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A. Aharony

Publications and source records attributed to A. Aharony.

89 records · Page 5Linked to original sources
arXiv2003

Fractal geometry of critical Potts clusters

Numerical simulations on the total mass, the numbers of bonds on the hull, external perimeter, singly connected bonds and gates into large fjords of the Fortuin-Kasteleyn clusters for two-dimensional q-state Potts models at criticality are presented. The data are found consistent with the recently derived corrections-to-scaling theory. However, the approach to the asymptotic region is slow, and the present range of the data does not allow a unique identification of the exact correction exponents

cond-mat.dis-nn↗
arXiv2003

Damped orbital excitations in the titanates

A possible mechanism for the removal of the orbital degeneracy in RTiO3 (where R=La, Y, ...) is considered. The calculation is based on the Kugel-Khomskii Hamiltonian for electrons residing in the t2g orbitals of the Ti ions, and uses a self-consistent pe rturbation expansion in the interaction between the orbital and the spin degrees of freedom. The latter are assumed to be ordered in a Neel state, brought about by delicate interactions that are not included in the Kugel-Khomskii Hamiltonian. Within our model calculations, each of the t2g bands is found to acquire a finite, temperature-dependent dispersion, that lifts the orbital degeneracy. The orbital excitations are found to be heavily damped over a rather wide band. Consequently, they do not participate as a separate branch of excitations in the low-temperature thermodynamics.e

cond-mat.str-el↗
arXiv2002

Quantized adiabatic quantum pumping due to interference

Recent theoretical calculations, demonstrating that quantized charge transfer due to adiabatically modulated potentials in mesoscopic devices can result purely from the interference of the electron wave functions (without invoking electron-electron interactions) are reviewed: (1) A new formula is derived for the pumped charge Q (per period); It reproduces the Brouwer formula without a bias, and also yields the effect of the modulating potential on the Landauer formula in the presence of a bias. (2) For a turnstile geometry, with time-dependent gate voltages V_L(t) and V_R(t), the magnitude and sign of Q are determined by the relative position and orientation of the closed contour traversed by the system in the {V_L-V_R} plane, relative to the transmission resonances in that plane. Integer values of Q (in units of e) are achieved when a transmission peak falls inside the contour, and are given by the winding number of the contour. (3) When the modulating potential is due to surface acoustic waves, Q exhibits a staircase structure, with integer values, reminiscent of experimental observations.

cond-mat.mes-hall↗
arXiv2002

Fractal dimensions and corrections to scaling for critical Potts clusters

Renormalization group and Coulomb gas mappings are used to derive theoretical predictions for the corrections to the exactly known asymptotic fractal masses of the hull, external perimeter, singly connected bonds and total mass of the Fortuin-Kasteleyn clusters for two-dimensional $q$-state Potts models at criticality. For q=4 these include exact logarithmic (as well as loglog) corrections.

cond-mat.stat-mech↗
arXiv2002

Which phase is measured in the mesoscopic Aharonov-Bohm interferometer?

Mesoscopic solid state Aharonov-Bohm interferometers have been used to measure the "intrinsic" phase, $α_{QD}$, of the resonant quantum transmission amplitude through a quantum dot (QD). For a two-terminal "closed" interferometer, which conserves the electron current, Onsager's relations require that the measured phase shift $β$ only "jumps" between 0 and $π$. Additional terminals open the interferometer but then $β$ depends on the details of the opening. Using a theoretical model, 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↗
arXiv2002

Broken unitarity and phase measurements in Aharonov-Bohm interferometers

Aharonov-Bohm mesoscopic solid-state interferometers yield a conductance which contains a term $\cos(ϕ+β)$, where $ϕ$ relates to the magnetic flux. Experiments with a quantum dot on one of the interfering paths aim to relate $β$ to the dot's intrinsic Friedel transmission phase, $α_1$. For closed systems, which conserve the electron current (unitarity), the Onsager relation requires that $β=0$. For open systems, we show that $β$ depends in general on the details of the broken unitarity. Although it gives information on the resonances of the dot, $β$ is generally not equal to $α_1$. A direct relation between $β$ and $α_1$ requires specific ways of opening the system, which are discussed.

cond-mat.mes-hall↗
arXiv2002

Adiabatic transport in nanostructures

A confined system of non-interacting electrons, subject to the combined effect of a time-dependent potential and different external chemical-potentials, is considered. The current flowing through such a system is obtained for arbitrary strengths of the modulating potential, using the adiabatic approximation in an iterative manner. A new formula is derived for the charge pumped through an un-biased system (all external chemical potentials are kept at the same value); It reproduces the Brouwer formula for a two-terminal nanostructure. The formalism presented yields the effect of the chemical potential bias on the pumped charge on one hand, and the modification of the Landauer formula (which gives the current in response to a constant chemical-potential difference) brought about by the modulating potential on the other. Corrections to the adiabatic approximation are derived and discussed.

cond-mat.mes-hall↗
arXiv2001

The Fano Effect in Aharonov-Bohm interferometers

After briefly reviewing the Fano effect, we explain why it may be relevant to various types of Aharonov-Bohm interferometers. We discuss both closed (electron conserving) and open interferometers, in which one path contains either a simple quantum dot or a decorated quantum dot (with more than one internal state or a parallel path). The possible relevance to some hitherto unexplained experimental features is also discussed.

cond-mat.mes-hall↗
arXiv2001

Quantum dots with two electrons: Singlet-triplet transitions

The magnetic character of the ground-state of two electrons on a double quantum dot, connected in series to left and right single-channel leads, is considered. By solving exactly for the spectrum of the two interacting electrons, it is found that the coupling to the continuum of propagating states on the leads, in conjunction with the electron-electron interactions, may result in a delocalization of the bound state of the two electrons. This, in turn, reduces significantly the range of the Coulomb interaction parameters over which singlet-triplet transitions can be realized. It is also found that the coupling to the leads favors the singlet ground-state.

cond-mat.mes-hall↗
arXiv2001

Dipolar ordering in Fe8?

We show that the low-temperature physics of molecular nanomagnets, contrary to the prevailing one-molecule picture, must be determined by the long-range magnetic ordering due to many-body dipolar interactions. The calculations here performed, using Ewald's summation, suggest a ferromagnetic ground state with a Curie temperature of about 130 mK. The energy of this state is quite close to those of an antiferromagnetic state and to a glass of frozen spin chains. The latter may be realized at finite temperature due to its high entropy.

cond-mat.mes-hall↗
arXiv2000

Neutron Scattering study of Sr_2Cu_3O_4Cl_2

We report a neutron scattering study on the tetragonal compound Sr_2Cu_3O_4Cl_2, which has two-dimensional (2D) interpenetrating Cu_I and Cu_{II} subsystems, each forming a S=1/2 square lattice quantum Heisenberg antiferromagnet (SLQHA). The mean-field ground state is degenerate, since the inter-subsystem interactions are geometrically frustrated. Magnetic neutron scattering experiments show that quantum fluctuations lift the degeneracy and cause a 2D Ising ordering of the Cu_{II} subsystem. Due to quantum fluctuations a dramatic increase of the Cu_I out-of-plane spin-wave gap is also observed. The temperature dependence and the dispersion of the spin-wave energy are quantitatively explained by spin-wave calculations which include quantum fluctuations explicitly. The values for the nearest-neighbor superexchange interactions between the Cu_I and Cu_{II} ions and between the Cu_{II} ions are determined experimentally to be J_{I-II} = -10(2)meV and J_{II}= 10.5(5)meV, respectively. Due to its small exchange interaction, J_{II}, the 2D dispersion of the Cu_{II} SLQHA can be measured over the whole Brillouin zone with thermal neutrons, and a novel dispersion at the zone boundary, predicted by theory, is confirmed. The instantaneous magnetic correlation length of the Cu_{II} SLQHA is obtained up to a very high temperature, T/J_{II}\approx 0.75. This result is compared with several theoretical predictions as well as recent experiments on the S=1/2 SLQHA.

cond-mat.str-el↗
arXiv2000

Quantum Fluctuations in the Frustrated Antiferromagnet Sr_2Cu_3O_4Cl_2

Sr_2Cu_3O_4Cl_2 is an antiferromagnet consisting of weakly coupled CuO planes which comprise two weakly interacting antiferromagnetic subsystems, I and II, which order at respective temperatures T_I \approx 390K and T_{II} \approx 40K. Except asymptotically near the ordering temperature, these systems are good representations of the two-dimensional quantum spin 1/2 Heisenberg model. For T< T_{II} there are four low-energy modes at zero wave vector, three of whose energies are dominated by quantum fluctuations. For T_{II} < T < T_I there are two low energy modes. The mode with lower energy is dominated by quantum fluctuations. Our calculations of the energies of these modes (including dispersion for wave vectors perpendicular to the CuO planes) agree extremely well with the experimental results of inelastic neutron scattering (in the accompanying paper) and for modes in the sub meV range observed by electron spin resonance. The parameters needed to describe quantum fluctuations are either calculated here or are taken from the literature. These results show that we have a reasonable qualitative understanding of the band structure of the lamellar cuprates needed to calculate the anisotropic exchange constants used here.

cond-mat.str-el↗
arXiv2000

Charge occupancy of two interacting electrons on artificial molecules - exact results

We present exact solutions for two interacting electrons on an artificial atom and on an artificial molecule made by one and two (single level) quantum dots connected by ideal leads. Specifically, we calculate the accumulated charge on the dots as function of the gate voltage, for various strengths of the electron-electron interaction and of the hybridization between the dots and the (one-dimensional) leads. With increasing of the (negative) gate voltage, the accumulated charge in the two-electron ground state increases in gradual steps from 0 to 1 and then to 2. The value 0 represents an "insulating" state, where both electrons are bound to shallow states on the impurities. The value of 1 corresponds to a "metal", with one electron localized on the dots and the other extended on the leads. The value of 2 corresponds to another "insulator", with both electrons strongly localized. The width of the "metallic" regime diverges with strength of the electron-electron interaction for the single dot, but remains very narrow for the double dot. These results are contrasted with the simple Coulomb blockade picture.

cond-mat.mes-hall↗
arXiv1999

Transmission of two interacting electrons

The transmission of two electrons through a region where they interact is found to be enhanced by a renormalization of the repulsive interaction. For a specific example of the single-particle Hamiltonian, which includes a strongly attractive potential, the renormalized interaction becomes attractive, and the transmission has a pronounced maximum as function of the depth of the single-electron attractive potential. The results apply directly to a simple model of scattering of two interacting electrons by a quantum dot.

cond-mat.mes-hall↗
arXiv1999

Exact solution for two interacting electrons on artificial atoms and molecules in solids

We present a general scheme for finding the exact eigenstates of two electrons, with on-site repulsive potentials U_i, on I impurities in a macroscopic crystal. The model describes impurities in doped semiconductors and artificial molecules in quantum dots. For quantum dots, the energy cost for adding two electrons is bounded by the single-electron spectrum, and does not diverge when U_i approaches infinity, implying limitations on the validity of the Coulomb blockade picture. Analytic applications on a one-dimensional chain yield quantum delocalization and magnetic transitions.

cond-mat↗
arXiv1999

Anisotropic Superexchange for nearest and next nearest coppers in chain, ladder and lamellar cuprates

We present a detailed calculation of the magnetic couplings between nearest-neighbor and next-nearest-neighbor coppers in the edge-sharing geometry, ubiquitous in many cuprates. In this geometry, the interaction between nearest neighbor coppers is mediated via two oxygens, and the Cu-O-Cu angle is close to 90 degrees. The derivation is based on a perturbation expansion of a general Hubbard Hamiltonian, and produces numerical estimates for the various magnetic energies. In particular we find the dependence of the anisotropy energies on the angular deviation away from the 90 degrees geometry of the Cu-O-Cu bonds. Our results are required for the correct analysis of the magnetic structure of various chain, ladder and lamellar cuprates.

cond-mat.mtrl-sci↗