arXiv subjects

Uzi Landman

Publications and source records attributed to Uzi Landman.

At least 73 records · Page 4Linked to original sources

arXiv2005

Electron localization and entanglement in a two-electron quantum dot

Calculations for two electrons in an elliptic quantum dot, using symmetry breaking at the unrestricted Hartree-Fock level and subsequent restoration of the broken parity via projection techniques, show that the electrons can localize and form a molecular dimer, described by a Heitler-London-type wave function. The calculated singlet-triplet splitting (J) as a function of the magnetic field (B) agrees with cotunneling measurements. Knowledge of the dot shape and of J(B) allows determination of the degree of entanglement in the ground state of the dot, which is of interest for the implementation of quantum logic gates. The theoretical value agrees with the experimental estimates.

cond-mat.mes-hall↗

arXiv2004

Structural properties of electrons in quantum dots in high magnetic fields: Crystalline character of cusp states and excitation spectra

The crystalline or liquid character of the downward cusp states in N-electron parabolic quantum dots (QD's) at high magnetic fields is investigated using conditional probability distributions obtained from exact diagonalization. These states are of crystalline character for fractional fillings covering both low and high values, unlike the liquid Jastrow-Laughlin wave functions, but in remarkable agreement with the rotating-Wigner-molecule ones [Phys. Rev. B 66, 115315 (2002)]. The crystalline arrangement consists of concentric polygonal rings that rotate independently of each other, with the electrons on each ring rotating coherently. We show that the rotation stabilizes the Wigner molecule relative to the static one defined by the broken-symmetry unrestricted-Hartree-Fock solution. We discuss the non-rigid behavior of the rotating Wigner molecule and pertinent features of the excitation spectrum, including the occurrence of a gap between the ground and first excited states that underlies the incompressibility of the system. This leads us to conjecture that the rotating crystal (and not the static one) remains the relevant ground state for low fractional fillings even at the thermodynamic limit.

cond-mat.mes-hall↗

arXiv2004

Crystalline boson phases in harmonic traps: Beyond the Gross-Pitaevskii mean field

Strongly repelling bosons in two-dimensional harmonic traps are described through breaking of rotational symmetry at the Hartree-Fock level and subsequent symmetry restoration via projection techniques, thus incorporating correlations beyond the Gross-Pitaevskii (GP) solution. The bosons localize and form polygonal-ring-like crystalline patterns, both for a repulsive contact potential and a Coulomb interaction, as revealed via conditional-probability-distribution analysis. For neutral bosons, the total energy of the crystalline phase saturates in contrast to the GP solution, and its spatial extent becomes smaller than that of the GP condensate. For charged bosons, the total energy and dimensions approach the values of classical point-like charges in their equilibrium configuration.

cond-mat.mes-hall↗

arXiv2004

Theoretical Considerations About the Generation and Properties of Narrow Electron Flows in Solid State Structures

A method for the evaluation of the angular width of an electron beam generated by a nanoconstriction is proposed and demonstrated. The approach is based on analysis of a narrow-width electron flow, that quantizes into modes inside a confining constriction which is described in the adiabatic approximation, evolving into a freely propagating electronic state after exiting the constriction. The method that we developed allows us to find the parameters and the shape of the constriction that are optimal for generation of extremely narrow electron beams. In the case of a constriction characterized by a linear widening shape an asymptotically exact solution for the injection problem is found. That solution verifies semi-quantitative results related to the angular characteristics of the beam, and it opens the way for determination of the distribution function of the electrons in the beam. We have found the relationship between the angular distribution of the electron density in the beam and the quantum states of the electrons inside the constriction. Such narrow electron beams may be employed in investigations of electronic systems and in data manipulations in electronic and spintronic devices.

cond-mat.mes-hall↗

arXiv2004

Phase-Controlled Force and Magnetization Oscillations in Superconducting Ballistic Nanowires

The emergence of superconductivity-induced phase-controlled forces in the (0.01-0.1) nN range, and of magnetization oscillations, in nanowire junctions, is discussed. A giant magnetic response to applied weak magnetic fields, is predicted in the ballistic Josephson junction formed by a superconducting tip and a surface, bridged by a normal metal nanowire where Andreev states form.

cond-mat.supr-con↗

arXiv2003

Unified description of floppy and rigid rotating Wigner molecules formed in quantum dots

Restoration of broken circular symmetry is used to explore the characteristics of the ground states and the excitation spectra of rotating Wigner molecules (RWM's) formed in two-dimensional parabolic N-electron quantum dots. In high magnetic fields, the RWM's are floppy rotors with the energies of the magic angular momentum (L) states obeying aL + b/L^{1/2}. Under such fields the ground-state energies (referenced to the kinetic energy in the lowest Landau level) approach the electrostatic energy of N point charges in the classical equilibrium molecular configuration. At zero field and strong interelectron repulsion, the RWM's behave like quasiclassical rigid rotors whose energies vary as L^2. The particular L-dependence in high B is inherent and natural to a floppy rotating WM, and it can be used as a crucial diagnostic tool for resolving the recently posed question whether the composite-fermion or the RWM picture is appropriate for QD's.

cond-mat.mes-hall↗

arXiv2003

A Magnetic-Field-Effect Transistor and Spin Transport

A magnetic-field-effect transistor is proposed that generates a spin-polarized current and exhibits a giant negative magnetoresitance. The device consists of a nonmagnetic conducting channel (wire or strip) wrapped, or sandwiched, by a grounded magnetic shell. The process underlying the operation of the device is the withdrawal of one of the spin components from the channel, and its dissipation through the grounded boundaries of the magnetic shell, resulting in a spin-polarized current in the nonmagnetic channel. The device may generate an almost fully spin-polarized current, and a giant negative magnetoresistance effect is predicted.

cond-mat.mes-hall↗

arXiv2003

Relaxation of high-energy quasiparticle distributions: electron-electron scattering in a two-dimensional electron gas

A theory is developed for the evolution of the non-equilibrium distribution of quasiparticles when the scattering rate decreases due to particle collisions. We propose a "modified one-collision approximation" which is most effective for high-energy quasiparticle distributions. This method is used to explain novel measurements of the non-monotonic energy dependence of the signal of scattered electrons in a 2D system. The observed effect is related to a crossover from the ballistic to the hydrodynamic regime of electron flow.

cond-mat.dis-nn↗

arXiv2003

Quantum dots in high magnetic fields: Rotating-Wigner-molecule versus composite-fermion approach

Exact diagonalization results are reported for the lowest rotational band of N=6 electrons in strong magnetic fields in the range of high angular momenta 70 <= L <= 140 (covering the corresponding range of fractional filling factors 1/5 >= nu >= 1/9). A detailed comparison of energetic, spectral, and transport properties (specifically, magic angular momenta, radial electron densities, occupation number distributions, overlaps and total energies, and exponents of current-voltage power law) shows that the recently discovered rotating-electron-molecule wave functions [Phys. Rev. B 66, 115315 (2002)] provide a superior description compared to the composite-fermion/Jastrow-Laughlin ones.

cond-mat.mes-hall↗

arXiv2003

Group theoretical analysis of symmetry breaking in two-dimensional quantum dots

We present a group theoretical study of the symmetry-broken unrestricted Hartree-Fock orbitals and electron densities in the case of a two-dimensional N-electron single quantum dot (with and without an external magnetic field). The breaking of rotational symmetry results in canonical orbitals that (1) are associated with the eigenvectors of a Hueckel hamiltonian having sites at the positions determined by the equilibrium molecular configuration of the classical N-electron problem, and (2) transform according to the irreducible representations of the point group specified by the discrete symmetries of this classical molecular configuration. Through restoration of the total-spin and rotational symmetries via projection techniques, we show that the point-group discrete symmetry of the unrestricted Hartree-Fock wave function underlies the appearance of magic angular momenta (familiar from exact-diagonalization studies) in the excitation spectra of the quantum dot. Furthermore, this two-step symmetry-breaking/symmetry-restoration method accurately describes the energy spectra associated with the magic angular momenta.

cond-mat.mes-hall↗

arXiv2003

Spin-Guide: A New Source of High Spin-Polarized Current

We propose a ``spin-guide'' source for generation of electric currents with a high degree of spin polarization, allowing long-distance transmission of the spin-polarization. In the spin-guide scheme proposed here, a non-magnetic conducting channel is wrapped by a magnetic shell which preferentially transmits electrons with a particular spin polarization. It is shown that this method is significantly more effective then the spin-filter-like scheme where the current flows perpendicular to the interface between a ferromagnetic metal to a non-magnetic conducting material. Under certain conditions a spin-guide may generate an almost perfectly spin-polarized current, even when the magnetic material used is not fully polarized. The spin-guide is predicted to allow the transport of spin polarization over long distances which may exceed significantly the spin-flip length in the channel. In addition, it readily permits detection and control of the spin-polarization of the current. The spin-guide may be employed for spin-flow manipulations in semiconductors used in spintronic devices.

cond-mat.mes-hall↗

arXiv2002

Thermal quenching of electronic shells and channel competition in cluster fission

Experimental and theoretical studies of fission of doubly-charged Li, Na, and K clusters in the low fissility regime reveal the strong influence of electronic shell effects on the fission products. The electronic entropy controls the quenching of the shell effects and the competition between magic-fragment channels, leading to a transition from favored channels of higher mass symmetry to the asymmetric channel involving the trimer cation at elevated temperatures.

cond-mat.mtrl-sci↗

arXiv2002

Trial wave functions with long-range Coulomb correlations for two-dimensional N-electron systems in high magnetic fields

A new class of analytic wave functions is derived for two dimensional N-electron (2 <= N < infinity) systems in high magnetic fields. These functions are constructed through breaking (at the Hartree-Fock level) and subsequent restoration (via post-Hartree-Fock methods) of the circular symmetry. They are suitable for describing long-range Coulomb correlations, while the Laughlin and composite-fermion functions describe Jastrow correlations associated with a short-range repulsion. Underlying our approach is a collectively-rotating-electron-molecule picture, yielding for all N an oscillatory radial electron density.

cond-mat.mes-hall↗

arXiv2002

Strongly correlated wave functions for artificial atoms and molecules

A method for constructing semianalytical strongly correlated wave functions for single and molecular quantum dots is presented. It employs a two-step approach of symmetry breaking at the Hartree-Fock level and of subsequent restoration of total spin and angular momentum symmetries via Projection Techniques. Illustrative applications are presented for the case of a two-electron helium-like single quantum dot and a hydrogen-like quantum dot molecule.

cond-mat.mes-hall↗

arXiv2002

Two-dimensional few electron systems in high magnetic fields: Composite-fermion or rotating-electron-molecule approach?

A new class of analytic and parameter-free, strongly correlated wave functions of simple functional form is derived for few electrons in two-dimensional quantum dots under high magnetic fields. These wave functions are constructed through breaking and subsequent restoration of the circular symmetry, and they offer a natural alternative to the Laughlin and composite-fermion functions. Underlying our approach is a collectively-rotating-electron-molecule picture. The angular momenta allowed by molecular symmetry correspond to the filling-factors' hierarchy of the fractional quantum Hall effect.

cond-mat.mes-hall↗

arXiv2001

Magnetic-field Manipulation of Chemical Bonding in Artificial Molecules

The effect of orbital magnetism on the chemical bonding of lateral, two-dimensional artificial molecules is studied in the case of a 2e double quantum dot (artificial molecular hydrogen). It is found that a perpendicular magnetic field reduces the coupling (tunneling) between the individual dots and, for sufficiently high values, it leads to complete dissociation of the artificial molecule. The method used is building on Lowdin's work on Projection Operators in Quantum Chemistry; it is a spin-and-space unrestricted Hartree-Fock method in conjunction with the companion step of the restoration of spin and space symmetries via Projection Techniques (when such symmetries are broken). This method is able to describe the full range of couplings in two-dimensional double quantum dots, from the strong-coupling regime exhibiting delocalized molecular orbitals to the weak-coupling and dissociation regimes associated with a Generalized Valence Bond combination of atomic-type orbitals localized on the individual dots.

cond-mat.mes-hall↗

arXiv2001

Coupling and Dissociation in Artificial Molecules

We show that the spin-and-space unrestricted Hartree-Fock method, in conjunction with the companion step of the restoration of spin and space symmetries via Projection Techniques (when such symmetries are broken), is able to describe the full range of couplings in two-dimensional double quantum dots, from the strong-coupling regime exhibiting delocalized molecular orbitals to the weak-coupling and dissociation regimes associated with a Generalized Valence Bond combination of atomic-type orbitals localized on the individual dots. The weak-coupling regime is always accompanied by an antiferromagnetic ordering of the spins of the individual dots. The cases of dihydrogen (H$_2$, $2e$) and dilithium (Li$_2$, $6e$) quantum dot molecules are discussed in detail.

cond-mat.mes-hall↗

arXiv2001

Photoabsorption Spectra of Na_n+ clusters: Thermal Line-Broadening Mechanisms

Photoabsorption cross sections of small sodium cluster cations (Na$_n^+$, n=3,5,7 and 9) were calculated at various temperatures with the time-dependent local-density-approximation (TDLDA) in conjunction with ab initio molecular dynamics simulations, yielding spectra that agree with measured ones without ad-hoc line broadening or renormalization. Three thermal line-broadening mechanisms are revealed: (I) lifting of level degeneracies caused by symmetry-breaking ionic motions, (II) oscillatory shifts of the entire spectrum caused by breathing vibrations, and (III) cluster structural isomerizations.

physics.atm-clus↗