport formulation with inelastic scattering

The Landauer transport formulation is generalized to the case of a dynamic scatterer with an arbitrary energy level structure, weakly coupled to a long ideal noninteracting wire. The two-terminal linear conductance of the device is expressed in terms of the (in general inelastic) scattering cross sections of the electrons off the scatterer, using a Fermi-liquid picture for the scattering states of the whole system. Assuming unitarity, i.e. the optical theorem, we obtain the same results from a Kubo linear response treatment and from a Keldysh technique approach.

cond-mat.mes-hall↗

arXiv2004

The magnetic structure of the Jahn-Teller System LaTiO3

We investigate the effect of the experimentally observed Jahn-Teller distortion of the oxygen octahedra in LaTiO3 on the magnetic exchange. We present a localized model for the effective hopping between nearest-neighbor Ti ions and the intra-site Coulomb interactions, based on a non-degenerate orbital ground state due to the static crystal field. The latter corresponds to an orbital order which recently has been confirmed experimentally. Using perturbation theory we calculate, in addition to the Heisenberg coupling, antisymmetric and symmetric anisotropy terms of the superexchange spin Hamiltonian which are caused by the spin-orbit interaction. Employing our superexchange Hamiltonian, we deduce the magnetic order at low temperatures which is found to be in good agreement with experiment.

cond-mat.str-el↗

arXiv2004

Quantized charge pumping by surface acoustic waves in ballistic quasi-1D channels

Adiabatic pumping of electrons induced by surface acoustic waves (SAWs) in a ballistic quasi-1D quantum channel is considered using an exactly solvable tight-binding model for non-interacting electrons. The single-electron degrees of freedom, responsible for acoustoelectric current quantization, are related to the transmission resonances. We study the influence of experimentally controllable parameters (SAW power, gate voltage, source-drain bias, amplitude and phase of a secondary SAW beam) on the plateau-like structure of the acoustoelectric current. The results are consistent with existing experimental observations.

cond-mat.mes-hall↗

arXiv2003

Adiabatic Quantum Pumping of Coherent Electrons

We review recent theoretical calculations of charge transfer through mesoscopic devices in response to slowly-oscillating, spatially-confined, potentials. The discussion is restricted to non-interacting electrons, and emphasizes the role of quantum interference and resonant transmission in producing almost integer values (in units of the electronic charge $e$) of the charge transmitted per cycle, $Q$. The expression for the pumped charge is derived from a systematic expansion of the system scattering states in terms of the temporal derivatives of the instantaneous solutions. This yields the effect of the modulating potential on the Landauer formula for the conductance in response to a constant bias on one hand, and the corrections to the widely-used adiabatic-limit formula (in which the modulation frequency is smaller than any electronic relaxation rate) on the other hand. The expression for $Q$ is used in connection with simple models to exemplify the intimate relationship between resonant transmission through the mesoscopic device and almost integral values of $Q$, and to analyze the charge pumped by a surface acoustic wave coupled to a quantum channel by the piezoelectric effect. \keywords{interference in nanostructures, quantum pumping, surface acoustic waves, resonant transmission.

cond-mat.mes-hall↗

arXiv2003

Phase measurements in Aharonov-Bohm interferometers

In this paper we address measurements of the resonant quantum transmission amplitude $t_{QD}=-i|t_{QD}|e^{iα_{QD}}$ through a quantum dot (QD), as function of the plunger gate voltage $V$. Mesoscopic solid state Aharonov-Bohm interferometers (ABI) have been used to measure the "intrinsic" phase, $α_{QD}$, when the QD is placed on one of the paths. In a "closed" interferometer, connected to two terminals, the electron current is conserved, and Onsager's relations require that the conductance ${\cal G}$ through the ABI is an even function of the magnetic flux $Φ=\hbar cϕ/e$ threading the ABI ring. Therefore, if one fits ${\cal G}$ to $A+B\cos(ϕ+β)$ then $β$ only "jumps" between 0 and $π$, with no relation to $α_{QD}$. Additional terminals open the ABI, break the Onsager relations and yield a non-trivial variation of $β$ with $V$. After reviewing these topics, we use theoretical models to derive three results on this problem: (i) For the one-dimensional leads, the relation $|t_{QD}|^2 \propto \sin^2(α_{QD})$ allows a direct measurement of $α_{QD}$. (ii) In many cases, the measured ${\cal G}$ in the closed ABI can be used to extract {\it both} $|t_{QD}|$ and $α_{QD}$. (iii) For open ABI's, $β$ depends on the details of the opening. We present quantitative criteria (which can be tested experimentally) for $β$ to be equal to the desired $α_{QD}$: the "lossy" channels near the QD should have both a small transmission and a small reflection.

cond-mat.mes-hall↗

arXiv2003

Resonance approximation and charge loading/unloading in adiabatic quantum pumping

Quantum pumping through mesoscopic quantum dots is known to be enhanced by resonant transmission. The pumped charge is close to an integer number of electrons when the pumping contour surrounds a resonance, but the transmission remains small on the contour. For non-interacting electrons, we give a quantitative account of the detailed exchange of electrons between the dot and the leads (to the electron reservoirs) during a pumping cycle. Near isolated distinct resonances, we use approximate Breit-Wigner expressions for the dot's Green function to discuss the loading/unloading picture of the pumping: the fractional charge exchanged between the dot and each lead through a single resonance point is related to the relative couplings of the dot and the leads at this resonance. If each resonance point along the pumping contour is dominated by the coupling to a single lead (which also implies a very small transmission), then the crossing of each such resonance results in a single electron exchange between the dot and that lead, ending up with a net quantized charge. When the resonance approximation is valid, the fractional charges can also be extracted from the peaks of the transmissions between the various leads.

cond-mat.mes-hall↗

arXiv2003

Unusual Symmetries in the Kugel-Khomskii Hamiltonian

The Kugel-Khomskii Hamiltonian for cubic titanates describes spin and orbital superexchange interactions between $d^1$ ions having three-fold degenerate $t_{2g}$ orbitals. Since orbitals do not couple along "inactive" axes, perpendicular to the orbital planes, the total number of electrons in $|α>$ orbitals in any such plane and the corresponding total spin are both conserved. A Mermin-Wagner construction shows that there is no long-range spin ordering at nonzero temperatures. Inclusion of spin-orbit coupling allows such ordering, but even then the excitation spectrum is gapless due to a continuous symmetry. Thus, the observed order and gap require more symmetry breaking terms.

cond-mat.str-el↗

arXiv2003

Measuring the transmission phase of a quantum dot in a closed interferometer

The electron transmission through a {\it closed} Aharonov-Bohm mesoscopic solid-state interferometer, with a quantum dot (QD) on one of the paths, is calculated exactly for a simple model. Although the conductance is an even function of the magnetic flux (due to Onsager's relations), in many cases one can use the measured conductance to extract both the amplitude and the phase of the "intrinsic" transmission amplitude $t_D=-i|t_D|e^{iα_D}$ through the "bare" QD. We also propose to compare this indirect measurement with the (hitherto untested) direct relation $\sin^2(α_D) \equiv |t_D|^2/\max(|t_D|^2)$.

cond-mat.mes-hall↗

arXiv2002

Measuring the transmission of a quantum dot using Aharonov-Bohm Interferometers

The conductance G through a closed Aharonov-Bohm mesoscopic solid-state interferometer (which conserves the electron current), with a quantum dot (QD) on one of the paths, depends only on cos(phi), where Phi= (hbar c phi)/e is the magnetic flux through the ring. The absence of a phase shift in the phi-dependence led to the conclusion that closed interferometers do not yield the phase of the "intrinsic" transmission amplitude t_D=|t_D|e^{i alpha} through the QD, and led to studies of open interferometers. Here we show that (a) for single channel leads, alpha can be deduced from |t_D|, with no need for interferometry; (b) the explicit dependence of G(phi) on cos(phi) (in the closed case) allows a determination of both |t_D| and alpha; (c) in the open case, results depend on the details of the opening, but optimization of these details can yield the two-slit conditions which relate the measured phase shift to alpha.

cond-mat.mes-hall↗

arXiv1997

Surface acoustic wave attenuation by a two-dimensional electron gas in a strong magnetic field

The propagation of a surface acoustic wave (SAW) on GaAs/AlGaAs heterostructures is studied in the case where the two-dimensional electron gas (2DEG) is subject to a strong magnetic field and a smooth random potential with correlation length Lambda and amplitude Delta. The electron wave functions are described in a quasiclassical picture using results of percolation theory for two-dimensional systems. In accordance with the experimental situation, Lambda is assumed to be much smaller than the sound wavelength 2*pi/q. This restricts the absorption of surface phonons at a filling factor \barν approx 1/2 to electrons occupying extended trajectories of fractal structure. Both piezoelectric and deformation potential interactions of surface acoustic phonons with electrons are considered and the corresponding interaction vertices are derived. These vertices are found to differ from those valid for three-dimensional bulk phonon systems with respect to the phonon wave vector dependence. We derive the appropriate dielectric function varepsilon(omega,q) to describe the effect of screening on the electron-phonon coupling. In the low temperature, high frequency regime T << Delta (omega_q*Lambda /v_D)^{alpha/2/nu}, where omega_q is the SAW frequency and v_D is the electron drift velocity, both the attenuation coefficient Gamma and varepsilon(omega,q) are independent of temperature. The classical percolation indices give alpha/2/nu=3/7. The width of the region where a strong absorption of the SAW occurs is found to be given by the scaling law |Delta \barν| approx (omega_q*Lambda/v_D)^{alpha/2/nu}. The dependence of the electron-phonon coupling and the screening due to the 2DEG on the filling factor leads to a double-peak structure for Gamma(\barν).

cond-mat.mes-hall↗

arXiv1997

Absorption of surface acoustic-waves by quantum dots: discrete spectrum limit

The absorption of surface acoustic waves (SAW's) by an array of quantum dots in which the mean level spacing Delta is larger than the sound frequency omega, the temperature T, and the phase breaking rate tau^{-1}_phi is considered. The direct and the intra-level (Debye) contributions to the SAW attenuation coefficient Gamma are evaluated, and it is shown that the sensitivity to weak magnetic fields and spin-orbit scattering (``weak localization effects'') is dramatically enhanced as compared to the case of a continuous spectrum, Delta < tau^{-1}_phi. It is argued that the non-invasive measurement of Gamma represents a new tool for the investigation of the temperature dependence of the energy relaxation rate, tau^{-1}_epsilon, and the phase breaking rate, tau^{-1}_phi$, of isolated electronic systems.

cond-mat.mes-hall↗

arXiv1996

Elastic scattering and absorption of surface acoustic waves by a quantum dot

We study theoretically the piezoelectric interaction of a surface acoustic wave (SAW) with a two-dimensional electron gas confined to an isolated quantum dot. The electron motion in the dot is diffusive. The electron-electron interaction is accounted for by solving the screening problem in real space. Since the screening in GaAs/AlGaAs heterostructures is strong, an approximate inversion of the dielectric function epsilon(r,r') can be utilized, providing a comprehensive qualitative picture of the screened SAW potential and the charge redistribution in the dot. We calculate the absorption and the scattering cross-sections for SAW's as a function of the area of the dot, A, the sound wave vector, q, and the diffusion coefficient D of the electrons. Approximate analytical expressions for the cross-sections are derived for all cases where the quantities A*q^2 and A*omega/D are much larger or smaller than unity; omega is the SAW frequency. Numerical results which include the intermediate regimes and show the sample-specific dependence of the cross-sections on the angles of incidence and scattering of surface phonons are discussed. The weak localization corrections to the cross-sections are found and discussed as a function of a weak magnetic field, the frequency, and the temperature. Due to the absence of current-carrying contacts, the phase coherence of the electron motion, and in turn the quantum corrections, increase as the size of the dot shrinks. This shows that scattering and absorption of sound as noninvasive probes may be advantageous in comparison to transport experiments for the investigation of very small electronic systems.

cond-mat↗