77 research outputs found
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.Comment: 5 pages, 3 figure
Ballistic electronic transport in Quantum Cables
We studied theoretically ballistic electronic transport in a proposed
mesoscopic structure - Quantum Cable. Our results demonstrated that Qauntum
Cable is a unique structure for the study of mesoscopic transport. As a
function of Fermi energy, Ballistic conductance exhibits interesting stepwise
features. Besides the steps of one or two quantum conductance units (),
conductance plateaus of more than two quantum conductance units can also be
expected due to the accidental degeneracies (crossings) of subbands. As
structure parameters is varied, conductance width displays oscillatory
properties arising from the inhomogeneous variation of energy difference
betweeen adjoining transverse subbands. In the weak coupling limits,
conductance steps of height becomes the first and second plateaus for
the Quantum Cable of two cylinder wires with the same width.Comment: 11 pages, 5 figure
Aharonov-Bohm effect and plasma oscillations in superconducting tubes and rings
Low frequency plasma oscillations in superconducting tubes are considered.
The emergence of two different dimensionality regimes of plasma oscillations in
tubes, exhibiting a crossover from one-dimensional to two-dimensional behavior,
depending on whether or , where is the plasmon wave
vector and is the radius of the tube, is discussed. The Aharonov-Bohm
effect pertaining to plasma oscillations in superconducting tubes and rings,
resulting in an oscillatory behavior of the plasmon frequency as a function of
the magnetic flux, with a flux quantum period (analog of the
Little-Parks effect), is studied. The amplitude of the oscillations is
proportional to , where is the superconducting coherence
length.Comment: 18 pages, 4 figure
Coherent quantum phenomena in mesoscopic metallic conductors (Review Article)
The quantum coherent phenomena in mesoscopic cylindrical metallic conductors have been considered. Pure double-and single-connected normal samples were placed in a longitudinal magnetic field, which generated interference phenomena depending on the magnetic flux through the cross-section of the conductor. The period of the induced oscillations is equal to the flux quantum hc/e of the normal metal. The quantum states are formed in the structures by collisions of the electrons with the dielectric boundary of the sample. The magnetic flux is included in the expression for the spectrum of quasiparticles. The proximity effect and its influence on the modification of the spectrum of quantum coherent phenomena have been investigated. The behavior of cylindrical samples consisting of a superconducting (S) metal with a deposited thin pure normal (N) metal layer has been analyzed. In this structure the electrons are localized in a well bounded by a dielectric on one side and by a superconductor on the other. The specific feature of the generated quantized Andreev levels is that in the varying field H (or temperature T) each of the levels in the well can coincide periodically with the chemical potential of the metal. As a result, the state of the system experiences strong degeneracy and the density of states exhibits resonance spikes of the energy of the NS sample. This makes a significant contribution to the magnetic moment. A theory of the reentrant effect for NS structures has been developed, which interprets the anomalous behavior of the magnetic susceptibility of such structures as a function of the magnetic field and temperatures
Persistent currents, flux quantization, and magnetomotive forces in normal metals and superconductors (Review Article)
The notion of persistent current comes back to orbital currents in normal metals, semiconductors and even insulators displaying diamagnetic behavior in weak magnetic fields, but came to focus at the discovery of current persistence and magnetic flux quantization at large fields in atomically big but macroscopically small (mesoscopic) objects. The phenomenon bears much similarity with supercurrents in superconductive metals. We will review progress in developing of our understanding of the physical and technological aspects of this phenomenon. The exact solution for currents, magnetic moments and magnetomotive forces (torques) in crossed magnetic fields are presented. Time-dependent phenomena in crossed magnetic and electric fields, and in possibility of spontaneous persistent currents and of work extraction from static and dynamic quantum states are discussed
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.Comment: 4 pages, 3 figure
The theory of the reentrant effect in susceptibility of cylindrical mesoscopic samples
A theory has been developed to explain the anomalous behavior of the magnetic
susceptibility of a normal metal-superconductor () structure in weak
magnetic fields at millikelvin temperatures. The effect was discovered
experimentally by A.C. Mota et al \cite{10}. In cylindrical superconducting
samples covered with a thin normal pure metal layer, the susceptibility
exhibited a reentrant effect: it started to increase unexpectedly when the
temperature lowered below 100 mK. The effect was observed in mesoscopic
structures when the and metals were in good electric contact. The
theory proposed is essentially based on the properties of the Andreev levels in
the normal metal. When the magnetic field (or temperature) changes, each of the
Andreev levels coincides from time to time with the chemical potential of the
metal. As a result, the state of the structure experiences strong
degeneracy, and the quasiparticle density of states exhibits resonance spikes.
This generates a large paramagnetic contribution to the susceptibility, which
adds up to the diamagnetic contribution thus leading to the reentrant effect.
The explanation proposed was obtained within the model of free electrons. The
theory provides a good description for experimental results [10]
Edge effects in graphene nanostructures: I. From multiple reflection expansion to density of states
We study the influence of different edge types on the electronic density of
states of graphene nanostructures. To this end we develop an exact expansion
for the single particle Green's function of ballistic graphene structures in
terms of multiple reflections from the system boundary, that allows for a
natural treatment of edge effects. We first apply this formalism to calculate
the average density of states of graphene billiards. While the leading term in
the corresponding Weyl expansion is proportional to the billiard area, we find
that the contribution that usually scales with the total length of the system
boundary differs significantly from what one finds in semiconductor-based,
Schr\"odinger type billiards: The latter term vanishes for armchair and
infinite mass edges and is proportional to the zigzag edge length, highlighting
the prominent role of zigzag edges in graphene. We then compute analytical
expressions for the density of states oscillations and energy levels within a
trajectory based semiclassical approach. We derive a Dirac version of
Gutzwiller's trace formula for classically chaotic graphene billiards and
further obtain semiclassical trace formulae for the density of states
oscillations in regular graphene cavities. We find that edge dependent
interference of pseudospins in graphene crucially affects the quantum spectrum.Comment: to be published in Phys. Rev.
Berry's Phases of Ground States of Interacting Spin-One Bosons: Chains of Monopoles and Monosegments
We study Berry's connection potentials of many-body ground states of spin-one
bosons with antiferromagnetic interactions in adiabatically varying magnetic
fields. We find that Berry's connection potentials are generally determined by,
instead of usual singular monopoles, linearly positioned monosegments each of
which carries one unit of topological charge; in the absence of a magnetic
field gradient this distribution of monosegments becomes a linear chain of
monopoles. Consequently, Berry's phases consist of a series of step functions
of magnetic fields; a magnetic field gradient causes rounding of these
step-functions. We also calculate Berry's connection fields, profiles of
monosegments and show that the total topological charge is conserved in a
parameter space
Quantum interference of surface states in bismuth nanowires probed by the Aharonov-Bohm oscillatory behavior of the magnetoresistance
We report the observation of a dependence of the low temperature resistance
of individual single-crystal bismuth nanowires on the Aharonov-Bohm phase of
the magnetic flux threading the wire. 55 and 75-nm wires were investigated in
magnetic fields of up to 14 T. For 55 nm nanowires, longitudinal
magnetoresistance periods of 0.8 and 1.6 T that were observed at magnetic
fields over 4 T are assigned to h/2e to h/e magnetic flux modulation. The same
modes of oscillation were observed in 75-nm wires. The observed effects are
consistent with models of the Bi surface where surface states give rise to a
significant population of charge carriers of high effective mass that form a
highly conducting tube around the nanowire. In the 55-nm nanowires, the Fermi
energy of the surface band is estimated to be 15 meV. An interpretation of the
magnetoresistance oscillations in terms of a subband structure in the surface
states band due to quantum interference in the tube is presented.Comment: 30 pages, 9 figure
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