408 research outputs found
Quantum integrability and nonintegrability in the spin-boson model
We study the spectral properties of a spin-boson Hamiltonian that depends on
two continuous parameters (interaction strength) and
(integrability switch). In the classical limit this
system has two distinct integrable regimes, and . For
each integrable regime we can express the quantum Hamiltonian as a function of
two action operators. Their eigenvalues (multiples of ) are the natural
quantum numbers for the complete level spectrum. This functional dependence
cannot be extended into the nonintegrable regime . Here level
crossings are prohibited and the level spectrum is naturally described by a
single (energy sorting) quantum number. In consequence, the tracking of
individual eigenstates along closed paths through both regimes leads to
conflicting assignments of quantum numbers. This effect is a useful and
reliable indicator of quantum chaos -- a diagnostic tool that is independent of
any level-statistical analysis
Writing electronic ferromagnetic states in a high-temperature paramagnetic nuclear spin system
In this paper we use the Nuclear Magnetic Resonance (NMR) to write eletronic
states of a ferromagnetic system into a high-temperature paramagnetic nuclear
spins. Through the control of phase and duration of radiofrequency pulses we
set the NMR density matrix populations, and apply the technique of quantum
state tomography to experimentally obtain the matrix elements of the system,
from which we calculate the temperature dependence of magnetization for
different magnetic fields. The effects of the variation of temperature and
magnetic field over the populations can be mapped in the angles of spins
rotations, carried out by the RF pulses. The experimental results are compared
to the Brillouin functions of ferromagnetic ordered systems in the mean field
approximation for two cases: the mean field is given by (i)
and (ii) , where is the external
magnetic field, and are mean field parameters. The
first case exhibits second order transition, whereas the second case has first
order transition with temperature hysteresis. The NMR simulations are in good
agreement with the magnetic predictions
Dynamic properties of quantum spin chains: Simple route to complex behavior
We examine dynamic structure factors of spin-1/2 chains with nearest-neighbor
interactions of XX and Dzyaloshinskii-Moriya type, and with periodic and random
changes in the sign of these interactions. This special kind of inhomogeneity
can be eliminated from the Hamiltonian by suitable transformation of the spin
variables. As a result, the dynamic structure factors of periodic or random
chains can be computed from those of the uniform chains. Using the exact
analytical and precise numerical results available for the uniform systems we
illustrate the effects of regular alternation or random disorder on dynamic
structure factors of quantum spin chains
VI3 - a new layered ferromagnetic semiconductor
Two-dimensional (2D) materials are promising candidates for next-generation
electronic devices. In this regime, insulating 2D ferromagnets, which remain
rare, are of special importance due to their potential for enabling new device
architectures. Here we report the discovery of ferromagnetism in a layered van
der Waals semiconductor, VI3, which is based on honeycomb vanadium layers
separated by an iodine-iodine van der Waals gap. It has a BiI3-type structure
(R-3, No.148) at room temperature, and our experimental evidence suggests that
it may undergo a subtle structural phase transition at 78 K. VI3 becomes
ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly
shows ferromagnetic domains, which can be manipulated by the applied external
magnetic field. The optical band gap determined by reflectance measurements is
0.6 eV, and the material is highly resistive
Charge and spin dynamics in the one-dimensional and models
The impact of the spin-flip terms on the (static and dynamic) charge and spin
correlations in the Luttinger-liquid ground state of the 1D model is
assessed by comparison with the same quantities in the 1D model, where
spin-flip terms are absent. We employ the recursion method combined with a
weak-coupling or a strong-coupling continued-fraction analysis. At
we use the Pfaffian representation of dynamic spin correlations. The changing
nature of the dynamically relevant charge and spin excitations on approach of
the transition to phase separation is investigated in detail. The
charge excitations (but not the spin excitations) at the transition have a
single-mode nature, whereas charge and spin excitations have a complicated
structure in the model. In the model, phase separation is
accompanied by N\'eel long-range order, caused by the condensation of electron
clusters with an already existing alternating up-down spin configuration
(topological long-range order). In the model, by contrast, the spin-flip
processes in the exchange coupling are responsible for continued strong spin
fluctuations (dominated by 2-spinon excitations) in the phase-separated state.Comment: 11 pages (RevTex). 14 Figures available from author
Dynamics of spin correlations in the spin-1/2 isotropic XY chain in a transverse field
Dynamic xx spin pair correlation functions for the isotropic spin-1/2 XY
chain are calculated numerically for long open chains in the presence of a
transverse magnetic field at finite temperature. As an application we discuss
the temperature dependence of the spin-spin relaxation time in PrCl_3.Comment: 2 pages, latex, 2 figures, abstract of the paper presented at Ampere
Summer School ``Applications of Magnetic Resonance in Novel Materials''
Nafplion, Greece, 3-9 September, 2000, partially published in J. Phys. A:
Math. Gen. 33, 3063 (2000
Random antiferromagnetic quantum spin chains: Exact results from scaling of rare regions
We study XY and dimerized XX spin-1/2 chains with random exchange couplings
by analytical and numerical methods and scaling considerations. We extend
previous investigations to dynamical properties, to surface quantities and
operator profiles, and give a detailed analysis of the Griffiths phase. We
present a phenomenological scaling theory of average quantities based on the
scaling properties of rare regions, in which the distribution of the couplings
follows a surviving random walk character. Using this theory we have obtained
the complete set of critical decay exponents of the random XY and XX models,
both in the volume and at the surface. The scaling results are confronted with
numerical calculations based on a mapping to free fermions, which then lead to
an exact correspondence with directed walks. The numerically calculated
critical operator profiles on large finite systems (L<=512) are found to follow
conformal predictions with the decay exponents of the phenomenological scaling
theory. Dynamical correlations in the critical state are in average
logarithmically slow and their distribution show multi-scaling character. In
the Griffiths phase, which is an extended part of the off-critical region
average autocorrelations have a power-law form with a non-universal decay
exponent, which is analytically calculated. We note on extensions of our work
to the random antiferromagnetic XXZ chain and to higher dimensions.Comment: 19 pages RevTeX, eps-figures include
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