825 research outputs found
Transmission Phase Through Two Quantum Dots Embedded in a Four-Terminal Quantum Ring
We use the Aharonov-Bohm effect in a four-terminal ring based on a Ga[Al]As
heterostructure for the measurement of the relative transmission phase. In each
of the two interfering paths we induce a quantum dot. The number of electrons
in the two dots can be controlled independently. The transmission phase is
measured as electrons are added to or taken away from the individual quantum
dots.Comment: 3 pages, 4 figure
Magnetic ordering in a doped frustrated spin-Peierls system
Based on a model of a quasi-one dimensional spin-Peierls system doped with
non-magnetic impurities, an effective two-dimensional Hamiltonian of randomly
distributed S=1/2 spins interacting via long-range pair-wise interaction is
studied using a stochastic series expansion quantum Monte Carlo method. The
susceptibility shows Curie-like behavior at the lowest temperatures reached
although the staggered magnetisation is found to be finite for . The
doping dependance of the corresponding three-dimensional Neel temperature is
also computed.Comment: Published version, 4 pages, 5 figure
Spin, charge and orbital fluctuations in a multi-orbital Mott insulator
The two-orbital degenerate Hubbard model with distinct hopping integrals is
studied by combining dynamical mean-field theory with quantum Monte Carlo
simulations. The role of orbital fluctuations for the nature of the Mott
transition is elucidated by examining the temperature dependence of spin,
charge and orbital susceptibilities as well as the one-particle spectral
function. We also consider the effect of the hybridization between the two
orbitals, which is important particularly close to the Mott transition points.
The introduction of the hybridization induces orbital fluctuations, resulting
in the formation of a Kondo-like heavy-fermion behavior, similarly to
electron systems, but involving electrons in bands of comparable width.Comment: 8 pages, 9 figure
Magnetic field dependent transmission phase of a double dot system in a quantum ring
The Aharonov-Bohm effect is measured in a four-terminal open ring geometry
based on a Ga[Al]As heterostructure. Two quantum dots are embedded in the
structure, one in each of the two interfering paths. The number of electrons in
the two dots can be controlled independently. The transmission phase is
measured as electrons are added to or taken away from the individual quantum
dots. Although the measured phase shifts are in qualitative agreement with
theoretical predictions, the phase evolution exhibits unexpected dependence on
the magnetic field. For example, phase lapses are found only in certain ranges
of magnetic field.Comment: 5 pages, 4 figure
Electronic properties of antidot lattices fabricated by atomic force lithography
Antidot lattices were fabricated by atomic force lithography using local
oxidation. High quality finite 20 x20 lattices are demonstrated with periods of
300 nm. The low temperature magnetoresistance shows well developed
commensurability oscillations as well as a quenching of the Hall effect around
zero magnetic field. In addition, we find B periodic oscillations superimposed
on the classical commensurability peaks at temperatures as high as 1.7 K. These
observations indicate the high electronic quality of our samples.Comment: Appl. Phys. Lett., in prin
Phenomenological theory of the 3 Kelvin phase in Sr2RuO4
We model the 3K-phase of Sr2RuO4 with Ru-metal inclusion as interface state
with locally enhanced transition temperatures. The resulting 3K-phase must have
a different pairing symmetry than the bulk phase of Sr2RuO4, because the
symmetry at the interface is lower than in the bulk. It is invariant under time
reversal and a second transition, in general, above the onset of bulk
superconductivity is expected where time reversal symmetry is broken. The
nucleation of the 3K-phase exhibits a ``capillary effect'' which can lead to
frustration phenomena for the superconducting states on different
Ru-inclusions. Furthermore, the phase structure of the pair wave function gives
rise to zero-energy quasiparticle states which would be visible in
quasiparticle tunneling spectra. Additional characteristic properties are
associated with the upper critical field Hc2. The 3K-phase has a weaker
anisotropy of Hc2 between the inplane and z-axis orientation than the bulk
superconducting phase. This is connected with the more isotropic nature
Ru-metal which yields a stronger orbital depairing effect for the inplane
magnetic field than in the strongly layered Sr$_2RuO4. An anomalous temperature
dependence for the z-axis critical field is found due to the coupling of the
magnetic field to the order parameter texture at the interface. Various other
experiments are discussed and new measurements are suggested.Comment: 10 pages, 5 figure
Order by disorder from non-magnetic impurities in a two-dimensional quantum spin liquid
We consider doping of non-magnetic impurities in the spin-1/2, 1/5-depleted
square lattice. This structure, whose undoped phase diagram offers both
magnetically ordered and spin-liquid ground states, is realized physically in
CaV_4O_9. Doping into the ordered phase results in a progressive loss of order,
which becomes complete at the percolation threshold. By contrast, non-magnetic
impurities introduced in the spin liquids create a phase of weak but
long-ranged antiferromagnetic order coexisting with the gapped state. The
latter may be viewed as a true order-by-disorder phenomenon. We study the phase
diagram of the doped system by computing the static susceptibility and
staggered magnetization using a stochastic series-expansion quantum Monte Carlo
technique.Comment: 4 pages, 5 figure
Spin-ladders with spin gaps: A description of a class of cuprates
We investigate the magnetic properties of the Cu-O planes in stoichiometric
SrCuO (n=3,5,7,...) which consist of CuO double chains
periodically intergrown within the CuO planes. The double chains break up
the two-dimensional antiferromagnetic planes into Heisenberg spin ladders with
rungs and legs and described by
the usual antiferromagnetic coupling J inside each ladder and a weak and
frustrated interladder coupling J. The resulting lattice is a new
two-dimensional trellis lattice. We first examine the spin excitation spectra
of isolated quasi one dimensional Heisenberg ladders which exhibit a gapless
spectra when is even and is odd ( corresponding to n=5,9,...) and a
gapped spectra when is odd and is even (corresponding to
n=3,7,...). We use the bond operator representation of quantum
spins in a mean field treatment with self-energy corrections and obtain a spin
gap of for the simplest single rung ladder (n=3), in
agreement with numerical estimates.Comment: 21 pages, 5 figures upon request, REVTEX, ETH-TH/93-3
Magnon Dispersion in the Field-Induced Magnetically Ordered Phase of TlCuCl3
The magnetic properties of the interacting dimer system TlCuCl3 are
investigated within a bond-operator formulation. The observed field-induced
staggered magnetic order perpendicular to the field is described as a Bose
condensation of magnons which are linear combinations of dimer singlet and
triplet modes. This technique accounts for the magnetization curve and for the
field dependence of the magnon dispersion curves observed by high-field neutron
scattering measurements.Comment: 4 pages, 4 figures, REVTeX
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