6,826 research outputs found
"Rare" Fluctuation Effects in the Anderson Model of Localization
We discuss the role of rare fluctuation effects in quantum condensed matter
systems. In particular, we present recent numerical results of the effect of
resonant states in Anderson's original model of electron localization. We find
that such resonances give rise to anomalous behavior of eigenstates not just
far in the Lifshitz tail, but rather for a substantial fraction of eigenstates,
especially for intermediate disorder. The anomalous behavior includes
non-analyticity in various properties as a characteristic. The effect of
dimensionality on the singularity, which is present in all dimensions, is
described, and the behavior for bounded and unbounded disorder is contrasted
Singular Behavior of Eigenstates in Anderson's Model of Localization
We observe a singularity in the electronic properties of the Anderson Model
of Localization with bounded diagonal disorder, which is clearly distinct from
the well-established mobility edge (localization-delocalization transition)
that occurs in dimensions . We present results of numerical calculations
for Anderson's original (box) distribution of onsite disorder in dimensions
= 1, 2 and 3. To establish this hitherto unreported behavior, and to understand
its evolution with disorder, we contrast the behavior of two different measures
of the localization length of the electronic wavefunctions - the averaged
inverse participation ratio and the Lyapunov exponent. Our data suggest that
Anderson's model exhibits richer behavior than has been established so far.Comment: Correction to v1: Fig.3 caption now displaye
Fast preparation of critical ground states using superluminal fronts
We propose a spatio-temporal quench protocol that allows for the fast
preparation of ground states of gapless models with Lorentz invariance.
Assuming the system initially resides in the ground state of a corresponding
massive model, we show that a superluminally-moving `front' that
quenches the mass, leaves behind it (in space) a state
to the ground state of the gapless model.
Importantly, our protocol takes time to produce
the ground state of a system of size ( spatial dimensions), while
a fully adiabatic protocol requires time
to produce a state with exponential accuracy in . The physics of the
dynamical problem can be understood in terms of relativistic rarefaction of
excitations generated by the mass front. We provide proof-of-concept by solving
the proposed quench exactly for a system of free bosons in arbitrary
dimensions, and for free fermions in . We discuss the role of
interactions and UV effects on the free-theory idealization, before numerically
illustrating the usefulness of the approach via simulations on the quantum
Heisenberg spin-chain.Comment: 4.25 + 10 pages, 3 + 2 figure
Ethics and Spirituality in Indian Thought
In response to problems of Globalization, we are faced with the need for an ethics which does not feed into the problems of materialism and egocentrism. The Indian tradition provides a possible solution, especially the tradition of spiritualistic organicism. Through its spiritualistic metaphysical premises one can work out an organicismic ethics; it would be an ethical system which can provide the norms to regulate human conduct in relation to other human beings, their social organizations, other living beings and the natural environment
Hopping Conduction in Uniaxially Stressed Si:B near the Insulator-Metal Transition
Using uniaxial stress to tune the critical density near that of the sample,
we have studied in detail the low-temperature conductivity of p-type Si:B in
the insulating phase very near the metal-insulator transition. For all values
of temperature and stress, the conductivity collapses onto a single universal
scaling curve. For large values of the argument, the scaling function is well
fit by the exponentially activated form associated with variable range hopping
when electron-electron interactions cause a soft Coulomb gap in the density of
states at the Fermi energy. The temperature dependence of the prefactor,
corresponding to the T-dependence of the critical curve, has been determined
reliably for this system, and is proportional to the square-root of T. We show
explicitly that nevlecting the prefactor leads to substantial errors in the
determination of the scaling parameters and the critical exponents derived from
them. The conductivity is not consistent with Mott variable-range hopping in
the critical region nor does it obey this form for any range of the parameters.
Instead, for smaller argument of the scaling function, the conductivity of Si:B
is well fit by an exponential form with exponent 0.31 related to the critical
exponents of the system at the metal- insulator transition.Comment: 13 pages, 6 figure
Temperature and magnetization-dependent band-gap renormalization and optical many-body effects in diluted magnetic semiconductors
We calculate the Coulomb interaction induced density, temperature and
magnetization dependent many-body band-gap renormalization in a typical diluted
magnetic semiconductor GaMnAs in the optimally-doped metallic regime as a
function of carrier density and temperature. We find a large (about 0.1 eV)
band gap renormalization which is enhanced by the ferromagnetic transition. We
also calculate the impurity scattering effect on the gap narrowing. We suggest
that the temperature, magnetization, and density dependent band gap
renormalization could be used as an experimental probe to determine the valence
band or the impurity band nature of carrier ferromagnetism.Comment: Revised versio
Disordered Quantum Spin Chains with Long-Range Antiferromagnetic Interactions
We investigate the magnetic susceptibility of quantum spin chains
of spins with power-law long-range antiferromagnetic coupling as a
function of their spatial decay exponent and cutoff length . The
calculations are based on the strong disorder renormalization method which is
used to obtain the temperature dependence of and distribution
functions of couplings at each renormalization step. For the case with only
algebraic decay () we find a crossover at
between a phase with a divergent low-temperature susceptibility
for to a phase with a vanishing
for . For finite cutoff lengths
, this crossover occurs at a smaller . Additionally we
study the localization of spin excitations for by evaluating
the distribution function of excitation energies and we find a delocalization
transition that coincides with the opening of the pseudo-gap at
.Comment: 6 pages, 7 figure
Exchange coupling between silicon donors: the crucial role of the central cell and mass anisotropy
Donors in silicon are now demonstrated as one of the leading candidates for
implementing qubits and quantum information processing. Single qubit
operations, measurements and long coherence times are firmly established, but
progress on controlling two qubit interactions has been slower. One reason for
this is that the inter donor exchange coupling has been predicted to oscillate
with separation, making it hard to estimate in device designs. We present a
multivalley effective mass theory of a donor pair in silicon, including both a
central cell potential and the effective mass anisotropy intrinsic in the Si
conduction band. We are able to accurately describe the single donor properties
of valley-orbit coupling and the spatial extent of donor wave functions,
highlighting the importance of fitting measured values of hyperfine coupling
and the orbital energy of the levels. Ours is a simple framework that can
be applied flexibly to a range of experimental scenarios, but it is nonetheless
able to provide fast and reliable predictions. We use it to estimate the
exchange coupling between two donor electrons and we find a smoothing of its
expected oscillations, and predict a monotonic dependence on separation if two
donors are spaced precisely along the [100] direction.Comment: Published version. Corrected b and B values from previous versio
Structural, Magnetic and Magneto-caloric studies of Ni50Mn30Sn20Shape Memory Alloy
We have synthesized a nominal composition of Ni50Mn30Sn20 alloy using arc
melting technique. Rietveld refinement confirms the austenite L21 structure in
Fm-3m space group. Electrical resistivity has been found to clearly exhibiting
two different phenomena viz. a magnetic transition from paramagnetic to
ferromagnetic and a structural transition from austenite to martensitic phase.
Thermo-magnetization measurements M(T) confirms ferromagnetic transition
temperature TC at 222 K and martensitic transition starting at 127 K(MS).
Magnetization measurement M(H) at 10 K confirms the ferromagnetic state.
Frequency dependence of ac susceptibility \c{hi}' at low temperature suggests
spin glass behavior in the system. The isothermal magnetic entropy change
values have been found to be 1.14 J/Kg.K, 2.69 J/Kg.K and 3.9 J/Kg.K, with
refrigeration capacities of 19.6 J/kg, 37.8 J/kg and 54.6 J/kg for the field
change of 1, 2 and 3 Tesla respectively at 227 K.Comment: 16 pages text + Figs. Ni50Mn30Sn20 alloy: reasonable refrigeration
capacity tunable to Room
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