2,195 research outputs found
Two-dimensional frustrated spin systems in high magnetic fields
We discuss our numerical results on the properties of the S = 1/2 frustrated
J1-J2 Heisenberg model on a square lattice as a function of temperature and
frustration angle phi = atan(J2/J1) in an applied magnetic field. We cover the
full phase diagram of the model in the range -pi <= phi <= pi. The discussion
includes the parameter dependence of the saturation field itself, and addresses
the instabilities associated with it. We also discuss the magnetocaloric effect
of the model and show how it can be used to uniquely determine the effective
interaction constants of the compounds which were investigated experimentally.Comment: 4 pages, 5 figures, proceedings of RHMF 200
Logarithmic correction to the Bekenstein-Hawking entropy of the BTZ black hole
We derive an exact expression for the partition function of the Euclidean BTZ
black hole. Using this, we show that for a black hole with large horizon area,
the correction to the Bekenstein-Hawking entropy is , in
agreement with that for the Schwarzschild black hole obtained in the canonical
gravity formalism and also in a Lorentzian computation of BTZ black hole
entropy. We find that the right expression for the logarithmic correction in
the context of the BTZ black hole comes from the modular invariance associated
with the toral boundary of the black hole.Comment: LaTeX, 10 pages, typos corrected, clarifications adde
Strong frustration due to competing ferromagnetic and antiferromagnetic interactions: magnetic properties of M(VO)2(PO4)2 (M = Ca and Sr)
We present a detailed investigation of the magnetic properties of complex
vanadium phosphates M(VO)2(PO4)2 (M = Ca, Sr) by means of magnetization,
specific heat, 31P NMR measurements, and band structure calculations.
Experimental data evidence the presence of ferro- and antiferromagnetic
interactions in M(VO)2(PO4)2 resulting in a nearly vanishing Curie-Weiss
temperature \theta_{CW} < 1 K that contrasts with the maximum of magnetic
susceptibility at 3 K. Specific heat and NMR measurements also reveal weak
exchange couplings with the thermodynamic energy scale J_c = 10-15 K.
Additionally, the reduced maximum of the magnetic specific heat indicates
strong frustration of the spin system. Band structure calculations show that
the spin systems of the M(VO)2(PO4)2 compounds are essentially
three-dimensional with the frustration caused by competing ferro- and
antiferromagnetic interactions. Both calcium and strontium compounds undergo
antiferromagnetic long-range ordering at T_N = 1.5 K and 1.9 K, respectively.
The spin model reveals an unusual example of controllable frustration in
three-dimensional magnetic systems.Comment: 14 pages, 13 figures, 3 tables; published versio
Spin correlations and exchange in square lattice frustrated ferromagnets
The J1-J2 model on a square lattice exhibits a rich variety of different
forms of magnetic order that depend sensitively on the ratio of exchange
constants J2/J1. We use bulk magnetometry and polarized neutron scattering to
determine J1 and J2 unambiguously for two materials in a new family of vanadium
phosphates, Pb2VO(PO4)2 and SrZnVO(PO4)2, and we find that they have
ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground
state is reduced, and the diffuse magnetic scattering is enhanced, as the
predicted bond-nematic region of the phase diagram is approached.Comment: 4 pages, 4 figure
Finite-size effects in amorphous Fe90Zr10/Al75Zr25 multilayers
The thickness dependence of the magnetic properties of amorphous Fe90Zr10
layers has been explored using Fe90Zr10/Al75Zr25 multilayers. The Al75Zr25
layer thickness is kept at 40 \AA, while the thickness of the Fe90Zr10 layers
is varied between 5 and 20 \AA. The thickness of the Al75Zr25 layers is
sufficiently large to suppress any significant interlayer coupling. Both the
Curie temperature and the spontaneous magnetization decrease non-linearly with
decreasing thickness of the Fe90Zr10 layers. No ferromagnetic order is observed
in the multilayer with 5 {\AA} Fe90Zr10 layers. The variation of the Curie
temperature with the Fe90Zr10 layer thickness is fitted with a
finite-size scaling formula [1-\Tc(t)/\Tc(\infty)]=[(t-t')/t_0]^{-\lambda},
yielding , and a critical thickness \AA, below which the
Curie temperature is zero.Comment: 8 pages, 8 figure
Observation of Spin-glass-like Behavior in SrRuO3 Epitaxial Thin Films
We report the observation of spin-glass-like behavior and strong magnetic
anisotropy in extremely smooth (~1-3 \AA) roughness) epitaxial (110) and (010)
SrRuO3 thin films. The easy axis of magnetization is always perpendicular to
the plane of the film (unidirectional) irrespective of crystallographic
orientation. An attempt has been made to understand the nature and origin of
spin-glass behavior, which fits well with Heisenberg model.Comment: 5 pages, 5 Figure
Influence of ion implantation on the magnetic and transport properties of manganite films
We have used oxygen ions irradiation to generate controlled structural
disorder in thin manganite films. Conductive atomic force microscopy CAFM),
transport and magnetic measurements were performed to analyze the influence of
the implantation process in the physical properties of the films. CAFM images
show regions with different conductivity values, probably due to the random
distribution of point defect or inhomogeneous changes of the local Mn3+/4+
ratio to reduce lattice strains of the irradiated areas. The transport and
magnetic properties of these systems are interpreted in this context.
Metal-insulator transition can be described in the frame of a percolative
model. Disorder increases the distance between conducting regions, lowering the
observed TMI. Point defect disorder increases localization of the carriers due
to increased disorder and locally enhanced strain field. Remarkably, even with
the inhomogeneous nature of the samples, no sign of low field magnetoresistance
was found. Point defect disorder decreases the system magnetization but doesn t
seem to change the magnetic transition temperature. As a consequence, an
important decoupling between the magnetic and the metal-insulator transition is
found for ion irradiated films as opposed to the classical double exchange
model scenario.Comment: 27 pages, 11 Figure
Ground-state phase diagram and magnetic properties of a tetramerized spin-1/2 J_1-J_2 model: BEC of bound magnons and absence of the transverse magnetization
We study the ground state and the magnetization process of a spin-1/2
- model with a plaquette structure by using various methods. For
small inter-plaquette interaction, this model is expected to have a spin-gap
and we computed the first- and the second excitation energies. If the gap of
the lowest excitation closes, the corresponding particle condenses to form
magnetic orders. By analyzing the quintet gap and magnetic interactions among
the quintet excitations, we find a spin-nematic phase around
due to the strong frustration and the quantum effect. When high magnetic moment
is applied, not the spin-1 excitations but the spin-2 ones soften and dictate
the magnetization process. We apply a mean-field approximation to the effective
Hamiltonian to find three different types of phases (a conventional BEC phase,
``striped'' supersolid phases and a 1/2-plateau). Unlike the BEC in spin-dimer
systems, this BEC phase is not accompanied by transverse magnetization.
Possible connection to the recently discovered spin-gap compound (CuCl)LaNb2O7
is discussed.Comment: 18pages, 17figures; title changed, typos correcte
Wall Crossing and Instantons in Compactified Gauge Theory
We calculate the leading weak-coupling instanton contribution to the
moduli-space metric of N=2 supersymmetric Yang-Mills theory with gauge group
SU(2) compactified on R^3 x S^1. The results are in precise agreement with the
semiclassical expansion of the exact metric recently conjectured by Gaiotto,
Moore and Neitzke based on considerations related to wall-crossing in the
corresponding four-dimensional theory.Comment: 24 pages, no figure
Magnetic properties of BaCdVO(PO_4)_2: a strongly frustrated spin-1/2 square lattice close to the quantum critical regime
We report magnetization and specific heat measurements on polycrystalline
samples of BaCdVO(PO_4)_2 and show that this compound is a S=1/2 frustrated
square lattice with ferromagnetic nearest-neighbor (J_1) and antiferromagnetic
next-nearest-neighbor (J_2) interactions. The coupling constants J_1\simeq -3.6
K and J_2\simeq 3.2 K are determined from a fitting of the susceptibility data
and confirmed by an analysis of the saturation field (H_s=4.2 T), the specific
heat, and the magnetic entropy. BaCdVO(PO_4)_2 undergoes magnetic ordering at
about 1 K, likely towards a columnar antiferromagnetic state. We find that
BaCdVO(PO_4)_2 with the frustration ratio \alpha=J_2/J_1\simeq -0.9 is closer
to a critical (quantum spin liquid) region of the frustrated square lattice
than any of the previously reported compounds. Positive curvature of the
magnetization curve is observed in agreement with recent theoretical
predictions for high-field properties of the frustrated square lattice close to
the critical regime.Comment: 8 pages, 6 figure
- …