Vector spin glasses with random short-range anisotropic couplings are studied using the defect-wall renormalization-group method. By calculating the correlation length exponent \ensuremath{\nu} and the fractal dimension of the defect wall,
ds, we obtain the first direct evidence that in two dimensions these systems have a zero-temperature transition in the same universality class as the Ising spin glass. We also present results which strongly suggest that these systems have a transition at nonzero temperature in three dimensions.
We study the anisotropic Heisenberg spin-glass model in a three-dimensional hierarchical lattice (designed to approximate the cubic lattice), within a real-space renormalization-group approach. Two different initial probability distributions for the exchange interaction (
Jij), Gaussian and uniform, are used, with zero mean and width
Jˉ. The
(kT/Jˉ)×Δ0 phase diagram is obtained, where
T is the temperature,
Δ0 is the first moment of the probability distribution for the uniaxial anisotropy, and
k is the Boltzmann constant. For the Ising model (
Δ0=1), there is a spin-glass phase at low temperatures (high
Jˉ) and a paramagnetic phase at high temperatures (low
Jˉ). For the isotropic Heisenberg model (
Δ0=0), our results indicate no spin-glass phase at finite temperatures. The transition temperature between the spin-glass and paramagnetic phase decreases with
Δ0, as expected, but goes to zero at a finite value of the anisotropy parameter, namely
Δ0=Δc∼0.59. Our results indicate that the whole transition line, between the paramagnetic and the spin-glass phases, for
Δc≤Δ0<1, belongs to the same universality class as the transition for the Ising spin glass.
The one-dimensional long-range Ising spin glass provides useful insights into the properties of finite-dimensional spin glasses with short-range interactions. The defect energy renormalization-group equations derived for it by Kotliar, Anderson, and Stein have been reexamined and a new fixed point has been found. The fixed points which have previously been studied are found to be inappropriate. It is shown that the renormalization-group equations themselves directly imply that that the spin-glass phase is replica symmetric, in agreement with droplet model expectations, when the range exponent $\ensuremath{\sigma}$ lies between the upper and lower critical values.
We compare the critical behavior of the short-range Ising spin glass with a spin glass with long-range interactions which fall off as a power $\ensuremath{\sigma}$ of the distance. We show that there is a value of $\ensuremath{\sigma}$ of the long-range model for which the critical behavior is very similar to that of the short-range model in four dimensions. We also study a value of $\ensuremath{\sigma}$ for which we find the critical behavior to be compatible with that of the three-dimensional model, although we have much less precision than in the four-dimensional case.
We investigate the
XY spin-glass model in two and three dimensions using the domain-wall renormalization-group method. The results for systems of linear sizes up to
L=12 (2D) and
L=8 (3D) strongly suggest that the lower critical dimension for spin-glass ordering may be ${d}_{\mathrm{c}}\ensuremath{\approx}3$, rather than 4 as is commonly believed. Our 3D data favor the scenario of a low but finite spin-glass ordering temperature below the chiral transition, but they are also compatible with the system being at or slightly below its lower critical dimension.
The transverse-field Ising models with random exchange interactions in finite dimensions are investigated by means of a real-space renormalization-group method. The scheme yields the exact values of the critical point and critical exponent $\ensuremath{\nu}$ in one dimension and some previous results in the case of random ferromagnetic interactions are reproduced in two and three dimensions. We apply the scheme to spin glasses in transverse fields in two and three dimensions, which have not been analyzed very extensively. The phase diagrams and the critical exponent $\ensuremath{\nu}$ are obtained and evidence for the existence of an infinite-randomness fixed point in these models is found.