77 research outputs found
Hydrodynamic Equation for the Breakdown of the Quantum Hall Effect in a Uniform Current
The hydrodynamic equation for the spatial and temporal evolution of the
electron temperature T_e in the breakdown of the quantum Hall effect at
even-integer filling factors in a uniform current density j is derived from the
Boltzmann-type equation, which takes into account electron-electron and
electron-phonon scatterings. The derived equation has a drift term, which is
proportional to j and to the first spatial derivative of T_e. Applied to the
spatial evolution of T_e in a sample with an abrupt change of the width along
the current direction, the equation gives a distinct dependence on the current
direction as well as a critical relaxation, in agreement with the recent
experiments.Comment: 4 pages, 1 Postscript figure, corrected equations, to be published in
J. Phys. Soc. Jpn. 70 (2001) No.
Electronic Processes at the Breakdown of the Quantum Hall Effect
Microscopic processes giving the energy gain and loss of a two-dimensional
electron system in long-range potential fluctuations are studied theoretically
at the breakdown of the quantum Hall effect in the case of even-integer filling
factors. The Coulomb scattering within a broadened Landau level is proposed to
give the gain, while the phonon scattering to give the loss. The energy balance
equation shows that the electron temperature T_e and the diagonal conductivity
sigma_{xx} exhibit a bistability above the lower critical electric field
E_{c1}. Calculated values of E_{c1} as well as T_e and sigma_{xx} at E_{c1} are
in agreement with the observed values in their orders of magnitude.Comment: 4 pages, 2 Postscript figures, submitted to the Journal of the
Physical Society of Japa
Hydrodynamic Equations in Quantum Hall Systems at Large Currents
Hydrodynamic equations (HDEQs) are derived which describe spatio-temporal
evolutions of the electron temperature and the chemical potential of
two-dimensional systems in strong magnetic fields in states with large diagonal
resistivity appearing at the breakdown of the quantum Hall effect. The
derivation is based on microscopic electronic processes consisting of drift
motions in a slowly-fluctuating potential and scattering processes due to
electron-electron and electron-phonon interactions. In contrast with the usual
HDEQs, one of the derived HDEQs has a term with an energy flux perpendicular to
the electric field due to the drift motions in the magnetic field. As an
illustration, the current distribution is calculated using the derived HDEQs.Comment: 10 pages, 2 Postscript figures, to be published in J. Phys. Soc. Jpn.
71 (2002) No.
Electrically controlled long-distance spin transport through an antiferromagnetic insulator
Spintronics uses spins, the intrinsic angular momentum of electrons, as an
alternative for the electron charge. Its long-term goal is in the development
of beyond-Moore low dissipation technology devices. Recent progress
demonstrated the long-distance transport of spin signals across ferromagnetic
insulators. Antiferromagnetically ordered materials are however the most common
class of magnetic materials with several crucial advantages over ferromagnetic
systems. In contrast to the latter, antiferromagnets exhibit no net magnetic
moment, which renders them stable and impervious to external fields. In
addition, they can be operated at THz frequencies. While fundamentally their
properties bode well for spin transport, previous indirect observations
indicate that spin transmission through antiferromagnets is limited to short
distances of a few nanometers. Here we demonstrate the long-distance, over tens
of micrometers, propagation of spin currents through hematite (\alpha-Fe2O3),
the most common antiferromagnetic iron oxide, exploiting the spin Hall effect
for spin injection. We control the spin current flow by the interfacial
spin-bias and by tuning the antiferromagnetic resonance frequency with an
external magnetic field. This simple antiferromagnetic insulator is shown to
convey spin information parallel to the compensated moment (N\'eel order) over
distances exceeding tens of micrometers. This newly-discovered mechanism
transports spin as efficiently as the net magnetic moments in the best-suited
complex ferromagnets. Our results pave the way to ultra-fast, low-power
antiferromagnet-insulator-based spin-logic devices that operate at room
temperature and in the absence of magnetic fields
Thermohydrodynamics in Quantum Hall Systems
A theory of thermohydrodynamics in two-dimensional electron systems in
quantizing magnetic fields is developed including a nonlinear transport regime.
Spatio-temporal variations of the electron temperature and the chemical
potential in the local equilibrium are described by the equations of
conservation with the number and thermal-energy flux densities. A model of
these flux densities due to hopping and drift processes is introduced for a
random potential varying slowly compared to both the magnetic length and the
phase coherence length. The flux measured in the standard transport experiment
is derived and is used to define a transport component of the flux density. The
equations of conservation can be written in terms of the transport component
only. As an illustration, the theory is applied to the Ettingshausen effect, in
which a one-dimensional spatial variation of the electron temperature is
produced perpendicular to the current.Comment: 10 pages, 1 figur
Superconductivity in diamond
We report the discovery of superconductivity in boron-doped diamond
synthesized at high pressure (8-9 GPa) and temperature (2,500-2,800 K).
Electrical resistivity, magnetic susceptibility, specific heat, and
field-dependent resistance measurements show that boron-doped diamond is a
bulk, type-II superconductor below the superconducting transition temperature
Tc=4 K; superconductivity survives in a magnetic field up to Hc2(0)=3.5 T. The
discovery of superconductivity in diamond-structured carbon suggests that Si
and Ge, which also form in the diamond structure, may similarly exhibit
superconductivity under the appropriate conditions.Comment: 13 pages, 4 figure
Some remarks on the low-energy excitations in glasses: interpretation of Boson peak data
Shot noise in mesoscopic systems
This is a review of shot noise, the time-dependent fluctuations in the
electrical current due to the discreteness of the electron charge, in small
conductors. The shot-noise power can be smaller than that of a Poisson process
as a result of correlations in the electron transmission imposed by the Pauli
principle. This suppression takes on simple universal values in a symmetric
double-barrier junction (suppression factor 1/2), a disordered metal (factor
1/3), and a chaotic cavity (factor 1/4). Loss of phase coherence has no effect
on this shot-noise suppression, while thermalization of the electrons due to
electron-electron scattering increases the shot noise slightly. Sub-Poissonian
shot noise has been observed experimentally. So far unobserved phenomena
involve the interplay of shot noise with the Aharonov-Bohm effect, Andreev
reflection, and the fractional quantum Hall effect.Comment: 37 pages, Latex, 10 figures (eps). To be published in "Mesoscopic
Electron Transport," edited by L. P. Kouwenhoven, G. Schoen, and L. L. Sohn,
NATO ASI Series E (Kluwer Academic Publishing, Dordrecht
Relativistic Dynamics and Extreme Mass Ratio Inspirals
It is now well-established that a dark, compact object (DCO), very likely a
massive black hole (MBH) of around four million solar masses is lurking at the
centre of the Milky Way. While a consensus is emerging about the origin and
growth of supermassive black holes (with masses larger than a billion solar
masses), MBHs with smaller masses, such as the one in our galactic centre,
remain understudied and enigmatic. The key to understanding these holes - how
some of them grow by orders of magnitude in mass - lies in understanding the
dynamics of the stars in the galactic neighbourhood. Stars interact with the
central MBH primarily through their gradual inspiral due to the emission of
gravitational radiation. Also stars produce gases which will subsequently be
accreted by the MBH through collisions and disruptions brought about by the
strong central tidal field. Such processes can contribute significantly to the
mass of the MBH and progress in understanding them requires theoretical work in
preparation for future gravitational radiation millihertz missions and X-ray
observatories. In particular, a unique probe of these regions is the
gravitational radiation that is emitted by some compact stars very close to the
black holes and which could be surveyed by a millihertz gravitational wave
interferometer scrutinizing the range of masses fundamental to understanding
the origin and growth of supermassive black holes. By extracting the
information carried by the gravitational radiation, we can determine the mass
and spin of the central MBH with unprecedented precision and we can determine
how the holes "eat" stars that happen to be near them.Comment: Update from the first version, 151 pages, accepted for publication @
Living Reviews in Relativit
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