1,145 research outputs found
Intrinsic temperature dependences of transport coefficients within the hot-spot model for normal state YBCO
The temperature dependences of the galvanomagnetic and thermoelectric
transport coefficients within a generic hot-spot model are reconsidered.
Despite the recent success in explaining ac Hall effect data in
YBa_{2}Cu_{3}O_{7}, a general feature of this model is a departure from the
approximately universal temperature dependences observed for normal state
transport in the optimally doped cuprates. In this paper, we discuss such
systematic deviations and illustrate some of their effects through a concrete
numerical example using the calculated band structure for YBa_{2}Cu_{3}O_{7}.Comment: 4 pages, LaTex, 2 EPS figure
Majorana bound states in two-channel time-reversal-symmetric nanowire systems
We consider time-reversal-symmetric two-channel semiconducting quantum wires
proximity coupled to an s-wave superconductor. We analyze the requirements for
a nontrivial topological phase and find that necessary conditions are 1) the
determinant of the pairing matrix in channel space must be negative, 2)
inversion symmetry must be broken, and 3) the two channels must have different
spin-orbit couplings. The first condition can be implemented in semiconducting
nanowire systems where interactions suppress intra-channel pairing, while the
inversion symmetry can be broken by tuning the chemical potentials of the
channels. For the case of collinear spin-orbit directions, we find a general
expression for the topological invariant by block diagonalization into two
blocks with chiral symmetry only. By projection to the low-energy sector, we
solve for the zero modes explicitly and study the details of the gap closing,
which in the general case happens at finite momenta.Comment: 6 pages. Corrected versio
Nematic Bond Theory of Heisenberg Helimagnets
We study classical two-dimensional frustrated Heisenberg models with
generically incommensurate groundstates. A new theory for the spin-nematic
"order by disorder" transition is developed based on the self-consistent
determination of the effective exchange coupling bonds. In our approach,
fluctuations of the constraint field imposing conservation of the local
magnetic moment drive nematicity at low temperatures. The critical temperature
is found to be highly sensitive to the peak helimagnetic wavevector, and
vanishes continuously when approaching rotation symmetric Lifshitz points.
Transitions between symmetry distinct nematic orders may occur by tuning the
exchange parameters, leading to lines of bicritical points.Comment: 4 pages, 4 figure
Interplay between Magnetic and Vestigial Nematic Orders in the Layered - Classical Heisenberg Model
We study the layered - classical Heisenberg model on the square
lattice using a self-consistent bond theory. We derive the phase diagram for
fixed as a function of temperature , and interplane coupling
. Broad regions of (anti)ferromagnetic and stripe order are found, and are
separated by a first-order transition near (in units of
). Within the stripe phase the magnetic and vestigial nematic
transitions occur simultaneously in first-order fashion for strong . For
weaker there is in addition, for , an intermediate
regime of split transitions implying a finite temperature region with nematic
order but no long-range stripe magnetic order. In this split regime, the order
of the transitions depends sensitively on the deviation from and
, with split second-order transitions predominating for . We find that the value of depends weakly on the
interplane coupling and is just slightly larger than for . In contrast the value of increases quickly from at
as the interplane coupling is further reduced. In
addition, the magnetic correlation length is shown to directly depend on the
nematic order parameter and thus exhibits a sharp increase (or jump) upon
entering the nematic phase. Our results are broadly consistent with predictions
based on itinerant electron models of the iron-based superconductors in the
normal-state, and thus help substantiate a classical spin framework for
providing a phenomenological description of their magnetic properties.Comment: 13 pages, 20 figure
Nonequilibrium Transport through a Kondo Dot: Decoherence Effects
We investigate the effects of voltage induced spin-relaxation in a quantum
dot in the Kondo regime. Using nonequilibrium perturbation theory, we determine
the joint effect of self-energy and vertex corrections to the conduction
electron T-matrix in the limit of transport voltage much larger than
temperature. The logarithmic divergences, developing near the different
chemical potentials of the leads, are found to be cut off by spin-relaxation
rates, implying that the nonequilibrium Kondo-problem remains at weak coupling
as long as voltage is much larger than the Kondo temperature.Comment: 16 pages, 4 figure
Cotunneling renormalization in carbon nanotube quantum dots
We determine the level-shifts induced by cotunneling in a Coulomb blockaded
carbon nanotube quantum dot using leading order quasi-degenerate perturbation
theory within a single nanotube quartet. It is demonstrated that otherwise
degenerate and equally tunnel-coupled and states are mixed by
cotunneling and therefore split up in energy except at the
particle/hole-symmetric midpoints of the Coulomb diamonds. In the presence of
an external magnetic field, we show that cotunneling induces a gate-dependent
-factor renormalization, and we outline different scenarios which might be
observed experimentally, depending on the values of both intrinsic
splitting and spin-orbit coupling.Comment: 12 pages, 7 figure
Sources of negative tunneling magneto-resistance in multilevel quantum dots with ferromagnetic contacts
We analyze distinct sources of spin-dependent energy level shifts and their
impact on the tunneling magnetoresistance (TMR) of interacting quantum dots
coupled to collinearly polarized ferromagnetic leads. Level shifts due to
virtual charge fluctuations can be quantitatively evaluated within a
diagrammatic representation of our transport theory. The theory is valid for
multilevel quantum dot systems and we exemplarily apply it to carbon nanotube
quantum dots, where we show that the presence of many levels can qualitatively
influence the TMR effect.Comment: 4 pages, 2 figures, supplemental materia
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