83,085 research outputs found
Giant Tunneling Magnetoresistance, Glassiness, and the Energy Landscape at Nanoscale Cluster Coexistence
We present microscopic results on the giant tunneling magnetoresistance that
arises from the nanoscale coexistence of ferromagnetic metallic (FMM) and
antiferromagnetic insulating (AFI) clusters in a disordered two dimensional
electron system with competing double exchange and superexchange interactions.
Our Monte Carlo study allows us to map out the different field regimes in
magnetotransport and correlate it with the evolution of spatial structures. At
coexistence, the isotropic O(3) model shows signs of slow relaxation, and has a
high density of low energy metastable states, but no genuine glassiness.
However, in the presence of weak magnetic anisotropy, and below a field
dependent irreversibility temperature , the response on field cooling
(FC) differs distinctly from that on zero field cooling (ZFC). We map out the
phase diagram of this `phase coexistence glass', highlight how its response
differs from that of a standard spin glass, and compare our results with data
on the manganites.Comment: Final published versio
Waves on Noncommutative Spacetimes
Waves on ``commutative'' spacetimes like R^d are elements of the commutative
algebra C^0(R^d) of functions on R^d. When C^0(R^d) is deformed to a
noncommutative algebra {\cal A}_\theta (R^d) with deformation parameter \theta
({\cal A}_0 (R^d) = C^0(R^d)), waves being its elements, are no longer
complex-valued functions on R^d. Rules for their interpretation, such as
measurement of their intensity, and energy, thus need to be stated. We address
this task here. We then apply the rules to interference and diffraction for d
\leq 4 and with time-space noncommutativity. Novel phenomena are encountered.
Thus when the time of observation T is so brief that T \leq 2 \theta w, where w
is the frequency of incident waves, no interference can be observed. For larger
times, the interference pattern is deformed and depends on \frac{\theta w}{T}.
It approaches the commutative pattern only when \frac{\theta w}{T} goes to 0.
As an application, we discuss interference of star light due to cosmic strings.Comment: 19 pages, 5 figures, LaTeX, added references, corrected typo
Conversion of glassy antiferromagnetic-insulating phase to equilibrium ferromagnetic-metallic phase by devitrification and recrystallization in Al substituted PrCaMnO
We show that PrCaMnO with 2.5% Al substitution and
LaCaMnO (LCMO) exhibit qualitatively similar and
visibly anomalous M-H curves at low temperature. Magnetic field causes a broad
first-order but irreversible antiferromagnetic (AF)-insulating (I) to
ferromagnetic (FM)-metallic (M) transition in both and gives rise to soft FM
state. However, the low temperature equilibrium state of
PrCaMnAlO (PCMAO) is FM-M whereas that
of LCMO is AF-I. In both the systems the respective equilibrium phase coexists
with the other phase with contrasting order, which is not in equilibrium, and
the cooling field can tune the fractions of the coexisting phases. It is shown
earlier that the coexisting FM-M phase behaves like `magnetic glass' in LCMO.
Here we show from specially designed measurement protocols that the AF-I phase
of PCMAO has all the characteristics of magnetic glassy states. It devitrifies
on heating and also recrystallizes to equilibrium FM-M phase after annealing.
This glass-like AF-I phase also shows similar intriguing feature observed in
FM-M magnetic glassy state of LCMO that when the starting coexisting fraction
of glass is larger, successive annealing results in larger fraction of
equilibrium phase. This similarity between two manganite systems with
contrasting magnetic orders of respective glassy and equilibrium phases points
toward a possible universality.Comment: Highlights potential of CHUF (Cooling and Heating in Unequal Fields),
a new measurement protoco
Dissipative dynamics of a Harmonic Oscillator : A non-perturbative approach
Starting from a microscopic theory, we derive a master equation for a
harmonic oscillator coupled to a bath of non-interacting oscillators. We follow
a non-perturbative approach, proposed earlier by us for the free Brownian
particle. The diffusion constants are calculated analytically and the
positivity of the Master Equation is shown to hold above a critical
temperature. We compare the long time behaviour of the average kinetic and
potential energies with known thermodynamic results. In the limit of vainishing
oscillator frequency of the system, we recover the results of the free Brownian
particle.Comment: 7 pages, 3 figure
Interacting Quantum Topologies and the Quantum Hall Effect
The algebra of observables of planar electrons subject to a constant
background magnetic field B is given by A_theta(R^2) x A_theta(R^2) the product
of two mutually commuting Moyal algebras. It describes the free Hamiltonian and
the guiding centre coordinates. We argue that A_theta(R^2) itself furnishes a
representation space for the actions of these two Moyal algebras, and suggest
physical arguments for this choice of the representation space. We give the
proper setup to couple the matter fields based on A_theta(R^2) to
electromagnetic fields which are described by the abelian commutative gauge
group G_c(U(1)), i.e. gauge fields based on A_0(R^2). This enables us to give a
manifestly gauge covariant formulation of integer quantum Hall effect (IQHE).
Thus, we can view IQHE as an elementary example of interacting quantum
topologies, where matter and gauge fields based on algebras A_theta^prime with
different theta^prime appear. Two-particle wave functions in this approach are
based on A_theta(R^2) x A_theta(R^2). We find that the full symmetry group in
IQHE, which is the semi-direct product SO(2) \ltimes G_c(U(1)) acts on this
tensor product using the twisted coproduct Delta_theta. Consequently, as we
show, many particle sectors of each Landau level have twisted statistics. As an
example, we find the twisted two particle Laughlin wave functions.Comment: 10 pages, LaTeX, Corrected typos, Published versio
Noncommutative BTZ Black Hole and Discrete Time
We search for all Poisson brackets for the BTZ black hole which are
consistent with the geometry of the commutative solution and are of lowest
order in the embedding coordinates. For arbitrary values for the angular
momentum we obtain two two-parameter families of contact structures. We obtain
the symplectic leaves, which characterize the irreducible representations of
the noncommutative theory. The requirement that they be invariant under the
action of the isometry group restricts to symplectic leaves,
where is associated with the Schwarzschild time. Quantization may then lead
to a discrete spectrum for the time operator.Comment: 10 page
Magnetic properties of EuPtSi single crystals
Single crystals of EuPtSi, which crystallize in the BaNiSn-type
crystal structure, have been grown by high temperature solution growth method
using molten Sn as the solvent. EuPtSi which lacks the inversion symmetry
and has only one Eu site in the unit cell is found to be an antiferromagnet
with two successive magnetic transitions at = 17 K and = 16 K, as inferred from magnetic susceptibility, heat capacity and
Eu M\"ossbauer measurements. The isothermal magnetization data for [001] reveal a metamagnetic transition at a critical field = 1 T. The magnetization saturates to a moment value of 6.43 /Eu above 5.9 T (9.2 T) for [001] ([100]), indicating that
these fields are spin-flip fields for the divalent Eu moments along the two
axes. The origin of this anisotropic behaviour is discussed. A magnetic (H, T)
phase diagram has been constructed from the temperature dependence of
isothermal magnetization data. The reduced jump in the heat capacity at indicates a transition to an incommensurate, amplitude modulated
antiferromagnetic structure. The shape of the hyperfine field split M\"ossbauer
spectrum at provides additional support for the proposed nature of
this magnetic transition.Comment: 6 pages, 6 figures. Submitted to Phys. Rev.
Modified 2D Proca Theory: Revisited Under BRST and (Anti-)Chiral Superfield Formalisms
Within the framework of Becchi-Rouet-Stora-Tyutin (BRST) approach, we discuss
mainly the fermionic (i.e. off-shell nilpotent) (anti-)BRST, (anti-)co-BRST and
some discrete dual-symmetries of the appropriate Lagrangian densities for a two
(1+1)-dimensional (2D) modified Proca (i.e. a massive Abelian 1-form) theory
without any interaction with matter fields. One of the novel observations of
our present investigation is the existence of some kinds of restrictions in the
case of our present St\"{u}ckelberg-modified version of the 2D Proca theory
which is not like the standard Curci-Ferrari (CF)-condition of a non-Abelian
1-form gauge theory. Some kinds of similarities and a few differences between
them have been pointed out in our present investigation. To establish the
sanctity of the above off-shell nilpotent (anti-)BRST and (anti-)co-BRST
symmetries, we derive them by using our newly proposed (anti-)chiral superfield
formalism where a few specific and appropriate sets of invariant quantities
play a decisive role. We express the (anti-)BRST and (anti-)co-BRST conserved
charges in terms of the superfields that are obtained after the applications of
(anti-)BRST and (anti-)co-BRST invariant restrictions and prove their off-shell
nilpotency and absolute anticommutativity properties, too. Finally, we make
some comments on (i) the novelty of our restrictions/obstructions, and (ii) the
physics behind the negative kinetic term associated with the pseudo-scalar
field of our present theory.Comment: LaTeX file, 58 pages, Journal reference give
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