327 research outputs found
Tilting of the magnetic field in Majorana nanowires: critical angle and zero-energy differential conductance
Semiconductor nanowires with strong spin-orbit coupling and proximity-induced
s-wave superconductivity in an external magnetic field have been the most
promising settings for approaches towards experimental evidence of topological
Majorana zero-modes. We investigate the effect of tilting the magnetic field
relative to the spin-orbit coupling direction in a simple continuum model and
provide an analytical derivation of the critical angle, at which the
topological states disappear. We also obtain the differential conductance
characteristic of a junction with a normal wire for different tilting angles
and propose the qualitative change of the dependence of the zero-energy
differential conductance on the tunnel barrier strength at the critical angle
as a new criterion for establishing the topological nature of the observed
signal.Comment: 6 pages, 6 figures, submitted to Physical Review B. V2: Published
versio
Thermodynamics of superconducting lattice fermions
We consider the Cooper-problem on a lattice model including onsite and
near-neighbor interactions. Expanding the interaction in basis functions for
the irreducible representation for the point group yields a
classification of the symmetry of the Cooper-pair wave function, which we
calculate in real-space. A change of symmetry upon doping, from s-wave at low
filling fractions, to at higher filling fractions, is found.
Fermi-surface details are thus important for the symmetry of the
superconducting wave function. Symmetry forbids mixing of s-wave and d-wave
symmetry in the Cooper-pair wavefunction on a square lattice, unless accidental
degeneracies occur. This conclusion also holds for the selfconsistent treatment
of the many-body problem, at the critical temperature . Below , we
find temperatures which are not critical points, where new superconducting
channels open up in the order parameter due to bifurcations in the solutions of
the nonlinear gap-equation. We calculate the free energy, entropy, coherence
length, critical magnetic fields, and Ginzburg-Landau parameter . The
model is of the extreme type-II variety. At the temperatures where subdominant
channels condense, we find cusps in the internal energy and entropy, as well as
as BCS-like discontinuities in the specific heat. The specific heat anomalies
are however weaker than at the true superconducting critical point, and argued
to be of a different nature.Comment: RevTex, 18 pages including 10 EPS figures; amstex, epsfig, subfigure
require
Properties of the Virial Expansion and Equation of State of Ideal Quantum Gases in Arbitrary Dimensions
The virial expansion of ideal quantum gases reveals some interesting and
amusing properties when considered as a function of dimensionality . In
particular, the convergence radius of the expansion is particulary
large at {\em exactly\/} dimensions, . The same phenomenon occurs in a few other special
(non-integer) dimensions. We explain the origin of these facts, and discuss
more generally the structure of singularities governing the asymptotic behavior
of the ideal gas virial expansion.Comment: 23 pages, 13 figure
Dirac-fermions and conductance-oscillations in (s,d)-wave superconductor/normal graphene junctions
We investigate quantum transport in a normal/superconductor graphene
heterostructure, including the possibility of an anisotropic pairing potential
in the superconducting region. We find that under certain circumstances, the
conductance displays an undamped, oscillatory behaviour as a function of
applied bias voltage. Also, we investigate how the conductance spectra are
affected by a d-wave pairing symmetry. These results combine unusual features
of the electronic structure of graphene with the unconventional pairing
symmetry found for instance in high-T_c superconductors.Comment: 4 pages, 2 figures. Accepted for publication in Phys. Rev. Let
Signatures of retroreflection and induced triplet electron-hole correlations in ferromagnet/s-wave superconductor structures
We present a theoretical study of a ferromagnet/s-wave superconductor
junction to investigate the signatures of induced triplet correlations in the
system. We apply the extended BTK-formalism and allow for an arbitrary
magnetization strength/direction of the ferromagnet, a spin-active barrier,
Fermi-vector mismatch, and different effective masses in the two systems. It is
found that the phase associated with the -components of the magnetization
in the ferromagnet couples with the superconducting phase and induces
spin-triplet pairing correlations in the superconductor, if the tunneling
barrier acts as a spin-filter. This feature leads to an induced spin-triplet
pairing correlation in the ferromagnet, along with a spin-triplet electron-hole
coherence due to an interplay between the ferromagnetic and superconducting
phase. As our main result, we investigate the experimental signatures of
retrorelection, manifested in the tunneling conductance of a ferromagnet/s-wave
superconductor junction with a spin-active interface.Comment: 13 pages, accepted for publication in Phys. Rev.
Current-loops, phase transitions, and the Higgs mechanism in Josephson-coupled multi-component superconductors
The -component London superconductor is expressed in terms
of integer-valued supercurrents. We show that the inclusion of inter-band
Josephson couplings introduces monopoles in the current fields, which convert
the phase transitions of the charge-neutral sector to crossovers. The monopoles
only couple to the neutral sector, and leave the phase transition of the
charged sector intact. The remnant non-critical fluctuations in the neutral
sector influence the one remaining phase transition in the charged sector, and
may alter this phase transition from a inverted phase transition into a
first-order phase transition depending on what the values of the gauge-charge
and the inter-component Josephson coupling are. This preemptive effect becomes
more pronounced with increasing number of components , since the number of
charge-neutral fluctuating modes that can influence the charged sector
increases with . We also calculate the gauge-field correlator, and by
extension the Higgs mass, in terms of current-current correlators. We show that
the onset of the Higgs-mass of the photon (Meissner-effect) is given in terms
of a current-loop blowout associated with going into the superconducting state
as the temperature of the system is lowered.Comment: 12 pages, 3 figures. To appear in Physical Review
Berry phases, current lattices, and suppression of phase transitions in a lattice gauge theory of quantum antiferromagnets
We consider a lattice model of two complex scalar matter fields under a CP1 constraint \abs{z_1}^2+\abs{z_2}^2=1, minimally coupled to
a compact gauge field, with an additional Berry phase term. This model has been
the point of origin for a large body of works addressing novel paradigms for
quantum criticality, in particular spin-quark (spinon) deconfinement in S=1/2
quantum antiferromagnets. We map the model exactly to a link-current model,
which permits the use of classical worm algorithms to study the model in
large-scale Monte Carlo simulations on lattices of size L^3, up to L=360. We
show that the addition of a Berry phase term to the lattice \CP-model
suppresses the phase transition in the \groupO{3} universality class of the
\CP-model. The link-current formulation of the model is useful in identifying
the mechanism by which the phase transition is suppressed.Comment: 12 pages, 9 figures. Published in Physical Review
Calculation of Drag and Superfluid Velocity from the Microscopic Parameters and Excitation Energies of a Two-Component Bose-Einstein Condensate on an Optical Lattice
We investigate a model of a two-component Bose-Einstein condensate residing
on an optical lattice. Within a Bogolioubov-approach at the mean-field level,
we derive exact analytical expressions for the excitation spectrum of the
two-component condensate when taking into account hopping and interactions
between arbitrary sites. Our results thus constitute a basis for works that
seek to clarify the effects of higher-order interactions in the system. We
investigate the excitation spectrum and the two branches of superfluid velocity
in more detail for two limiting cases of particular relevance. Moreover, we
relate the hopping and interaction parameters in the effective Bose-Hubbard
model to microscopic parameters in the system, such as the laserlight
wavelength and atomic masses of the components in the condensate. These results
are then used to calculate analytically and numerically the drag coefficient
between the components of the condensate. We find that the drag is most
effective close to the symmetric case of equal masses between the components,
regardless of the strength of the intercomponent interaction and the lattice
well depth.Comment: 11 pages, 5 figures. Accepted for publication in Phys. Rev.
Derivation of a Ginzburg-Landau free energy density containing mixed gradient terms of a superconductor with spin-orbit coupling
A Ginzburg-Landau free energy for a superconducting chiral p-wave order
parameter is derived from a two-dimensional tight binding lattice model with
weak spin-orbit coupling included as a general symmetry-breaking field.
Superconductivity is accounted for by a BCS-type nearest neighbor opposite-spin
interaction where we project the potential onto the -wave irreducible
representation of the square lattice symmetry group and assume this to be the
dominating order. The resulting free energy contains kinetic terms that mix
components of the order parameter as well as directional gradients --- so
called mixed gradient terms --- as a virtue of the symmetry of the order
parameter. Spin-orbit coupling and electron-hole anisotropy lead to additional
contributions to the coefficients of these terms, increasing the number of
necessary phenomenological parameters by one compared to previous work, and
leading to an increase in the coefficient measuring Fermi surface anisotropy
for Rashba spin-orbit coupling in the continuum limit
Fluctuation effects in phase-frustrated multiband superconductors
We compare the phase-diagrams of an effective theory of a three-dimensional
multi-band superconductor obtained within standard and cluster mean-field
theories, and in large-scale Monte Carlo simulations. In three dimensions, mean
field theory fails in locating correctly the positions of the phase
transitions, as well as the character of the transitions between the different
states. A cluster mean-field calculations taking into account order-parameter
fluctuations in a local environment improves the results considerably for the
case of extreme type-II superconductors where gauge-field fluctuations are
negligible. The large fluctuations in the multi-component superconducting order
parameter originate with strong frustration due to interband
Josephson-couplings. A novel chiral metallic phase found in previous works
using large scale Monte-Carlo computations, is not obtained either within the
single-site mean-field theory or the improved cluster mean-field theory of
order parameter fluctuations. In three-dimensional superconductors, this
unusual metallic phase originates with gauge-field fluctuations.Comment: 11 pages, 3 Figures. Submitted to Physical Review
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