Superconducting Plate in Transverse Magnetic Field: New State

A model to describe Cooper pairs near the transition point (on temperature and magnetic field), when the distance between them is big compared to their sizes, is proposed. A superconducting plate whose thickness is less than the pair size in the transverse magnetic field near the critical value $H_{c2}$ is considered as an application of the model. A new state that is energetically more favourable than that of Abrikosov vortex state within an interval near the transition point was obtained. The system's wave function in this state looks like that of Laughlin's having been used in fractional quantum Hall effect (naturally, in our case - for Cooper pairs as Bose-particles) and it corresponds to homogeneous incompressible liquid. The state energy is proportional to the first power of value $(1 - H/H_{c2})$, unlike the vortex state energy having this value squared. The interval of the new state existence is greater for dirty specimens.Comment: 7 page

Commensurate and incommensurate ground states of Cs_2CuCl_4 in a magnetic field

We present calculations of the magnetic ground state of Cs_2CuCl_4 in an applied magnetic field, with the aim of understanding the commensurately ordered state that has been discovered in recent experiments. This layered material is a realization of a Heisenberg antiferromagnet on an anisotropic triangular lattice. Its behavior in a magnetic field depends on field orientation, because of weak Dzyaloshinskii-Moriya interactions.We study the system by mapping the spin-1/2 Heisenberg Hamiltonian onto a Bose gas with hard core repulsion. This Bose gas is dilute, and calculations are controlled, close to the saturation field. We find a zero-temperature transition between incommensurate and commensurate phases as longitudinal field strength is varied, but only incommensurate order in a transverse field. Results for both field orientations are consistent with experiment.Comment: 5 Pages, 3 Figure

Bose-Einstein condensation of magnons in magnets with predominant ferromagnetic interaction

We discuss Bose-Einstein condensation of magnons (BEC) in magnets with predominant ferromagnetic (FM) interaction in magnetic field $H$ near saturation ($H_c$). Because $H_c$ is independent of FM couplings, magnetic materials of this type can have small $H_c$ that makes them promising candidates for experimental investigation of BEC. Ferromagnets with easy-plane anisotropy and antiferromagnets (AFs) containing weakly coupled FM planes or chains are discussed in detail. We observe small effective interaction between magnons near the QCP in such magnets, in contrast to AFs with strong AF coupling previously discussed. In particular, this smallness allows us to find crossovers in the critical temperature $T_c(H)\propto (H_c-H)^{1/\phi}$ from $\phi=3/2$ to $\phi=1$ in quasi-1D magnets, and from $\phi=3/2$ to $\phi\approx1$ ($T_c\ln T_c\propto H_c-H$) in quasi-2D ones.Comment: 9 pages, 4 figure. The paper has been rewritten considerably. In particular, discussion of crossovers in the critical temperature in quasi-low-dimensional magnets is adde

Dilute-Bose-Gas Approach to ground state phases of 3D quantum helimagnets under high magnetic field

We study high-field phase diagram and low-energy excitations of three-dimensional quantum helimagnets. Slightly below the saturation field, the emergence of magnetic order may be mathematically viewed as Bose-Einstein condensation (BEC) of magnons. The method of dilute Bose gas enables an unbiased quantitative analysis of quantum effects in three-dimensional helimagnets and thereby three phases are found: cone, coplanar fan and an attraction-dominant one. To investigate the last phase, we extend the usual BEC approach so that we can handle 2-magnon bound states. In the case of 2-magnon BEC, the transverse magnetization vanishes and long-range order occurs in the quadrupolar channel (spin-nematic phase). As an application, we map out the phase diagram of a 3D helimagnet which consists of frustrated J1-J2 chains coupled by an interchain interaction J3.Comment: 4pages, 3figures, International Conference on Magnetism (ICM) 2009 (Karlsruhe, Germany, July 26-31, 2009)

Bose-Einstein condensation in antiferromagnets close to the saturation field

At zero temperature and strong applied magnetic fields the ground sate of an anisotropic antiferromagnet is a saturated paramagnet with fully aligned spins. We study the quantum phase transition as the field is reduced below an upper critical $H_{c2}$ and the system enters a XY-antiferromagnetic phase. Using a bond operator representation we consider a model spin-1 Heisenberg antiferromagnetic with single-ion anisotropy in hyper-cubic lattices under strong magnetic fields. We show that the transition at $H_{c2}$ can be interpreted as a Bose-Einstein condensation (BEC) of magnons. The theoretical results are used to analyze our magnetization versus field data in the organic compound $NiCl_2$-$4SC(NH_2)_2$ (DTN) at very low temperatures. This is the ideal BEC system to study this transition since $H_{c2}$ is sufficiently low to be reached with static magnetic fields (as opposed to pulsed fields). The scaling of the magnetization as a function of field and temperature close to $H_{c2}$ shows excellent agreement with the theoretical predictions. It allows to obtain the quantum critical exponents and confirm the BEC nature of the transition at $H_{c2}$.Comment: 4 pages, 1 figure. Accepted for publication in PRB

Time-dependent spin-wave theory

We generalize the spin-wave expansion in powers of the inverse spin to time-dependent quantum spin models describing rotating magnets or magnets in time-dependent external fields. We show that in these cases, the spin operators should be projected onto properly defined rotating reference frames before the spin components are bosonized using the Holstein-Primakoff transformation. As a first application of our approach, we calculate the reorganization of the magnetic state due to Bose-Einstein condensation of magnons in the magnetic insulator yttrium-iron garnet; we predict a characteristic dip in the magnetization which should be measurable in experiments.Comment: 6 pages, 5 figures, final version published in PR

Double zigzag spin chain in a strong magnetic field close to saturation

We study the ground state phase diagram of a frustrated spin tube in a strong external magnetic field. This model can be viewed as two coupled zigzag spin chains, or as a two-leg spin ladder with frustrating next-nearest-neighbor couplings along the legs, and its study is motivated by the physics of such materials as sulfolane-Cu2Cl4 and BiCu2PO6. In magnetic fields right below the saturation, the system can be effectively represented as a dilute gas of two species of bosonic quasiparticles that correspond to magnons with inequivalent incommensurate momenta at two degenerate minima of the magnon dispersion. Using the method previously proposed and tested for frustrated spin chains, we calculate effective interactions in this two-component Bose gas. On this basis, we establish the phase diagram of nearly saturated frustrated spin tube, which is shown to include the two-component Luttinger liquid, two types of vector chiral phases, and phases whose physics is determined by the presence of bound magnons. We study the phase diagram of the model numerically by means of the density matrix renormalization group technique, and find a good agreement with our analytical predictions

Bose-Einstein condensation of magnons in Cs2_{2}CuCl4_{4}: a dilute gas limit near the saturation magnetic field

Based on a realistic spin Hamiltonian for a frustrated quasi-two dimensional spin-1/2 antiferromagnet Cs$_{2}$CuCl$_{4}$, a three-dimensional spin ordering in the applied magnetic field $B$ near the saturation value $B_{c}$ is studied within the magnon Bose-Einstein condensation (BEC) scenario. With the use of a hard-core boson formulation of the spin model, a strongly anysotropic magnon dispersion in Cs$_{2}$CuCl$_{4}$ is calculated. In the dilute magnon limit near $B_{c}$, the hard-core boson constraint is resulted in an effective magnon interaction which is treated in the Hartree-Fock approximation. The critical temperature $T_{c}$ is calculated as a function of a magnetic field $B$ and compared with the phase boundary $T_{c}(B)$ experimentally determined in Cs$_{2}$CuCl$_{4}$ [Phys. Rev. Lett. \textbf{95}, 127202 (2005)]