619 research outputs found
Dissipative Quantum Ising model in a cold atomic spin-boson mixture
Using cold bosonic atoms with two (hyperfine) ground states, we introduce a
spin-boson mixture which allows to implement the quantum Ising model in a
tunable dissipative environment. The first specie lies in a deep optical
lattice with tightly confining wells and forms a spin array; spin-up/down
corresponds to occupation by one/no atom at each site. The second specie forms
a superfluid reservoir. Different species are coupled coherently via laser
transitions and collisions. Whereas the laser coupling mimics a transverse
field for the spins, the coupling to the reservoir sound modes induces a
ferromagnetic (Ising) coupling as well as dissipation. This gives rise to an
order-disorder quantum phase transition where the effect of dissipation can be
studied in a controllable manner.Comment: 4 pages, 2 figures, 1 table; Title modified and cosmetic change
Kondo effect in a one dimensional d-wave superconductor
We derive a solvable resonant-level type model, to describe an impurity spin
coupled to zero-energy bound states localized at the edge of a one dimensional
d-wave superconductor. This results in a two-channel Kondo effect with a quite
unusual low-temperature thermodynamics. For instance, the local impurity
susceptibility yields a finite maximum at zero temperature (but no
logarithmic-divergence) due to the splitting of the impurity in two Majorana
fermions. Moreover, we make comparisons with the Kondo effect occurring in a
two dimensional d-wave superconductor.Comment: 9 pages, final version; To be published in Europhysics Letter
Probing spin and orbital Kondo effects with a mesoscopic interferometer
We investigate theoretically the transport properties of a closed
Aharonov-Bohm interferometer containing two quantum dots in the strong coupling
regime. We find two distinct physical scenarios depending on the strength of
the interdot Coulomb interaction. When the interdot Coulomb interaction is
negligible only spin fluctuations are important and each dot develops a Kondo
resonance at the Fermi level independently of the applied magnetic flux. The
transport is characterized by the interference of these two independent Kondo
resonances. On the contrary, for large interdot interaction, only one electron
can be accommodated onto the double dot system. In this situation, not only the
spin can fluctuate but also the orbital degree of freedom (the pseudo-spin). As
a result, we find different ground states depending on the value of the applied
flux. When (mod ) (, where is
applied flux, and the flux quantum) the electronic transport can
take place via simultaneous correlations in the spin and pseudo-spin sectors,
leading to the highly symmetric SU(4) Kondo state. Nevertheless, we find
situations with (mod ) where the pseudo-spin quantum number is
not conserved during tunneling events, giving rise to the common SU(2) Kondo
state with an enhanced Kondo temperature. We investigate the crossover between
both ground states and discuss possible experimental signatures of this physics
as a function of the applied magnetic flux.Comment: 12 pages, 3 figures; extended discussions, improved presentatio
Electric field effects on magnetotransport properties of multiferroic Py/YMnO3/Pt heterostructures
We report on the exchange bias between antiferromagnetic and ferroelectric
hexagonal YMnO3 epitaxial thin films sandwiched between a metallic electrode
(Pt) and a soft ferromagnetic layer (Py). Anisotropic magnetoresistance
measurements are performed to monitor the presence of an exchange bias field.
When the heteroestructure is biased by an electric field, it turns out that the
exchange bias field is suppressed. We discuss the dependence of the observed
effect on the amplitude and polarity of the electric field. Particular
attention is devoted to the role of current leakage across the ferroelectric
layer.Comment: Accepted for publication in Philosophical Magazine Letters (Special
issue on multiferroics
Multifractal scaling in the Bak-Tang-Wiesenfeld Sandpile and edge events
An analysis of moments and spectra shows that, while the distribution of
avalanche areas obeys finite size scaling, that of toppling numbers is
universally characterized by a full, nonlinear multifractal spectrum. Rare,
large avalanches dissipating at the border influence the statistics very
sensibly. Only once they are excluded from the sample, the conditional toppling
distribution for given area simplifies enough to show also a well defined,
multifractal scaling. The resulting picture brings to light unsuspected, novel
physics in the model.Comment: 5 pages, 4 figure
Two-channel Kondo effect and renormalization flow with macroscopic quantum charge states
Many-body correlations and macroscopic quantum behaviors are fascinating
condensed matter problems. A powerful test-bed for the many-body concepts and
methods is the Kondo model which entails the coupling of a quantum impurity to
a continuum of states. It is central in highly correlated systems and can be
explored with tunable nanostructures. Although Kondo physics is usually
associated with the hybridization of itinerant electrons with microscopic
magnetic moments, theory predicts that it can arise whenever degenerate quantum
states are coupled to a continuum. Here we demonstrate the previously elusive
`charge' Kondo effect in a hybrid metal-semiconductor implementation of a
single-electron transistor, with a quantum pseudospin-1/2 constituted by two
degenerate macroscopic charge states of a metallic island. In contrast to other
Kondo nanostructures, each conduction channel connecting the island to an
electrode constitutes a distinct and fully tunable Kondo channel, thereby
providing an unprecedented access to the two-channel Kondo effect and a clear
path to multi-channel Kondo physics. Using a weakly coupled probe, we reveal
the renormalization flow, as temperature is reduced, of two Kondo channels
competing to screen the charge pseudospin. This provides a direct view of how
the predicted quantum phase transition develops across the symmetric quantum
critical point. Detuning the pseudospin away from degeneracy, we demonstrate,
on a fully characterized device, quantitative agreement with the predictions
for the finite-temperature crossover from quantum criticality.Comment: Letter (5 pages, 4 figures) and Methods (10 pages, 6 figures
Interaction-induced Fermi surface deformations in quasi one-dimensional electronic systems
We consider serious conceptual problems with the application of standard
perturbation theory, in its zero temperature version, to the computation of the
dressed Fermi surface for an interacting electronic system. In order to
overcome these difficulties, we set up a variational approach which is shown to
be equivalent to the renormalized perturbation theory where the dressed Fermi
surface is fixed by recursively computed counterterms. The physical picture
that emerges is that couplings that are irrelevant tend to deform the Fermi
surface in order to become more relevant (irrelevant couplings being those that
do not exist at vanishing excitation energy because of kinematical constraints
attached to the Fermi surface). These insights are incorporated in a
renormalization group approach, which allows for a simple approximate
computation of Fermi surface deformation in quasi one-dimensional electronic
conductors. We also analyze flow equations for the effective couplings and
quasiparticle weights. For systems away from half-filling, the flows show three
regimes corresponding to a Luttinger liquid at high energies, a Fermi liquid,
and a low-energy incommensurate spin-density wave. At half-filling Umklapp
processes allow for a Mott insulator regime where the dressed Fermi surface is
flat, implying a confined phase with vanishing effective transverse
single-particle coherence. The boundary between the confined and Fermi liquid
phases is found to occur for a bare transverse hopping amplitude of the order
of the Mott charge gap of a single chain.Comment: 38 pages, 39 figures. Accepted for publication in Phys. Rev.
Stochastic processes and conformal invariance
We discuss a one-dimensional model of a fluctuating interface with a dynamic
exponent . The events that occur are adsorption, which is local, and
desorption which is non-local and may take place over regions of the order of
the system size. In the thermodynamic limit, the time dependence of the system
is given by characters of the conformal field theory of percolation. This
implies in a rigorous way a connection between CFT and stochastic processes.
The finite-size scaling behavior of the average height, interface width and
other observables are obtained. The avalanches produced during desorption are
analyzed and we show that the probability distribution of the avalanche sizes
obeys finite-size scaling with new critical exponents.Comment: 4 pages, 6 figures, revtex4. v2: change of title and minor
correction
Successive opening of the Fermi surface in doped N-leg Hubbard ladders
We study the effect of doping away from half-filling in weakly (but finitely)
interacting N-leg Hubbard ladders using renormalization group and bosonization
techniques. For a small on-site repulsion U, the N-leg Hubbard ladders are
equivalent to a N-band model, where at half-filling the Fermi velocities are
v_{1}=v_{N}<v_{2}=v_{N-1}<... We then obtain a hierarchy of energy-scales,
where the band pairs (j,N+1-j) are successively frozen out. The low-energy
Hamiltonian is then the sum of N/2 (or (N-1)/2 for N odd) two-leg ladder
Hamiltonians without gapless excitations (plus a single chain for N odd with
one gapless spin mode), similar to the N-leg Heisenberg spin-ladders. The
energy-scales lead to a hierarchy of gaps. Upon doping away from half-filling,
the holes enter first the band(s) with the smallest gap: For odd N, the holes
enter first the nonbonding band (N+1)/2 and the phase is a Luttinger liquid,
while for even N, the holes enter first the band pair (N/2,N/2+1) and the phase
is a Luther-Emery liquid, similar to numerical treatments of the t-J model,
i.e., at and close to half-filling, the phases of the Hubbard ladders for small
and large U are the same. For increasing doping, hole-pairs subsequently enter
at critical dopings the other band pairs (j,N+1-j) (accompanied by a diverging
compressibility): The Fermi surface is successively opened by doping, starting
near the wave vector (pi/2,pi/2). Explicit calculations are given for the cases
N=3,4.Comment: 10 pages, 4 figures, to be published in Phys. Rev.
New and noteworthy lichen-forming and lichenicolous fungi 5
Data on 54 new for China, India, Korea and Russia species of lichen-forming and lichenicolous fungi, including 22 new for science taxa of lichen-forming and lichenicolous fungi, i.e.: Acarospora ulleungdoensis, Amandinea trassii, Aspicilia geumodoensis, Biatora ivanpisutii, Caloplaca patwolseleyae, Catillaria ulleungdoensis, Coenogonium agonimieoides, Gyalidea austrocoreana, G. ropalosporoides, Opegrapha briancoppinsii, O. ulleungdoensis, Phyllopsora loekoesii, Psoroglaena coreana, Psorotichia gyelnikii, Rinodina oxneriana, Scoliciosporum jasonhurii, Staurothele oxneri, Stigmidium coarctatae, Thelocarpon ulleungdoense, Thelopsis loekoesii, Toninia poeltiana, Unguiculariopsis helmutii, and and 7 new species to China (Caloplaca ussuriensis, Megaspora rimisorediata, Rinodina xanthophaea, Rusavskia dasanensis, Xanthoria splendens, Zeroviella coreana, Z. esfahanensis), and 1 new species to India (Zeroviella esfahanensis), and 24 new species to Korea (Agonimia blumii, Arthonia rinodinicola, Buelliella minimula, Dactylospora australis, Endococcus propinguus, Halecania santessonii, Laeviomyces aff. fallaciosus, Lecanora albescens, L. layana, Lecidella scabra, Micarea farinosa, Minutoexcipula aff. mariana, Opegrapha anomaea, O. aff. xerica, Phoma aff. lecanorina, Polycoccum rubellianae, Porina nucula, Pyrenidium actinellum, Rhexophiale rhexoblephara, Rimularia badioatra, Rinodina confragosa, R. milvina, R. occulta, Tremella phaeophysciae), as well as 1 new species to Russia (Verseghya klarae) are provided. Furthermore new for science species of lichenicolous fungus Polycoccum clauderouxii from China is described. Four new combinations, i.e.: Biatora pseudosambuci (Basionym: Lecanora pseudosambuci S. Y. Kondr., L. Lőkös et J.-S. Hur), Buellia pseudosubnexa (Basionym: Hafellia pseudosubnexa S. Y. Kondr., L. Lőkös et J.-S. Hur), Buellia extremoorientalis (Basionym: Hafellia extremorientalis S. Y. Kondr., L. Lőkös et J.-S. Hur), and Sagedia nunatakkorum (Basionym: Lecanora nunatakkorum Poelt) are proposed. Data on conidiomata and conidia for lichenicolous fungus Opegrapha anomea Nyl are for the first time provided
- …