740 research outputs found
Thermodynamic Limit for the Ising Model on the Cayley Tree
While the Ising model on the Cayley tree has no spontaneous magnetization at
nonzero temperatures in the thermodynamic limit, we show that finite systems of
astronomical sizes remain magnetically ordered in a wide temperature range, if
the symmetry is broken by fixing an arbitrary single (bulk or surface) spin. We
compare the behavior of the finite size magnetization of this model with that
of the Ising model on both the Sierpinski Gasket, and the one-dimensional
linear chain. This comparison reveals the analogy of the behavior of the
present model with the Sierpinski Gasket case.Comment: 5 pages, 4 figure
Chiral spin currents and spectroscopically accessible single merons in quantum dots
We provide unambiguous theoretical evidence for the formation of
correlation-induced isolated merons in rotationally-symmetric quantum dots. Our
calculations rely on neither the lowest-Landau-level approximation, nor on the
maximum-density-droplet approximation, nor on the existence of a spin-polarized
state. For experimentally accessible system parameters, unbound merons condense
in the ground state at magnetic fields as low as T and for as few
as N = 3 confined fermions. The four-fold degenerate ground-state at
corresponds to four orthogonal merons characterized by their
topological chirality and charge . This degeneracy is lifted by the
Rashba and Dresselhaus spin-orbit interaction, which we include perturbatively,
yielding spectroscopic accessibility to individual merons. We further derive a
closed-form expression for the topological chirality in the form of a chiral
spin current and use it to both characterize our states and predict the
existence of other topological textures in other regions of phase space, for
example, at N=5. Finally, we compare the spin textures of our numerically exact
meron states to ansatz wave-functions of merons in quantum Hall droplets and
find that the ansatz qualitatively describes the meron states.Comment: 4 pages, 5 figures; minor title change, typos fixe
Anomalous Behavior of the Zero Field Susceptibility of the Ising Model on the Cayley Tree
It is found that the zero field susceptibility chi of the Ising model on the
Cayley tree exhibits unusually weak divergence at the critical point Tc. The
susceptibility amplitude is found to diverge at Tc proportionally to the tree
generation level n, while the behavior of chi is otherwise analytic in the
vicinity of Tc, with the critical exponent gamma=0.Comment: 3 pages, 2 figure
Critical behavior of 2 and 3 dimensional ferro- and antiferromagnetic spin ice systems in the framework of the Effective Field Renormalization Group technique
In this work we generalize and subsequently apply the Effective Field
Renormalization Group technique to the problem of ferro- and
antiferromagnetically coupled Ising spins with local anisotropy axes in
geometrically frustrated geometries (kagome and pyrochlore lattices). In this
framework, we calculate the various ground states of these systems and the
corresponding critical points. Excellent agreement is found with exact and
Monte Carlo results. The effects of frustration are discussed. As pointed out
by other authors, it turns out that the spin ice model can be exactly mapped to
the standard Ising model but with effective interactions of the opposite sign
to those in the original Hamiltonian. Therefore, the ferromagnetic spin ice is
frustrated, and does not order. Antiferromagnetic spin ice (in both 2 and 3
dimensions), is found to undergo a transition to a long range ordered state.
The thermal and magnetic critical exponents for this transition are calculated.
It is found that the thermal exponent is that of the Ising universality class,
whereas the magnetic critical exponent is different, as expected from the fact
that the Zeeman term has a different symmetry in these systems. In addition,
the recently introduced Generalized Constant Coupling method is also applied to
the calculation of the critical points and ground state configurations. Again,
a very good agreement is found with both exact, Monte Carlo, and
renormalization group calculations for the critical points. Incidentally, we
show that the generalized constant coupling approach can be regarded as the
lowest order limit of the EFRG technique, in which correlations outside a
frustrated unit are neglected, and scaling is substituted by strict equality of
the thermodynamic quantities.Comment: 28 pages, 9 figures, RevTeX 4 Some minor changes in the conclussions.
One reference adde
Evidence for the Sr2RuO4 intercalations in the Sr3Ru2O7 region of the Sr3Ru2O7-Sr2RuO4 eutectic system
Although Sr3Ru2O7 has not been reported to exhibit superconductivity so far,
ac susceptibility measurements revealed multiple superconducting transitions
occurring in the Sr3Ru2O7 region cut from Sr3Ru2O7-Sr2RuO4 eutectic crystals.
Based on various experimental results, some of us proposed the scenario in
which Sr2RuO4 thin slabs with a few layers of the RuO2 plane are embedded in
the Sr3Ru2O7 region as stacking faults and multiple superconducting transitions
arise from the distribution of the slab thickness. To examine this scenario, we
measured the resistivity along the ab plane (rho_ab) using a Sr3Ru2O7-region
sample cut from the eutectic crystal, as well as along the c axis (rho_c) using
the same crystal. As a result, we detected resistance drops associated with
superconductivity only in rho_ab, but not in rho_c. These results support the
Sr2RuO4 thin-slab scenario. In addition, we measured the resistivity of a
single crystal of pure Sr3Ru2O7 with very high quality and found that pure
Sr3Ru2O7 does not exhibit superconductivity down to 15 mK.Comment: 4 pages, 2 figures, Int. Conf. on Low Temperature Physics (LT25
Tunnelling spectroscopy of the interface between Sr2RuO4 and a single Ru micro-inclusion in eutectic crystals
The understanding of the zero bias conductance peak (ZBCP) in the tunnelling
spectra of S/N junctions involving d-wave cuprate superconductors has been
important in the determination of the phase structure of the superconducting
order parameter. In this context, the involvement of a p-wave superconductor
such as Sr2RuO4 in tunnelling studies is indeed of great importance. We have
recently succeeded in fabricating devices that enable S/N junctions forming at
interfaces between Sr2RuO4 and Ru micro-inclusions in eutectic crystals to be
investigated.3 We have observed a ZBCP and have interpreted it as due to the
Andreev bound state, commonly seen in unconventional superconductors. Also we
have proposed that the onset of the ZBCP may be used to delineate the phase
boundary for the onset of a time reversal symmetry broken (TRSB) state within
the superconducting state, which does not always coincide with the onset of the
superconducting state. However, these measurements always involved two
interfaces between Sr2RuO4 and Ru. In the present study, we have extended the
previous measurements to obtain a deeper insight into the properties of a
single interface between Sr2RuO4 and Ru.Comment: To appear in J. Phys. Soc. Jpn. Vol. 75 No.12 issu
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