390 research outputs found
Spin-Glass State in
Magnetic susceptibility, magnetization, specific heat and positive muon spin
relaxation (\musr) measurements have been used to characterize the magnetic
ground-state of the spinel compound . We observe a spin-glass
transition of the S=1/2 spins below characterized
by a cusp in the susceptibility curve which suppressed when a magnetic field is
applied. We show that the magnetization of depends on the
magnetic histo Well below , the muon signal resembles the dynamical
Kubo-Toyabe expression reflecting that the spin freezing process in results Gaussian distribution of the magnetic moments. By means of
Monte-Carlo simulati we obtain the relevant exchange integrals between the spins in this compound.Comment: 6 pages, 16 figure
Signatures of correlated magnetic phases in the local two-particle density matrix
Experiments with quantum gas microscopes have started to explore the
antiferromagnetic phase of the two-dimensional Fermi-Hubbard model and effects
of doping with holes away from half filling. In this work we show how direct
measurements of the system averaged two-spin density matrix and its full
counting statistics can be used to identify different correlated magnetic
phases with or without long-range order. We discuss examples of phases which
are potentially realized in the Hubbard model close to half filling, including
antiferrromagnetically ordered insulators and metals, as well as insulating
spin-liquids and metals with topological order. For these candidate states we
predict the doping- and temperature dependence of local correlators, which can
be directly measured in current experiments.Comment: 15 pages, 7 figure
Bell's Theorem Versus Local Realism in a Quaternionic Model of Physical Space
In the context of EPR-Bohm type experiments and spin detections confined to spacelike hypersurfaces, a local, deterministic and realistic model within a Friedmann-Robertson-Walker spacetime with a constant spatial curvature (S^3 ) is presented that describes simultaneous measurements of the spins of two fermions emerging in a singlet state from the decay of a spinless boson. Exact agreement with the probabilistic predictions of quantum theory is achieved in the model without data rejection, remote contextuality, superdeterminism or backward causation. A singularity-free Clifford-algebraic representation of S^3 with vanishing spatial curvature and non-vanishing torsion is then employed to transform the model in a more elegant form. Several event-by-event numerical simulations of the model are presented, which confirm our analytical results with the accuracy of 4 parts in 10^4 . Possible implications of our results for practical applications such as quantum security protocols and quantum computing are briefly discussed
What is the gravity dual of a chiral primary?
In the AdS/CFT correspondence a chiral primary is described by a supergravity
solution with mass equaling angular momentum. For AdS_3 X S^3 we are led to
consider three special families of metrics with this property: metrics with
conical defects, Aichelburg-Sexl type metrics generated by rotating particles,
and metrics generated by giant gravitons. We find that the first two of these
are special cases of the complete family of chiral primary metrics which can be
written down using the general solution in hep-th/0109154, but they correspond
to two extreme limits - the conical defect metrics map to CFT states generated
by twist operators that are all identical, while the Aichelburg-Sexl metrics
yield a wide dispersion in the orders of these twists. The giant graviton
solutions in contrast do not represent configurations of the D1-D5 bound state;
they correspond to fragmenting this system into two or more pieces. We look at
the large distance behavior of the supergravity fields and observe that the
excitation of these fields is linked to the existence of dispersion in the
orders and spins of the twist operators creating the chiral primary in the CFT.Comment: 33 pages, Latex, 3 figure
Spontaneous symmetry breaking in a generalized orbital compass model
We introduce a generalized two-dimensional orbital compass model, which
interpolates continuously from the classical Ising model to the orbital compass
model with frustrated quantum interactions, and investigate it using the
multiscale entanglement renormalization ansatz (MERA). The results demonstrate
that increasing frustration of exchange interactions triggers a second order
quantum phase transition to a degenerate symmetry broken state which minimizes
one of the interactions in the orbital compass model. Using boson expansion
within the spin-wave theory we unravel the physical mechanism of the symmetry
breaking transition as promoted by weak quantum fluctuations and explain why
this transition occurs only surprisingly close to the maximally frustrated
interactions of the orbital compass model. The spin waves remain gapful at the
critical point, and both the boson expansion and MERA do not find any
algebraically decaying spin-spin correlations in the critical ground state.Comment: 9 pages, 6 figures, improved presentation, version to appear in Phys.
Rev.
Microstructure from ferroelastic transitions using strain pseudospin clock models in two and three dimensions: a local mean-field analysis
We show how microstructure can arise in first-order ferroelastic structural
transitions, in two and three spatial dimensions, through a local meanfield
approximation of their pseudospin hamiltonians, that include anisotropic
elastic interactions. Such transitions have symmetry-selected physical strains
as their -component order parameters, with Landau free energies that
have a single zero-strain 'austenite' minimum at high temperatures, and
spontaneous-strain 'martensite' minima of structural variants at low
temperatures. In a reduced description, the strains at Landau minima induce
temperature-dependent, clock-like hamiltonians, with
-component strain-pseudospin vectors pointing to
discrete values (including zero). We study elastic texturing in five such
first-order structural transitions through a local meanfield approximation of
their pseudospin hamiltonians, that include the powerlaw interactions. As a
prototype, we consider the two-variant square/rectangle transition, with a
one-component, pseudospin taking values of , as in a
generalized Blume-Capel model. We then consider transitions with two-component
() pseudospins: the equilateral to centred-rectangle ();
the square to oblique polygon (); the triangle to oblique ()
transitions; and finally the 3D cubic to tetragonal transition (). The
local meanfield solutions in 2D and 3D yield oriented domain-walls patterns as
from continuous-variable strain dynamics, showing the discrete-variable models
capture the essential ferroelastic texturings. Other related hamiltonians
illustrate that structural-transitions in materials science can be the source
of interesting spin models in statistical mechanics.Comment: 15 pages, 9 figure
- …