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In cluster-based quasicrystals, tetrahedra located in conventional Tsai clusters may be replaced by single rare-earth (R) ions at the cluster centers (pseudo-Tsai clusters). In this study, we investigate the effect of the pseudo-Tsai cluster incorporation on the magnetic structures of two approximants, the Tsai-type Tb-Au-Si [denoted TAS(0)] and Ho-Au-Si [denoted HAS(52)] with partial replacement of conventional Tsai clusters by pseudo-Tsai clusters, up to 52%. The mixture of Tsai and pseudo-Tsai clusters can be considered a different source of randomness/disorder other than the conventional chemical mix sites (Au/Si). The effect of the latter has been previously discussed regarding the origin/cause of spin-glass-like ordering and Anderson localization of electronic states in quasicrystals and approximant crystals. Single crystal neutron diffraction experiments at 2 K were performed and bulk physical properties (magnetization and specific heat) were investigated. In addition, earlier collected powder neutron diffraction data of TAS(14) with 14% replacement was reanalyzed in light of the results on TAS(0) and HAS(52). We find that the arrangement of ordered magnetic spins in the icosahedral shells of these phases is similar, while the cluster-center R magnetic states are different. In the case of TAS(14), the cluster-center Tb magnetic moments seem to affect the arrangement of surrounding icosahedral magnetic moments, and the magnetic structure of the icosahedral shell deviates from that of TAS(0). In the case of HAS(52), however, the icosahedral R magnetic moments are less affected by the cluster-center R, while the averaged cluster-center R magnetic moments are significantly diminished. We discuss these results considering the magnetic ordering effect on the bulk physical properties.

We present a comprehensive experimental and theoretical study of the kagome ice Coulomb phase that explores the fine-tuning of critical correlations by applied field, temperature, and crystal orientation. The continuous modification of algebraic correlations is observed by polarized neutron scattering experiments and is found to be well described by numerical simulations of an idealized model. We further clarify the thermodynamics of field-tuned Kasteleyn transitions and demonstrate some dramatic finite-size-scaling properties that depend on how topological string defects wind around the system boundaries. We conclude that kagome ice is a remarkable example of a critical and topological state in a real system that may be subject to fine experimental control., QC 20220330

We report neutron scattering, magnetic susceptibility and Monte Carlo theoretical analysis to verify the short-range nature of the magnetic structure and spin-spin correlations in a Yb3Ga5O12 single crystal. The quantum spin state of Yb3+ in Yb3Ga5O12 is verified. The quantum spins organize into a short-ranged emergent director state for T < 0.6 K derived from anisotropy and near-neighbor exchange. We derive the magnitude of the nearneighbor exchange interactions 0.6 < J(1) < 0.7 K, J(2) = 0.12 K and the magnitude of the dipolar exchange interaction, D, in the range 0.18 < D < 0.21 K. Certain aspects of the broad experimental dataset can be modeled using a J(1)D model with ferromagnetic near-neighbor spin-spin correlations while other aspects of the data can be accurately reproduced using a J(1)J(2)D model with antiferromagnetic near-neighbor spin-spin correlation. As such, although we do not quantify all the relevant exchange interactions, we nevertheless provide a strong basis for the understanding of the complex Hamiltonian required to fully describe the magnetic state of Yb3Ga5O12., QC 20210831

Spin ice may be considered to be a model system for the investigation of pinch point scattering. We present very-high-resolution numerical simulations and an analytical theory of the pinch point profiles of the near-neighbor and dipolar spin ice models and find these to be in excellent agreement with each other and with existing theory. Most importantly, the pinch points of the dipolar spin ice model are infinitely sharp, as a result of unscreened dipolar fields. These results are compared to polarized neutron scattering measurements of the pinch point profiles in Ho2Ti2O7, considered to be an accurate realization of dipolar spin ice. In contrast to the numerical and analytical results, the experimental pinch point profiles are shown to be broadened in a manner that is quantitatively consistent with fully screened dipolar fields. This striking paradox is not easily resolved: Possible resolutions implicate quantum fluctuations or fundamental corrections to the theory of simulation or polarized neutron scattering. We further discuss our results in the context of spin ice's role as a model Coulomb fluid., QC 20210127

Determining the fate of the Pauling entropy in the classical spin ice material Dy2Ti2O7 with respect to the third law of thermodynamics has become an important test case for understanding the existence and stability of ice-rule states in general. The standard model of spin ice-the dipolar spin ice model-predicts an ordering transition at T approximate to 0.15 K, but recent experiments by Pomaranski et al. suggest an entropy recovery over long timescales at temperatures as high as 0.5 K, much too high to be compatible with the theory. Using neutron scattering and specific heat measurements at low temperatures and with long timescales ( 0.35 K/10(6) s and 0.5 K/10(5) s, respectively) on several isotopically enriched samples, we find no evidence of a reduction of ice-rule correlations or spin entropy. High-resolution simulations of the neutron structure factor show that the spin correlations remain well described by the dipolar spin ice model at all temperatures. Furthermore, by careful consideration of hyperfine contributions, we conclude that the original entropy measurements of Ramirez et al. are, after all, essentially correct: The short-time relaxation method used in that study gives a reasonably accurate estimate of the equilibrium spin ice entropy due to a cancellation of contributions., QC 20180919

The description and detection of unconventional magnetic states, such as spin liquids, is a recurring topic in condensed matter physics. While much of the efforts have traditionally been directed at geometrically frustrated antiferromagnets, recent studies reveal that systems featuring competing antiferromagnetic and ferromagnetic interactions are also promising candidate materials. We find that this competition leads to the notion of special temperatures, analogous to those of gases, at which the competing interactions balance, and the system is quasi-ideal. Although induced by weak perturbing interactions, these special temperatures are surprisingly high and constitute an accessible experimental diagnostic of eventual order or spin-liquid properties. The well characterised Hamiltonian and extended low-temperature susceptibility measurement of the canonical frustrated ferromagnet Dy2Ti2O7 enables us to formulate both a phenomenological and microscopic theory of special temperatures for magnets. Other members of this class of magnets include kapellasite Cu3Zn(OH)6Cl2 and the spinel GeCo2O4., QC 20180601

Spin ices, frustratedmagnetic materials analogous to common water ice, have emerged over the past 15 years as exemplars of high frustration in three dimensions. Recent experimental developments aimed at interrogating anew the low-temperature properties of these systems, in particular whether the predicted transition to long-range order occurs, behoove researchers to scrutinize our current dipolar spin ice model description of these materials. In this work, we do so by combining extensive Monte Carlo simulations and mean-field theory calculations to analyze data from previous magnetization, diffuse neutron scattering, and specific-heat measurements on the paradigmatic Dy2Ti2O7 spin ice material. In this work, we also reconsider the possible importance of the nuclear specific heat C-nuc in Dy2Ti2O7. We find that C-nuc is not entirely negligible below a temperature similar to 0.5 K and must therefore be taken into account in a quantitative analysis of the calorimetric data of this compound below that temperature. We find that in this material, small effective spin-spin exchange interactions compete with the magnetostatic dipolar interaction responsible for the main spin ice phenomenology. This causes an unexpected " refrustration" of the long-range order that would be expected from the incompletely self-screened dipolar interaction and which positions the material at the boundary between two competing classical long-range-ordered ground states. This allows for the manifestation of new physical low-temperature phenomena in Dy2Ti2O7, as exposed by recent specific-heat measurements. We show that among the four most likely causes for the observed upturn of the specific heat at low temperature [an exchange-induced transition to long-range order, quantum non-Ising (transverse) terms in the effective spin Hamiltonian, the nuclear hyperfine contribution, and random disorder], only the last appears to be reasonably able to explain the calorimetric data., QC 20160205

Electrical resistance is a crucial and well-understood property of systems ranging from computer microchips to nerve impulse propagation in the human body. Here we study the motion of magnetic charges in spin ice and find that extra spins inserted in Dy2Ti2O7 trap magnetic monopole excitations and provide the first example of how defects in a spin-ice material obstruct the flow of monopoles-a magnetic version of residual resistance. We measure the time-dependent magnetic relaxation in Dy2Ti2O7 and show that it decays with a stretched exponential followed by a very slow long-time tail. In a Monte Carlo simulation governed by Metropolis dynamics we show that surface effects and a very low level of stuffed spins (0.30%)-magnetic Dy ions substituted for non-magnetic Ti ions-cause these signatures in the relaxation. In addition, we find evidence that the rapidly diverging experimental timescale is due to a temperature-dependent attempt rate proportional to the monopole density., QC 20130121

In magnetic systems the combined effects of disorder and frustration may cause the moments to freeze into a disordered state at a spin-glass transition. Recent experiments have shown that the rare earth compound LiHo0.045Y0.955F4 freezes, but that the transition is unreachable because of dynamics that are 10(7) times slower than in ordinary spin-glass materials. This conclusion refutes earlier investigations reporting a speed-up of the dynamics into an exotic anti-glass phase caused by entanglement of quantum dipoles. Here we present a theory, backed by numerical simulations, which describes the material in terms of classical dipoles governed by Glauber dynamics. The dipoles freeze and we find that the ultra-slow dynamics are caused by rare, strongly ordered clusters, which give rise to a previously predicted, but hitherto unobserved, Griffths phase between the paramagnetic and spin-glass phases. In addition, the hyperfine interaction creates a high energy barrier to flipping the electronic spin, resulting in a clear signature in the dynamic correlation function., QC 20120625

In LiHo xY 1-xF 4, the magnetic Holmium Ho 3+ ions behave as effective Ising spins that can point parallel or antiparallel to the crystalline c-axis. The predominant inter-Ho 3+ interaction is dipolar, while the Y 3+ ions are non-magnetic. The application of a magnetic field B x transverse to the c-axis Ising direction leads to quantum spin-flip fluctuations, making this material a rare physical realization of the celebrated transverse field Ising model. The problems of classical and transverse-field-induced quantum phase transitions in LiHo xY 1-xF 4 in the dipolar ferromagnetic (x = 1), diluted ferromagnetic (0.25 ≤∼ x < 1) and highly diluted x ≤∼ 0.25 dipolar spin glass regimes have attracted much experimental and theoretical interest over the past twenty-five years. Two questions have received particular attention: (i) is there an antiglass (quantum disordered) phase at low Ho 3+ concentration and (ii) what is the mechanism responsible for the fast B x-induced destruction of the ferromagnetic (0.25 ≤∼ x < 1) and spin glass (x ≤∼ 0.25) phases? This paper reviews some of the recent theoretical and experimental progress in our understanding of the collective phenomena at play in LiHo xY 1-xF 4, in both zero and nonzero B x., QC 20140909

Using Monte Carlo simulations we verify that the rare-earth compound LiHoF4 is a very good realization of a dipolar Ising model. With only one free parameter our calculations for the magnetization, specific heat and inverse susceptibility match experimental data at a quantitative level in the 0.5-3 kelvin range, including the ferromagnetic transition at 1.53 K. Using parallel tempering methods and reaching system sizes up to 32000 dipoles with periodic boundary conditions, we are able to give evidence of the logarithmic corrections predicted in renormalization group theory. Due to the long range and angular dependence of the dipolar model, sample shape and domains play a crucial role in the ordered state. We consider surface corrections to Griffiths's theorem, which arise infinite macroscopic samples and lead to a theory of magnetic domains. We find a domain wall energy of 0.059 erg/cm(2) and predict that the ground-state domain structure for cylinders with a demagnetization factor N > 0 consists of thin parallel sheets of opposite magnetization, with a width depending on the demagnetization factor., QC 20100705

Using a Monte Carlo (MC) method, we study an effective model for the Fe(II)Fe(III) bimetallic oxalates. Within a hybrid quantum-classical MC algorithm, the Heisenberg S=2 and S-'=5/2 spins on the Fe(II) and Fe(III) sites are updated using a quantum MC loop while the Ising-type orbital angular momenta on the Fe(II) sites are updated using a single-spin classical MC flip. The effective field acting on the orbital angular momenta depends on the quantum state of the system. We find that the mean-field phase diagram for the model is surprisingly robust with respect to fluctuations. In particular, the region displaying two compensation points shifts and shrinks but remains finite., QC 20100525

We analyze recent experiments on the dilute rare-earth compound LiHoxY1-xF4 in the context of an effective Ising dipolar model. Using a Monte Carlo method we calculate the low-temperature behavior of the specific heat and linear susceptibility and compare our results to measurements. In our model the susceptibility follows a Curie-Weiss law at high temperature, X similar to 1 / (T- T-cw), with a Curie-Weiss temperature that scales with dilution, T-cw similar to x, consistent with early experiments. We also find that the peak in the specific heat scales linearly with dilution, C-max(T)similar to x, in disagreement with recent experiments. This difference could be caused by the hyperfine interaction which is not included in our calculation. Experimental studies do not reach a consensus on the functional form of the susceptibility and specific heat, and in particular, we do not see reported scalings of the form X similar to T-0.75 and X similar to exp(-T/T-0). Furthermore, we calculate the ground-state magnetization as a function of dilution and re-examine the phase diagram around the critical dilution x, = 0.24 +/- 0.03. We find that the spin-glass susceptibility for the Ising model does not diverge below x, while some recent experiments give strong evidence for a stable spin-glass phase in LiHo0.167Y0.833F4., QC 20100705

We study the effective long-range Ising dipole model with a local exchange interaction appropriate for the dilute magnetic compound LiHoxY1-xF4. Our calculations yield a value of 0.12 K for the nearest-neighbor exchange interaction. Using a Monte Carlo method, we calculate the phase boundary T-c(x) between the ferromagnetic and paramagnetic phases. We demonstrate that the experimentally observed linear decrease in T-c with dilution is not the simple mean-field result, but a combination of the effects of fluctuations and the exchange interaction. Furthermore, we find a critical dilution x(c)=0.21(2), below which there is no ordering. In agreement with recent Monte Carlo simulations on a similar model, we find no evidence of the experimentally observed freezing of the glassy state in our calculation. We apply the theory of Stephen and Aharony to LiHoxY1-xF4 and find that the theory does predict a finite-temperature freezing of the spin glass. Reasons for the discrepancies are discussed., QC 20100705

The random antiferromagnetic spin-1/2 XX and XXZ chain is studied numerically for varying strength of the disorder, using exact diagonalization and stochastic series expansion methods. The spin-spin correlation function as well as the stiffness display a clear crossover from the pure behavior (no disorder) to the infinite randomness fixed point or random singlet behavior predicted by the real space renormalization group. The crossover length scale is shown to diverge as xisimilar toD(-gamma), where D is the variance of the random bonds. Our estimates for the exponent gamma agrees well within the error bars with the one for the localization length exponent emerging within an analytical bosonization calculation. Exact diagonalization and stochastic series expansion results for the string correlation function are also presented., QC 20100525 QC 20110926

The properties of LiHoF4 are believed to be well described by a long-range dipolar Ising model. We go beyond mean-field theory and calculate the phase diagram of the Ising model in a transverse field using a quantum Monte Carlo method. The relevant Ising degrees of freedom are obtained using a nonperturbative projection onto the low-lying crystal-field eigenstates. We explicitly take the domain structure into account, and the strength of the near-neighbor exchange interaction is obtained as a fitting parameter. The on-site hyperfine interaction is approximately taken into account through a renormalization of the transverse applied magnetic field. Finally, we propose a spectroscopy experiment to precisely measure the most important parameter controlling the location of the phase boundary., QC 20100525 QC 20110922

The stochastic series expansion quantum Monte Carlo method is used to study thin ferromagnetic films, described by a Heisenberg model including local anisotropies. The magnetization curve is calculated, and the results compared to Schwinger boson and many-body Green's function calculations. A transverse field is introduced in order to study the reorientation effect, in which the magnetization changes from out of plane to in plane. Since the approximate theoretical approaches above differ significantly from each other, and the Monte Carlo method is free of systematic errors, the calculation provides an unbiased check of the approximate treatments. By studying quantum spin models with local anisotropies, varying spin size, and a transverse field, we also demonstrate the general applicability of the recent cluster-loop formulation of the stochastic series expansion quantum Monte Carlo method., QC 20100525

We discuss the: sign problem arising in Monte Carlo simulations of frustrated quantum spin systems. We show that for a class of semifrustrated systems [Heisenberg models with ferromagnetic couplings J(z)(r) < 0 along the; axis and antiferromagnetic couplings J(yx)(r) = -J(z)(r) in the xy plane, for arbitrary distances, 1. the sign problem present for algorithms operating in the z basis can be solved within a recent operator-loop formulation of the stochastic series expansion method [a cluster algorithm for sampling the diagonal matrix elements of the power series expansion of exp(-beta H) to all orders]. The solution relies on the identification of operator loops which change the configuration sign when updated (merons) and is similar to the meron-cluster algorithm recently proposed by Chandrasekharan and Wiese for solving the sign problem fur a class of fermion models [Phys. Rev. Lett. 83, 3116 (1999]. Some important expectation values, e.g., the internal energy, can be evaluated in the subspace with no merons, where the weight function is positive definite. Calculations of other expectation values require sampling of configurations with only a small number of merons (typically zero or two), with an accompanying sign problem which is not serious. We also discuss problems which arise in applying the meron concept to more general quantum spin models with frustrated interactions., QC 20100525

We present quantum Monte Carlo results for the field and temperature dependence of the magnetization and the spin lattice relaxation rate 1/T-1 of a two-dimensional S = 1/2 quantum Heisenberg ferromagnet. The Monte Carlo method, which yields results free of systematic errors, is described in detail. The high accuracy of the calculated magnetization allows for stringent tests of recent approximate analytical calculations. We also compare our results with recent experimental data for a nu = 1 quantum Hall ferromagnet, which is expected to be well described by the Heisenberg model. The dynamic response function needed to extract 1/T-1 is obtained using maximum-entropy analytic continuation of the corresponding imaginary-time dependent correlation function. We discuss the reliability of this approach., QC 20100525