arXiv subjects

T. Vekua

Publications and source records attributed to T. Vekua.

31 records · Page 2Linked to original sources

arXiv2011

Resonances in a dilute gas of magnons and metamagnetism of isotropic frustrated ferromagnetic spin chains

We show that spin-S chains with SU(2)-symmetric, ferromagnetic nearest-neighbor and frustrating antiferromagnetic next-nearest-neighbor exchange interactions exhibit metamagnetic behavior under the influence of an external magnetic field for small S, in the form of a first-order transition to the fully polarized state. The corresponding magnetization jump increases gradually starting from an S-dependent critical value of exchange couplings and takes a maximum in the vicinity of a ferromagnetic Lifshitz point. The metamagnetism results from resonances in the dilute magnon gas caused by an interplay between quantum fluctuations and frustration.

cond-mat.str-el↗

arXiv2010

Magnetic phases of spin-3/2 fermions on a spatially anisotropic square lattice

We study the magnetic phase diagram of spin-3/2 fermions in a spatially anisotropic square optical lattice at quarter filling (corresponding to one particle per lattice site). In the limit of the large on-site repulsion the system can be mapped to the so-called Sp(N) Heisenberg spin model with N=4. We analyze the Sp(N) spin model with the help of the large-N field-theoretical approach and show that the effective theory corresponds to the Sp(N) extension of the CP^{N-1} model, with the Lorentz invariance generically broken. We obtain the renormalization flow of the model couplings and show that although the Sp(N) terms are seemingly irrelevant, their presence leads to a renormalization of the CP^{N-1} part of the action, driving a phase transition. We further consider the influence of the external magnetic field (the quadratic Zeeman effect), and present the qualitative analysis of the ground state phase diagram.

cond-mat.quant-gas↗

arXiv2010

Field-induced phase transitions of repulsive spin-1 bosons in optical lattices

We study the phase diagram of repulsively interacting spin-1 bosons in optical lattices at unit filling, showing that an externally induced quadratic Zeeman effect may lead to a rich physics characterized by various phases and phase transitions. We find that the main properties of the system may be described by an effective field model, which provides the precise location of the phase boundaries for any dimension, being in excellent agreement with our numerical calculations for one-dimensional systems. Our work provides a quantitative guide for the experimental analysis of various types of field-induced quantum phase transitions in spin-1 lattice bosons. These transitions, which are precluded in spin-1/2 systems, may be realized using an externally modified quadratic Zeeman coupling, similar to recent experiments with spinor condensates in the continuum.

cond-mat.quant-gas↗

arXiv2008

Critical properties of 1D spin-gapped fermions at the onset of magnetization

We develop an effective field theory for finding critical properties of 1D spin gapped fermions at the onset of magnetization. It is shown how the spin-charge interaction leads to a linear critical behavior and finite susceptibility for a wide range of models. We also discuss possible manifestations of spin-charge coupling in cold atomic gases.

cond-mat.str-el↗

arXiv2007

Correlation functions and excitation spectrum of the frustrated ferromagnetic spin-1/2 chain in an external magnetic field

Magnetic field effects on the one-dimensional frustrated ferromagnetic chain are studied by means of effective field theory approaches in combination with numerical calculations utilizing Lanczos diagonalization and the density matrix renormalization group method. The nature of the ground state is shown to change from a spin-density-wave region to a nematic-like one upon approaching the saturation magnetization. The excitation spectrum is analyzed and the behavior of the single spin-flip excitation gap is studied in detail, including the emergent finite-size corrections.

cond-mat.str-el↗

arXiv2007

Quantum phase transitions in trimerized zig-zag spin ladders

We analyze the effects of a trimerized modulation in a quantum spin $S=\frac12$ zig-zag ladder at the magnetization plateau $M=1/3$. Such periodicity is argued to be stemmed from lattice deformations by phonons. The interplay between frustration and exchange modulation is well described by an effective triple sine-Gordon field theory close to the homogeneous ladder and by block-spin perturbation theory in the weakly coupled trimers regime. The characteristic triple degeneracy of the ground state for homogeneous ladders gives place to modulation driven quantum phase transitions, leading to a rich phase diagram including up-up-down, quantum plateau and gapless plateau states.

cond-mat.str-el↗

arXiv2007

Excitations with fractional spin less than 1/2 in frustrated magnetoelastic chains

We study the magnetic excitations on top of the plateaux states recently discovered in spin-Peierls systems in a magnetic field. We show by means of extensive density matrix renormalization group (DMRG) computations and an analytic approach that one single spin-flip on top of $M=1-\frac2N$ ($N=3,4,...$) plateau decays into $N$ elementary excitations each carrying a fraction $\frac1N$ of the spin. This fractionalization goes beyond the well-known decay of one magnon into two spinons taking place on top of the M=0 plateau. Concentrating on the $\frac13$ plateau (N=3) we unravel the microscopic structure of the domain walls which carry fractional spin-$\frac13$, both from theory and numerics. These excitations are shown to be noninteracting and should be observable in x-ray and nuclear magnetic resonance experiments.

cond-mat.str-el↗

arXiv2006

Frustrated ferromagnetic spin-1/2 chain in a magnetic field: The phase diagram and thermodynamic properties

The frustrated ferromagnetic spin-1/2 Heisenberg chain is studied by means of a low-energy field theory as well as the density-matrix renormalization group and exact diagonalization methods. Firstly, we study the ground-state phase diagram in a magnetic field and find an `even-odd' (EO) phase characterized by bound pairs of magnons in the region of two weakly coupled antiferromagnetic chains. A jump in the magnetization curves signals a first-order transition at the boundary of the EO phase, but otherwise the curves are smooth. Secondly, we discuss thermodynamic properties at zero field, where we confirm a double-peak structure in the specific heat for moderate frustrating next-nearest neighbor interactions.

cond-mat.str-el↗

arXiv2005

Magnetization plateaux induced by a coupling to the lattice

We investigate a simple model of a frustrated spin-1/2 Heisenberg chain coupled to adiabatic phonons under an external magnetic field. Using field theoretic methods complemented by extensive Density Matrix Renormalisation Group techniques generalized to include self-consistent lattice distortions, we show that magnetization plateaux at non-trivial rational values of the magnetization can be stabilized by the lattice coupling. We suggest that such a scenario could be relevant for some low dimensional frustrated spin-Peierls compounds.

cond-mat.str-el↗

arXiv2004

Magnetic field induced phase transitions in spin ladders with ferromagnetic legs

We study the phase diagram of spin ladders with ferromagnetic legs under the influence of a symmetry breaking magnetic field in the weak coupling effective field theory by bosonization. For antiferromagnetic interleg coupling we identify two phase transitions introduced by the external magnetic field. In order to establish the universality of the phases we supplement the bosonization approach by results from a strong coupling (rung) expansion and from spin wave analysis.

cond-mat.str-el↗

arXiv2002

Phase diagrams of spin ladders with ferromagnetic legs

The low-temperature properties of the spin S=1/2 ladder with anisotropic ferromagnetic legs are studied using the continuum limit bosonization approach. The weak-coupling ground state phase diagram of the model is obtained for a wide range of coupling constants and several unconventional gapless ''spin-liquid'' phases are shown to exist for ferromagnetic coupling. The behavior of the ladder system in the vicinity of the ferromagnetic instability point is discussed in detail.

cond-mat.str-el↗

arXiv2002

Perturbation theories for the S=1/2 spin ladder with four-spin ring exchange

The isotropic S=1/2 antiferromagnetic spin ladder with additional four-spin ring exchange is studied perturbatively in the strong coupling regime with the help of cluster expansion technique, and by means of bosonization in the weak coupling limit. It is found that a sufficiently large strength of ring exchange leads to a second-order phase transition, and the shape of the boundary in the vicinity of the known exact transition point is obtained. The critical exponent for the gap is found to be $η\simeq1$, in agreement both with exact results available for the dimer line and with the bosonization analysis. The phase emerging for high values of the ring exchange is argued to be gapped and spontaneously dimerized. The results for the transition line from strong coupling and from weak coupling match with each other naturally.

cond-mat.str-el↗

arXiv2000

Magnetic monopoles vs. Hopf defects in the Laplacian (Abelian) gauge

We investigate the Laplacian Abelian gauge on the sphere S^4 in the background of a single `t Hooft instanton. To this end we solve the eigenvalue problem of the covariant Laplace operator in the adjoint representation. The ground state wave function serves as an auxiliary Higgs field. We find that the ground state is always degenerate and has nodes. Upon diagonalisation, these zeros induce toplogical defects in the gauge potentials. The nature of the defects crucially depends on the order of the zeros. For first-order zeros one obtains magnetic monopoles. The generic defects, however, arise from zeros of second order and are pointlike. Their topological invariant is the Hopf index S^3 -> S^2. These findings are corroborated by an analysis of the Laplacian gauge in the fundamental representation where similar defects occur. Possible implications for the confinement scenario are discussed.

hep-th↗