1,777 research outputs found
Breathing Modes and Hidden Symmetry of Trapped Atoms in 2D
Atoms confined in a harmonic potential show universal oscillations in 2D. We
point out the connection of these ''breathing'' modes to the presence of a
hidden symmetry. The underlying symmetry SO(2,1), i.e. the two dimensional
Lorentz group, allows pulsating solutions to be constructed for the interacting
quantum system and for the corresponding nonlinear Gross-Pitaevskii equation.
We point out how this symmetry can be used as a probe for recently proposed
experiments of trapped atoms in 2D.Comment: 4 pages, small changes in title and text, references adde
Weak spin-orbit interactions induce exponentially flat mini-bands in magnetic metals without inversion symmetry
In metallic magnets like MnSi the interplay of two very weak spin-orbit
coupling effects can strongly modify the Fermi surface. In the absence of
inversion symmetry even a very small Dzyaloshinsky-Moriya interaction of
strength delta<<1 distorts a ferromagnetic state into a chiral helix with a
long pitch of order 1/delta. We show that additional small spin-orbit coupling
terms of order delta in the band structure lead to the formation of
exponentially flat minibands with a bandwidth of order exp(-1/sqrt(delta))
parallel to the direction of the helix. These flat minibands cover a rather
broad belt of width sqrt(delta) on the Fermi surface where electron motion
parallel to the helix practically stops. We argue that these peculiar
band-structure effects lead to pronounced features in the anomalous skin
effect.Comment: 7 pages, minor corrections, references adde
Wilson chains are not thermal reservoirs
Wilson chains, based on a logarithmic discretization of a continuous
spectrum, are widely used to model an electronic (or bosonic) bath for Kondo
spins and other quantum impurities within the numerical renormalization group
method and other numerical approaches. In this short note we point out that
Wilson chains can not serve as thermal reservoirs as their temperature changes
by a number of order Delta E when a finite amount of energy Delta E is added.
This proves that for a large class of non-equilibrium problems they cannot be
used to predict the long-time behavior.Comment: 2 page
Vibrational Sidebands and Kondo-effect in Molecular Transistors
Electron transport through molecular quantum dots coupled to a single
vibrational mode is studied in the Kondo regime. We apply a generalized
Schrieffer-Wolff transformation to determine the effective low-energy
spin-spin-vibron-interaction. From this model we calculate the nonlinear
conductance and find Kondo sidebands located at bias-voltages equal to
multiples of the vibron frequency. Due to selection rules, the side-peaks are
found to have strong gate-voltage dependences, which can be tested
experimentally. In the limit of weak electron-vibron coupling, we employ a
perturbative renormalization group scheme to calculate analytically the
nonlinear conductance.Comment: 4 page
Sign change of the Grueneisen parameter and magnetocaloric effect near quantum critical points
We consider the Grueneisen parameter and the magnetocaloric effect near a
pressure and magnetic field controlled quantum critical point, respectively.
Generically, the Grueneisen parameter (and the thermal expansion) displays a
characteristic sign change close to the quantum-critical point signaling an
accumulation of entropy. If the quantum critical point is the endpoint of a
line of finite temperature phase transitions, T_c \propto (p_c-p)^Psi, then we
obtain for p<p_c, (1) a characteristic increase \Gamma \sim T^{-1/(\nu z)} of
the Grueneisen parameter Gamma for T>T_c, (2) a sign change in the Ginzburg
regime of the classical transition, (3) possibly a peak at T_c, (4) a second
increase Gamma \sim -T^{-1/(nu z)} below T_c for systems above the upper
critical dimension and (5) a saturation of Gamma \propto 1/(p_c-p). We argue
that due to the characteristic divergencies and sign changes the thermal
expansion, the Grueneisen parameter and magnetocaloric effect are excellent
tools to detect and identify putative quantum critical points.Comment: 10 pages, 7 figures; final version, only minor change
Recommended from our members
Semantic memory redux: an experimental test of hierarchical category representation
Four experiments investigated the classic issue in semantic memory of whether people organize categorical information in hierarchies and use inference to retrieve information from them, as proposed by Collins & Quillian (1969). Past evidence has focused on RT to confirm sentences such as “All birds are animals” or “Canaries breathe.” However, confounding variables such as familiarity and associations between the terms have led to contradictory results. Our experiments avoided such problems by teaching subjects novel materials. Experiment 1 tested an implicit hierarchical structure in the features of a set of studied objects (e.g., all brown objects were large). Experiment 2 taught subjects nested categories of artificial bugs. In Experiment 3, subjects learned a tree structure of novel category hierarchies. In all three, the results differed from the predictions of the hierarchical inference model. In Experiment 4, subjects learned a hierarchy by means of paired associates of novel category names. Here we finally found the RT signature of hierarchical inference. We conclude that it is possible to store information in a hierarchy and retrieve it via inference, but it is difficult and avoided whenever possible. The results are more consistent with feature comparison models than hierarchical models of semantic memory
Fragility of String Orders
One-dimensional gapped systems are often characterized by a 'hidden'
non-local order parameter, the so-called string order. Due to the gap,
thermodynamic properties are robust against a weak higher-dimensional coupling
between such chains or ladders. To the contrary, we find that the string order
is not stable and decays for arbitrary weak inter-chain or inter-ladder
coupling. We investigate the vanishing of the order for three different
systems: spin-one Haldane chains, band insulators, and the transverse-field
Ising model. Using perturbation theory and bosonization, we show that the
fragility of the string order arises from non-local commutation relations
between the non-local order parameter and the perturbation.Comment: 7 pages, 3 figures. Published versio
Crystalline phases in chiral ferromagnets: Destabilization of helical order
In chiral ferromagnets, weak spin-orbit interactions twist the ferromagnetic
order into spirals, leading to helical order. We investigate an extended
Ginzburg-Landau theory of such systems where the helical order is destabilized
in favor of crystalline phases. These crystalline phases are based on periodic
arrangements of double-twist cylinders and are strongly reminiscent of blue
phases in liquid crystals. We discuss the relevance of such blue phases for the
phase diagram of the chiral ferromagnet MnSi.Comment: 6 pages, 5 figures (published version
Some remarks about pseudo gap behavior of nearly antiferromagnetic metals
In the antiferromagnetically ordered phase of a metal, gaps open on parts of
the Fermi surface if the Fermi volume is sufficiently large. We discuss simple
qualitative and heuristic arguments under what conditions precursor effects,
i.e. pseudo gaps, are expected in the paramagnetic phase of a metal close to an
antiferromagnetic quantum phase transition. At least for weak interactions, we
do not expect the formation of pseudo gaps in a three dimensional material.
According to our arguments, the upper critical dimension d_c for the formation
of pseudo gaps is d_c=2. However, at the present stage we cannot rule out a
higher upper critical dimension, 2 < d_c <= 3. We also discuss briefly the role
of statistical interactions in pseudo gap phases.Comment: 6 pages, accepted in PRB, relevant references added, several small
change
Low-temperature ordered phases of the spin- XXZ chain system CsCoCl
In this study the magnetic order of the spin-1/2 XXZ chain system
CsCoCl in a temperature range from 50 mK to 0.5 K and in applied
magnetic fields up to 3.5 T is investigated by high-resolution measurements of
the thermal expansion and the specific heat. Applying magnetic fields along a
or c suppresses completely at about 2.1 T. In addition, we find
an adjacent intermediate phase before the magnetization saturates close to 2.5
T. For magnetic fields applied along b, a surprisingly rich phase diagram
arises. Two additional transitions are observed at critical fields T and T, which we propose to
arise from a two-stage spin-flop transition.Comment: 10 pages, 10 figure
- …