4,238 research outputs found

### Validity and failure of some entropy inequalities for CAR systems

Basic properties of von Neumann entropy such as the triangle inequality and
what we call MONO-SSA are studied for CAR systems.
We show that both inequalities hold for any even state. We construct a
certain class of noneven states giving counter examples of those inequalities.
It is not always possible to extend a set of prepared states on disjoint
regions to some joint state on the whole region for CAR systems.
However, for every even state, we have its `symmetric purification' by which
the validity of those inequalities is shown.
Some (realized) noneven states have peculiar state correlations among
subsystems and induce the failure of those inequalities.Comment: 14 pages, latex, to appear in JMP. Some typos are correcte

### Markov property and strong additivity of von Neumann entropy for graded quantum systems

It is easy to verify the equivalence of the quantum Markov property and the
strong additivity of entropy for graded quantum systems as well. However, the
structure of Markov states for graded systems is different from that for tensor
product systems. For three-composed graded systems there are U(1)-gauge
invariant Markov states whose restriction to the pair of marginal subsystems is
non-separable.Comment: 14 pages, to appear J. Math. Phy

### Superconductivity and Pseudogap in Quasi-Two-Dimensional Metals around the Antiferromagnetic Quantum Critical Point

Spin fluctuations (SF) and SF-mediated superconductivity (SC) in
quasi-two-dimensional metals around the antiferrromagnetic (AF) quantum
critical point (QCP) are investigated by using the self-consistent
renormalization theory for SF and the strong coupling theory for SC. We
introduce a parameter y0 as a measure for the distance from the AFQCP which is
approximately proportional to (x-xc), x being the electron (e) or hole (h)
doping concentration to the half-filled band and xc being the value at the
AFQCP. We present phase diagrams in the T-y0 plane including contour maps of
the AF correlation length and AF and SC transition temperatures TN and Tc,
respectively. The Tc curve is dome-shaped with a maximum at around the AFQCP.
The calculated one-electron spectral density shows a pseudogap in the
high-density-of-states region near (pi,0) below around a certain temperature T*
and gives a contour map at the Fermi energy reminiscent of the Fermi arc. These
results are discussed in comparison with e- and h-doped high-Tc cuprates.Comment: 5 pages, 3 figure

### Episodic modulations in supernova radio light curves from luminous blue variable supernova progenitor models

Ideally, one would like to know which type of core-collapse SNe is
produced by different progenitors and the channels of stellar evolution leading
to these progenitors. These links have to be very well known to use the
observed frequency of different types of SN events for probing the star
formation rate and massive star evolution in different types of galaxies.
We investigate the link between LBV as SN progenitors and the appearance
of episodic radio light curve modulations of the SN event. We use the
20Msun and 25Msun models with rotation at solar metallicity, part of an
extended grid of stellar models computed by the Geneva team. At their pre-SN
stage, these two models have recently been shown to have spectra similar to
those of LBV stars and possibly explode as Type IIb SNe. Based on the wind
properties before the explosion, we derive the density structure of their
circumstellar medium. This structure is used as input for computing the SN
radio light curve. We find that the 20Msun model shows radio light
curves with episodic luminosity modulations, similar to those observed in some
Type IIb SNe. This occurs because the evolution of the 20Msun model terminates
in a region of the HR diagram where radiative stellar winds present strong
density variations, caused by the bistability limit. The 25Msun model, ending
its evolution in a zone of the HR diagram where no change of the mass-loss
rates is expected, presents no such modulations in its radio SN light curve.
Our results reinforce the link between SN progenitors and LBV
stars. We also confirm the existence of a physical mechanism for a single star
to have episodic radio light curve modulations. In the case of the 25Msun
progenitors, we do not obtain modulations in the radio light curve, but our
models may miss some outbursting behavior in the late stages of massive stars.Comment: 5 pages, 3 figures, accepted by Astronomy & Astrophysics Letter

### Current-induced magnetization reversal in a (Ga,Mn)As-based magnetic tunnel junction

We report current-induced magnetization reversal in a ferromagnetic
semiconductor-based magnetic tunnel junction (Ga,Mn)As/AlAs/(Ga,Mn)As prepared
by molecular beam epitaxy on a p-GaAs(001) substrate. A change in
magneto-resistance that is asymmetric with respect to the current direction is
found with the excitation current of 10^6 A/cm^2. Contributions of both
unpolarized and spin-polarized components are examined, and we conclude that
the partial magnetization reversal occurs in the (Ga,Mn)As layer of smaller
magnetization with the spin-polarized tunneling current of 10^5 A/cm^2.Comment: 13 pages, 3 figure

### Frequency-dependent spin susceptibility in the two-dimensional Hubbard model

A Quantum Monte Carlo calculation of dynamical spin susceptibility in the
half-filled 2D Hubbard model is presented for temperature $T=0.2t$ and an
intermediate on-site repulsion $U=4t$. Using the singular value decomposition
technique we succeed in analytically continuing the Matsubara Green's function
into the real frequency domain and in deriving the spectral representation for
the longitudinal and transverse spin susceptibility. The simulation results,
while contradicting the random-phase approximation prediction of
antiferromagnetic long-range order at this temperature, are in agreement with
an extension of a self-consistent renormalization approach of Moriya. The
static susceptibility calculated using this technique is qualitatively
consistent with the $\omega \rightarrow 0$ simulation results.Comment: 4 pages, Revtex, encoded figs.uu file with 3 figures enclose

### Ferromagnetic Quantum Critical Fluctuations and Anomalous Coexistence of Ferromagnetism and Superconductivity in UCoGe Revealed by Co-NMR and NQR Studies

Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR)
studies were performed in the recently discovered UCoGe, in which the
ferromagnetic and superconducting (SC) transitions were reported to occur at
$T_{\rm Curie} \sim 3$ K and $T_S \sim 0.8$ K (N. T. Huy {\it et al.}, Phys.
Rev. Lett. {\bf 99} (2007) 067006), in order to investigate the coexistence of
ferromagnetism and superconductivity as well as the normal-state and SC
properties from a microscopic point of view. From the nuclear spin-lattice
relaxation rate $1/T_1$ and Knight-shift measurements, we confirmed that
ferromagnetic fluctuations which possess a quantum critical character are
present above $T_{\rm Curie}$ and the occurrence of ferromagnetic transition at
2.5 K in our polycrystalline sample. The magnetic fluctuations in the normal
state show that UCoGe is an itinerant ferromagnet similar to ZrZn$_2$ and
YCo$_2$. The onset SC transition was identified at $T_S \sim 0.7$ K, below
which $1/T_1$ of 30 % of the volume fraction starts to decrease due to the
opening of the SC gap. This component of $1/T_1$, which follows a $T^3$
dependence in the temperature range of $0.3 - 0.1$ K, coexists with the
magnetic components of $1/T_1$ showing a $\sqrt{T}$ dependence below $T_S$.
From the NQR measurements in the SC state, we suggest that the self-induced
vortex state is realized in UCoGe.Comment: 5 pages, 7 figures. submitted to J. Phys. Soc. Jpn. To appear in J.
Phys. Soc. Jp

### Towards compact and portable sub-kHz AlGaInP semiconductor disk lasers for cold atom experiments

Stable lasers are crucial for experiments that target narrow atomic transitions (kHz down to Hz linewidth). Such transitions are used, for example, to cool and trap atoms in magneto-optical traps down to the μK regime, in particular for optical clock systems. In this context, semiconductor disk lasers (SDLs) have demonstrated great potential due to their spectral flexibility, high brightness, and low intensity and frequency noise. Here we report our recent progress in frequency stabilisation of an AlGaInP SDL designed for ultra-narrow linewidth at 689 nm for a strontium clock, achieving sub-kHz RMS frequency noise, relative to a reference Fabry-Perot resonator

### Scaling of the magnetic response in doped antiferromagnets

A theory of the anomalous $\omega/T$ scaling of the dynamic magnetic response
in cuprates at low doping is presented. It is based on the memory function
representation of the dynamical spin suceptibility in a doped antiferromagnet
where the damping of the collective mode is constant and large, whereas the
equal-time spin correlations saturate at low $T$. Exact diagonalization results
within the t-J model are shown to support assumptions. Consequences, both for
the scaling function and the normalization amplitude, are well in agreement
with neutron scattering results.Comment: 4 pages, 4 figure

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