8,897 research outputs found
Cuprates as doped U(1) spin liquids
We explore theoretically the notion that the underdoped cuprates may be
viewed as doped U(1) spin liquid Mott insulators. We pursue a conceptually
clear version of this idea that naturally incorporates several aspects of the
phenomenology of the cuprates. We argue that the low doping region may be
fruitfully discussed in terms of the universal physics associated with a
chemical potential tuned Mott transition between a U(1) spin liquid insulator
and a d-wave superconductor. A precise characterization of the deconfinement in
the U(1) spin liquid is provided by the emergence of a conserved gauge flux.
This extra conservation law should hold at least approximately in the
underdoped materials. Experiments that could possibly detect this conserved
gauge flux are proposed.Comment: 11 pages, 7 figure
Majorana Fermion Induced Resonant Andreev Reflection
We describe experimental signatures of Majorana fermion edge states, which
form at the interface between a superconductor and the surface of a topological
insulator. If a lead couples to the Majorana fermions through electron
tunneling, the Majorana fermions induce \textit{resonant} Andreev reflections
from the lead to the grounded superconductor. The linear tunneling conductance
is () if there is an even (odd) number of vortices in the
superconductor. Similar resonance occurs for tunneling into the zero mode in
the vortex core. We also study the current and noise of a two-lead device.Comment: 4 pages, 3 figures. Discussion on STM tunneling into the Majorana
zero mode in the vortex core is adde
Overscreened Kondo fixed point in S=1 spin liquid
We propose a possible realization of the overscreened Kondo impurity problem
by a magnetic s=1/2 impurity embedded in a two-dimensional S=1 U(1) spin liquid
with a Fermi surface. This problem contains an interesting interplay between
non-Fermi-liquid behavior induced by a U(1) gauge field coupled to fermions and
a non-Fermi-liquid fixed point in the overscreened Kondo problem. Using a
large-N expansion together with an expansion in the dynamical exponent of the
gauge field, we find that the coupling to the gauge field leads to weak but
observable changes in the physical properties of the system at the overscreened
Kondo fixed point. We discuss the extrapolation of this result to a physical
case and argue that the realization of overscreened Kondo physics could lead to
observations of effects due to gauge fields.Comment: 10 pages, 5 figure
Vortex description of the fractionalized phase in exciton bose condensate
As a sequel to the previous work [Phys. Rev. B 72, 235104 (2005)] we present
a vortex description of the fractionalized phase in exciton bose condensate.
Magnetic flux line and monopole of the 3+1D emergent U(1) gauge theory are
identified in the exciton picture. A bundle of vortex/anti-vortex pairs of all
flavors of excitons corresponds to the magnetic flux line and a point at which
the vortices and anti-vortices recombine is identified as magnetic monopole.
This completes the magnetic sector of the low energy excitation in the
fractionalized phase.Comment: 8 pages, 6 figures; clarification made in introductio
Paired chiral spin liquid with a Fermi surface in S=1 model on the triangular lattice
Motivated by recent experiments on Ba3NiSb2O9, we investigate possible
quantum spin liquid ground states for spin S=1 Heisenberg models on the
triangular lattice. We use Variational Monte Carlo techniques to calculate the
energies of microscopic spin liquid wave functions where spin is represented by
three flavors of fermionic spinon operators. These energies are compared with
the energies of various competing three-sublattice ordered states. Our approach
shows that the antiferromagnetic Heisenberg model with biquadratic term and
single-ion anisotropy does not have a low-temperature spin liquid phase.
However, for an SU(3)-invariant model with sufficiently strong ring-exchange
terms, we find a paired chiral quantum spin liquid with a Fermi surface of
deconfined spinons that is stable against all types of ordering patterns we
considered. We discuss the physics of this exotic spin liquid state in relation
with the recent experiment and suggest new ways to test this scenario.Comment: 18 pages, 6 figures; replaced with published versio
Weak Mott insulators on the triangular lattice: possibility of a gapless nematic quantum spin liquid
We study the energetics of Gutzwiller projected BCS states of various
symmetries for the triangular lattice antiferromagnet with a four particle ring
exchange using variational Monte Carlo methods. In a range of parameters the
energetically favored state is found to be a projected paired
state which breaks lattice rotational symmetry. We show that the properties of
this nematic or orientationally ordered paired spin liquid state as a function
of temperature and pressure can account for many of the experiments on organic
materials. We also study the ring-exchange model with ferromagnetic Heisenberg
exchange and find that amongst the studied ans\"atze, a projected wave
state is the most favorable.Comment: Longer version, 7+ pages, 5 figure
Selective Equal-Spin Andreev Reflections Induced by Majorana Fermions
In this work, we find that Majorana fermions induce selective equal spin
Andreev reflections (SESARs), in which incoming electrons with certain spin
polarization in the lead are reflected as counter propagating holes with the
same spin. The spin polarization direction of the electrons of this Andreev
reflected channel is selected by the Majorana fermions. Moreover, electrons
with opposite spin polarization are always reflected as electrons with
unchanged spin. As a result, the charge current in the lead is spin-polarized.
Therefore, a topological superconductor which supports Majorana fermions can be
used as a novel device to create fully spin-polarized currents in paramagnetic
leads. We point out that SESARs can also be used to detect Majorana fermions in
topological superconductors.Comment: 5 pages, 3 figures. Comments are welcome. Title changed to match
published versio
Robustness of Majorana Fermion induced Fractional Josephson Effect
It is shown in previous works that the coupling between two Majorana end
states in superconducting quantum wires leads to fractional Josephson effect.
However, in realistic experimental conditions, multiple bands of the wires are
occupied and the Majorana end states are accompanied by other fermionic end
states. This raises the question concerning the robustness of fractional
Josephson effect in these situations. In this work, we show that the absence of
the avoided energy crossing which gives rise to the fractional Josephson effect
is robust, even when the Majorana fermions are coupled with arbitrary strengths
to other fermions. Moreover, we calculate the temperature dependence of the
fractional Josephson current and show that it is suppressed by thermal
excitations to the other fermion bound states.Comment: 4+ pages, 3 figure
Zero-bias peaks in spin-orbit coupled superconducting wires with and without Majorana end-states
One of the simplest proposed experimental probes of a Majorana bound-state is
a quantized (2e^2/h) value of zero-bias tunneling conductance. When temperature
is somewhat larger than the intrinsic width of the Majorana peak, conductance
is no longer quantized, but a zero-bias peak can remain. Such a non-quantized
zero-bias peak has been recently reported for semiconducting nanowires with
proximity induced superconductivity. In this paper we analyze the relation of
the zero-bias peak to the presence of Majorana end-states, by simulating the
tunneling conductance for multi-band wires with realistic amounts of disorder.
We show that this system generically exhibits a (non-quantized) zero-bias peak
even when the wire is topologically trivial and does not possess Majorana
end-states. We make comparisons to recent experiments, and discuss the
necessary requirements for confirming the existence of a Majorana state.Comment: 5 pages, 4 Figure
Spinon Fermi surface in a cluster Mott insulator model on a triangular lattice and possible application to 1T-TaS
1T-TaS is a cluster Mott insulator on the triangular lattice with 13 Ta
atoms forming a star of David cluster as the unit cell. We derive a two
dimensional XXZ spin-1/2 model with four-spin ring exchange term to describe
the effective low energy physics of a monolayer 1T-TaS, where the effective
spin-1/2 degrees of freedom arises from the Kramers degenerate spin-orbital
states on each star of David. A large scale density matrix renormalization
group simulation is further performed on this effective model and we find a
gapless spin liquid phase with spinon Fermi surface at moderate to large
strength region of four-spin ring exchange term. All peaks in the static spin
structure factor are found to be located on the "" surface of half-filled
spinon on the triangular lattice. Experiments to detect the spinon Fermi
surface phase in 1T-TaS are discussed.Comment: 5+11 pages, 4+13 figure
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