Quantum Fluctuations In Antiferromagnets Research Articles

Phase Diagrams of One-Dimensional Ising and XY Models with Fully Connected Ferromagnetic and Anti-Ferromagnetic Quantum Fluctuations

We analytically investigated the phase diagrams of one-dimensional Ising and XY models. A second-order phase transition occurs in both models only with the transverse magnetic field. In this study,...

Experimental observation of Bethe strings

Almost a century ago, string states-complex bound states of magnetic excitations-were predicted to exist in one-dimensional quantum magnets. However, despite many theoretical studies, the experimental realization and identification of string states in a condensed-matter system have yet to be achieved. Here we use high-resolution terahertz spectroscopy to resolve string states in the antiferromagnetic Heisenberg-Ising chain SrCo2V2O8 in strong longitudinal magnetic fields. In the field-induced quantum-critical regime, we identify strings and fractional magnetic excitations that are accurately described by the Bethe ansatz. Close to quantum criticality, the string excitations govern the quantum spin dynamics, whereas the fractional excitations, which are dominant at low energies, reflect the antiferromagnetic quantum fluctuations. Today, Bethe's result is important not only in the field of quantum magnetism but also more broadly, including in the study of cold atoms and in string theory; hence, we anticipate that our work will shed light on the study of complex many-body systems in general.

Quantum annealing with antiferromagnetic fluctuations

We introduce antiferromagnetic quantum fluctuations into quantum annealing in addition to the conventional transverse-field term. We apply this method to the infinite-range ferromagnetic p-spin model, for which the conventional quantum annealing has been shown to have difficulties in finding the ground state efficiently due to a first-order transition. We study the phase diagram of this system both analytically and numerically. Using the static approximation, we find that there exists a quantum path to reach the final ground state from the trivial initial state that avoids first-order transitions for intermediate values of p. We also study numerically the energy gap between the ground state and the first excited state and find evidence for intermediate values of p for which the time complexity scales polynomially with the system size at a second-order transition point along the quantum path that avoids first-order transitions. These results suggest that quantum annealing would be able to solve this problem with intermediate values of p efficiently, in contrast to the case with only simple transverse-field fluctuations.

Kondo ground states and non-Fermi-liquid behavior inCeNi1−xCoxGe2

We report measurements of the magnetic susceptibility, specific heat, and electrical resistivity of the heavy fermion alloy series $\mathrm{Ce}{\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}{\mathrm{Ge}}_{2}$. With increasing $x$, hybridization between the localized $4f$ and conduction band electrons is enhanced. The magnetic order observed at $x=0$ is completely suppressed at $x=0.3$ and non-Fermi-liquid behavior appears at the critical concentration, which is analyzed in terms of two-dimensional antiferromagnetic quantum fluctuations. Specific heat and magnetic susceptibility data are quantitatively explained by the Coqblin-Schrieffer model with degenerate impurity spin $j=1∕2$, $3∕2$, and $5∕2$ for Co concentration range of $x\ensuremath{\leqslant}0.6$, $0.7\ensuremath{\leqslant}x\ensuremath{\leqslant}0.8$, and $x\ensuremath{\geqslant}0.9$, respectively.

Excitations and quantum fluctuations in site-diluted two-dimensional antiferromagnets

We study the effect of site dilution and quantum fluctuations in an antiferromagnetic spin system on a square lattice within the linear spin-wave approximation. By performing numerical diagonalization in real space and finite-size scaling, we characterize the nature of the low-energy spin excitations for different dilution fractions up to the classical percolation threshold. We find nontrivial signatures of fractonlike excitations at high frequencies. Our simulations also confirm the existence of an upper bound for the amount of quantum fluctuations in the ground state of the system, leading to the persistence of long-range order up to the percolation threshold. This result is in agreement with recent neutron-scattering experimental data and quantum Monte Carlo numerical calculations. We also show that the absence of a quantum critical point below the classical percolation threshold holds for a large class of systems whose Hamiltonians can be mapped onto a system of coupled noninteracting massless bosons.

Statics and dynamics of the magnetic soliton lattice in the high-field phase of CuGeO 3

Driven by antiferromagnetic quantum fluctuations on a deformable lattice, spin-Peierls systems dimerize magnetically and structurally at low temperatures. In this dimer phase all excitations with non-zero magnetization have a finite energy. The energy gain by a finite induced magnetization overcomes the energy gap at a critical magnetic field strength H c. Above H c, an incommensurately modulated magnetic and distortive domain wall pattern is formed. The magnetic modulation pattern and the low-energy excitation spectrum in this high-field phase of the inorganic spin-Peierls compound CuGeO 3 are fully determined by elastic and inelastic neutron scattering and discussed together with theoretical predictions as well as other experimental results.

Quantum fluctuations in antiferromagnets

Dynamic properties of antiferromagnets at zero temperature are explored. Going beyond RPA a nonlinear set of coupled equations for dynamic susceptibilities and correlation functions is derived. New results are presented for nondiagonal elements of the dynamic susceptibility tensor, longitudinal fluctuations and their influence on transverse fluctuations.

Quantum fluctuations in antiferromagnets of the BX2 family (abstract)

The hexagonal antiferromagnets of the BX2 family, where B is a magnetic ion, and X a halogen, are characterized by an in plane exchange interaction J much stronger than the exchange interaction J′ along the c axis, so that some features of the triangular antiferromagnet (TA) could survive in these real compounds. In particular we are interested to investigate a possible ‘‘planar’’ phase with the spins lying in the c plane supported by quantum fluctuations when an external magnetic field H is applied perpendicular to the c axis. We find that such a phase is stable for sufficiently small interplane coupling owing to the zero point motion energy, whereas an ‘‘umbrella’’ phase would be expected on the basis of the classical approximation for any nonzero interplane coupling. Notice that for the TA model the planar and umbrella configurations are degenerate in classical approximation for any H and infinite isoenergetic planar configurations exist. Quantum fluctuations select a planar configuration with a spin over three opposite to the field. We find that the classical scenario itself is substantially different for any j=J′/J≠0, because only a planar configuration with a spin over three nearly perpendicular to the field and the umbrella configuration minimize the energy of the model and the umbrella is stable. However, quantum fluctuations stabilize the planar configuration for any H below a critical interplane coupling j*. For intermediate j the planar configuration is stable for low and high fields, whereas only the umbrella phase is stable of j large enough.

Quantum fluctuations in antiferromagnets of the family

The hexagonal antiferromagnet, convenient for describing magnetic compounds of the type , where B is a magnetic ion and X a halogen, is characterized by having an in-plane exchange interaction much stronger than the out-of-plane exchange interaction . The interest in the class of compounds arises because they are expected to manifest some features peculiar to the triangular antiferromagnets. In particular, we intend to investigate the existence of possible planar phases supported by quantum fluctuations when an external magnetic field is applied perpendicular to the c axis. We find that a planar phase is stable for sufficiently small interplane coupling and small single-ion easy-axis anisotropy, whereas an umbrella phase would be expected on the basis of the classical approximation.