405 research outputs found
A superconductor to superfluid phase transition in liquid metallic hydrogen
Although hydrogen is the simplest of atoms, it does not form the simplest of
solids or liquids. Quantum effects in these phases are considerable (a
consequence of the light proton mass) and they have a demonstrable and often
puzzling influence on many physical properties, including spatial order. To
date, the structure of dense hydrogen remains experimentally elusive. Recent
studies of the melting curve of hydrogen indicate that at high (but
experimentally accessible) pressures, compressed hydrogen will adopt a liquid
state, even at low temperatures. In reaching this phase, hydrogen is also
projected to pass through an insulator-to-metal transition. This raises the
possibility of new state of matter: a near ground-state liquid metal, and its
ordered states in the quantum domain. Ordered quantum fluids are traditionally
categorized as superconductors or superfluids; these respective systems feature
dissipationless electrical currents or mass flow. Here we report an analysis
based on topological arguments of the projected phase of liquid metallic
hydrogen, finding that it may represent a new type of ordered quantum fluid.
Specifically, we show that liquid metallic hydrogen cannot be categorized
exclusively as a superconductor or superfluid. We predict that, in the presence
of a magnetic field, liquid metallic hydrogen will exhibit several phase
transitions to ordered states, ranging from superconductors to superfluids.Comment: for a related paper see cond-mat/0410425. A correction to the front
page caption appeared in Oct 14 issue of Nature:
http://www.nature.com/nature/links/041014/041014-11.htm
Violation of the London Law and Onsager-Feynman quantization in multicomponent superconductors
Non-classical response to rotation is a hallmark of quantum ordered states
such as superconductors and superfluids. The rotational responses of all
currently known single-component "super" states of matter (superconductors,
superfluids and supersolids) are largely described by two fundamental
principles and fall into two categories according to whether the systems are
composed of charged or neutral particles: the London law relating the angular
velocity to a subsequently established magnetic field and the Onsager-Feynman
quantization of superfluid velocity. These laws are theoretically shown to be
violated in a two-component superconductor such as the projected liquid
metallic states of hydrogen and deuterium at high pressures. The rotational
responses of liquid metallic hydrogen or deuterium identify them as a new class
of dissipationless states; they also directly point to a particular
experimental route for verification of their existence.Comment: Nature Physics in print. This is an early version of the paper. The
final version will be posted 6 months after its publication Nature Physics,
according to the journal polic
A quantum fluid of metallic hydrogen suggested by first-principles calculations
It is generally assumed that solid hydrogen will transform into a metallic
alkali-like crystal at sufficiently high pressure. However, some theoretical
models have also suggested that compressed hydrogen may form an unusual
two-component (protons and electrons) metallic fluid at low temperature, or
possibly even a zero-temperature liquid ground state. The existence of these
new states of matter is conditional on the presence of a maximum in the melting
temperature versus pressure curve (the 'melt line'). Previous measurements of
the hydrogen melt line up to pressures of 44 GPa have led to controversial
conclusions regarding the existence of this maximum. Here we report ab initio
calculations that establish the melt line up to 200 GPa. We predict that subtle
changes in the intermolecular interactions lead to a decline of the melt line
above 90 GPa. The implication is that as solid molecular hydrogen is
compressed, it transforms into a low-temperature quantum fluid before becoming
a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and
its isotopes bears analogies with the familiar phases of 3He and 4He, the only
known zero-temperature liquids, but the long-range Coulombic interactions and
the large component mass ratio present in hydrogen would ensure dramatically
different propertiesComment: See related paper: cond-mat/041040
Experimental observation of moving intrinsic localized modes in germanium
Deep level transient spectroscopy shows that defects created by alpha
irradiation of germanium are annealed by low energy plasma ions up to a depth
of several thousand lattice units. The plasma ions have energies of 2-8eV and
therefore can deliver energies of the order of a few eV to the germanium atoms.
The most abundant defect is identified as the E-center, a complex of the dopant
antimony and a vacancy with and annealing energy of 1.3eV as determined by our
measurements. The inductively coupled plasma has a very low density and a very
low flux of ions. This implies that the ion impacts are almost isolated both in
time and at the surface of the semiconductor. We conclude that energy of the
order of an eV is able to travel a large distance in germanium in a localized
way and is delivered to the defects effectively. The most likely candidates are
vibrational nonlinear wave packets known as intrinsic localized modes, which
exist for a limited range of energies. This property is coherent with the fact
that more energetic ions are less efficient at producing the annealing effect.Comment: 20 pages, 10 figure
Ewald method for polytropic potentials in arbitrary dimensionality
The Ewald summation technique is generalised to power-law 1/|r|^k potentials
in three-, two- and one-dimensional geometries with explicit formulae for all
the components of the sums. The cases of short-range, long-range and "marginal"
interactions are treated separately. The jellium model, as a particular case of
a charge-neutral system, is discussed and the explicit forms of the Ewald sums
for such system are presented. A generalised form of the Ewald sums for a
noncubic (nonsquare) simulation cell for three- (two-) dimensional geometry is
obtained and its possible field of application is discussed. A procedure for
the optimisation of the involved parameters in actual simulations is developed
and an example of its application is presented.Comment: 41 pages, 3 figure
The origin of defects induced in ultra-pure germanium by Electron Beam Deposition
The creation of point defects in the crystal lattices of various
semiconductors by subthreshold events has been reported on by a number of
groups. These observations have been made in great detail using sensitive
electrical techniques but there is still much that needs to be clarified.
Experiments using Ge and Si were performed that demonstrate that energetic
particles, the products of collisions in the electron beam, were responsible
for the majority of electron-beam deposition (EBD) induced defects in a
two-step energy transfer process. Lowering the number of collisions of these
energetic particles with the semiconductor during metal deposition was
accomplished using a combination of static shields and superior vacuum
resulting in devices with defect concentrations lower than cm, the measurement limit of our deep level transient
spectroscopy (DLTS) system. High energy electrons and photons that samples are
typically exposed to were not influenced by the shields as most of these
particles originate at the metal target thus eliminating these particles as
possible damage causing agents. It remains unclear how packets of energy that
can sometimes be as small of 2eV travel up to a m into the material while
still retaining enough energy, that is, in the order of 1eV, to cause changes
in the crystal. The manipulation of this defect causing phenomenon may hold the
key to developing defect free material for future applications.Comment: 18 pages, 9 figure
Electron-hole symmetry in a semiconducting carbon nanotube quantum dot
Optical and electronic phenomena in solids arise from the behaviour of
electrons and holes (unoccupied states in a filled electron sea). Electron-hole
symmetry can often be invoked as a simplifying description, which states that
electrons with energy above the Fermi sea behave the same as holes below the
Fermi energy. In semiconductors, however, electron-hole symmetry is generally
absent since the energy band structure of the conduction band differs from the
valence band. Here we report on measurements of the discrete, quantized-energy
spectrum of electrons and holes in a semiconducting carbon nanotube. Through a
gate, an individual nanotube is filled controllably with a precise number of
either electrons or holes, starting from one. The discrete excitation spectrum
for a nanotube with N holes is strikingly similar to the corresponding spectrum
for N electrons. This observation of near perfect electron-hole symmetry
demonstrates for the first time that a semiconducting nanotube can be free of
charged impurities, even in the limit of few-electrons or holes. We furthermore
find an anomalously small Zeeman spin splitting and an excitation spectrum
indicating strong electron-electron interactions.Comment: 12 pages, 4 figure
The phases of deuterium at extreme densities
We consider deuterium compressed to higher than atomic, but lower than
nuclear densities. At such densities deuterium is a superconducting quantum
liquid. Generically, two superconducting phases compete, a "ferromagnetic" and
a "nematic" one. We provide a power counting argument suggesting that the
dominant interactions in the deuteron liquid are perturbative (but screened)
Coulomb interactions. At very high densities the ground state is determined by
very small nuclear interaction effects that probably favor the ferromagnetic
phase. At lower densities the symmetry of the theory is effectively enhanced to
SU(3), and the quantum liquid enters a novel phase, neither ferromagnetic nor
nematic. Our results can serve as a starting point for investigations of the
phase dynamics of deuteron liquids, as well as exploration of the stability and
dynamics of the rich variety of topological objects that may occur in phases of
the deuteron quantum liquid, which range from Alice strings to spin skyrmions
to Z_2 vortices.Comment: 9 pages, 6 figures; v2: fixed typo
Crossover from mesoscopic to universal phase for electron transmission in quantum dots
Measuring phase in coherent electron systems (mesoscopic systems) provides
ample information not easily revealed by conductance measurements. Phase
measurements in relatively large quantum dots (QDs) recently demonstrated a
universal like phase evolution independent of dot size, shape, and occupancy.
Explicitly, in Coulomb blockaded QDs the transmission phase increased
monotonically by pi throughout each conductance peak, thereafter, in the
conductance valleys the phase returned sharply to its base value. Expected
mesoscopic features in the phase, related to spin degeneracy or to exchange
effects, were never observed. Presently, there is no satisfactory full
explanation for the observed phase universality. Unfortunately, the phase in a
few-electron QDs, where it can be better understood was never measured. Here we
report on such measurements on a small QD that occupy only 1-20 electrons. Such
dot was embedded in one arm of a two path electron interferometer, with an
electron counter near the dot. Unlike the repetitive behavior found in larger
dots we found now mesoscopic features for dot occupation of less than some 10
electrons. An unexpected feature in this regime is a clear observation of the
occupation of two different orbital states by the first two electrons -
contrary to the recent publications. As the occupation increased the phase
evolved and turned universal like for some 14 electrons and higher. The present
measurements allowed us to determine level occupancy and parity. More
importantly, they suggest that QDs go through a phase transition, from
mesoscopic to universal like behavior, as the occupancy increases. These
measurements help in singling out potential few theoretical models among the
many proposed.Comment: 12 pages, 6 figure
Dynamic screening of a localized hole during photoemission from a metal cluster
Recent advances in attosecond spectroscopy techniques have fueled the
interest in the theoretical description of electronic processes taking place in
the subfemtosecond time scale. Here we study the coupled dynamic screening of a
localized hole and a photoelectron emitted from a metal cluster using a
semi-classical model. Electron density dynamics in the cluster is calculated
with Time-Dependent Density Functional Theory and the motion of the
photoemitted electron is described classically. We show that the dynamic
screening of the hole by the cluster electrons affects the motion of the
photoemitted electron. At the very beginning of its trajectory, the
photoemitted electron interacts with the cluster electrons that pile up to
screen the hole. Within our model, this gives rise to a significant reduction
of the energy lost by the photoelectron. Thus, this is a velocity dependent
effect that should be accounted for when calculating the average losses
suffered by photoemitted electrons in metals.Comment: 15 pages, 5 figure
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