2,030 research outputs found

### Thermal radiation and amplified spontaneous emission from a random medium

We compute the statistics of thermal emission from systems in which the
radiation is scattered chaotically, by relating the photocount distribution to
the scattering matrix - whose statistical properties are known from
random-matrix theory. We find that the super-Poissonian noise is that of a
black body with a reduced number of degrees of freedom. The general theory is
applied to a disordered slab and to a chaotic cavity, and is extended to
include amplifying as well as absorbing systems. We predict an excess noise of
amplified spontaneous emission in a random laser below the laser threshold.Comment: 4 pages, including 2 figure

### Random-matrix theory of Majorana fermions and topological superconductors

I. Introduction (What is new in RMT, Superconducting quasiparticles,
Experimental platforms)
II. Topological superconductivity (Kitaev chain, Majorana operators, Majorana
zero-modes, Phase transition beyond mean-field)
III. Fundamental symmetries (Particle-hole symmetry, Majorana representation,
Time-reversal and chiral symmetry)
IV. Hamiltonian ensembles (The ten-fold way, Midgap spectral peak, Energy
level repulsion)
V. Scattering matrix ensembles (Fundamental symmetries, Chaotic scattering,
Circular ensembles, Topological quantum numbers)
VI. Electrical conduction (Majorana nanowire, Counting Majorana zero-modes,
Conductance distribution, Weak antilocalization, Andreev resonances, Shot noise
of Majorana edge modes)
VII. Thermal conduction (Topological phase transitions, Super-universality,
Heat transport by Majorana edge modes, Thermopower and time-delay matrix,
Andreev billiard with chiral symmetry)
VIII. Josephson junctions (Fermion parity switches, 4{\pi}-periodic Josephson
effect, Discrete vortices)
IX. ConclusionComment: V1: 18 pages, 16 figures; pre-submission version, for feedback; V2:
33 pages, 31 figures; greatly expanded in response to feedback, thank you!;
V3: minor corrections, version to be published in Reviews of Modern Physic

### Exactly Solvable Scaling Theory of Conduction in Disordered Wires

Recent developments are reviewed in the scaling theory of phase-coherent
conduction through a disordered wire. The Dorokhov-Mello-Pereyra-Kumar equation
for the distribution of transmission eigenvalues has been solved exactly, in
the absence of time-reversal symmetry. Comparison with the previous prediction
of random-matrix theory shows that this prediction was highly accurate --- but
not exact: The repulsion of the smallest eigenvalues was overestimated by a
factor of two. This factor of two resolves several disturbing discrepancies
between random-matrix theory and microscopic calculations, notably in the
magnitude of the universal conductance fluctuations in the metallic regime, and
in the width of the log-normal conductance distribution in the insulating
regime. ***To be published as a "Brief Review" in Modern Physics Letters B.****Comment: 9 pages, LATEX, INLO-PUB-940309

### Switching of electrical current by spin precession in the first Landau level of an inverted-gap semiconductor

We show how the quantum Hall effect in an inverted-gap semiconductor (with
electron- and hole-like states at the conduction- and valence-band edges
interchanged) can be used to inject, precess, and detect the electron spin
along a one-dimensional pathway. The restriction of the electron motion to a
single spatial dimension ensures that all electrons experience the same amount
of precession in a parallel magnetic field, so that the full electrical current
can be switched on and off. As an example, we calculate the magnetoconductance
of a p-n interface in a HgTe quantum well and show how it can be used to
measure the spin precession due to bulk inversion asymmetry.Comment: 5 pages, 4 figures, extended versio

### Reentrance effect in a graphene n-p-n junction coupled to a superconductor

We study the interplay of Klein tunneling (= interband tunneling) between
n-doped and p-doped regions in graphene and Andreev reflection (= electron-hole
conversion) at a superconducting electrode. The tunneling conductance of an
n-p-n junction initially increases upon lowering the temperature, while the
coherence time of the electron-hole pairs is still less than their lifetime,
but then drops back again when the coherence time exceeds the lifetime. This
reentrance effect, known from diffusive conductors and ballistic quantum dots,
provides a method to detect phase coherent Klein tunneling of electron-hole
pairs.Comment: 4 pages, 3 figure

### Ballistic transmission through a graphene bilayer

We calculate the Fermi energy dependence of the (time-averaged) current and
shot noise in an impurity-free carbon bilayer (length $L\ll$ width $W$), and
compare with known results for a monolayer. At the Dirac point of charge
neutrality, the bilayer transmits as two independent monolayers in parallel:
Both current and noise are resonant at twice the monolayer value, so that their
ratio (the Fano factor) has the same 1/3 value as in a monolayer -- and the
same value as in a diffusive metal. The range of Fermi energies around the
Dirac point within which this pseudo-diffusive result holds is smaller,
however, in a bilayer than in a monolayer (by a factor $l_{\perp}/L$, with
$l_{\perp}$ the interlayer coupling length).Comment: 6 pages, 7 figures, version to appear in PR

### Photon shot noise

A recent theory is reviewed for the shot noise of coherent radiation
propagating through a random medium. The Fano factor P/I (the ratio of the
noise power and the mean transmitted current) is related to the scattering
matrix of the medium. This is the optical analogue of Buttiker's formula for
electronic shot noise. Scattering by itself has no effect on the Fano factor,
which remains equal to 1 (as for a Poisson process). Absorption and
amplification both increase the Fano factor above the Poisson value. For strong
absorption P/I has the universal limit 1+3f/2 with f the Bose-Einstein function
at the frequency of the incident radiation. This is the optical analogue of the
one-third reduction factor of electronic shot noise in diffusive conductors. In
the amplifying case the Fano factor diverges at the laser threshold, while the
signal-to-noise ratio I^2/P reaches a finite, universal limit.Comment: 11 pages, 4 figures (caption to figure 3 corrected

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