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PubMed2001

Two-dimensional randomly frustrated spin-1/2 Heisenberg model.

We investigate the properties of S = 1/2 Heisenberg clusters with random frustration using exact diagonalizations. This is a model for a quantum spin glass. We show that the average ground state spin is S proportional to the square root of N, where N is the number of sites. We also calculate the magnetic susceptibility and the spin stiffness and low-energy excitations and discuss these in terms of a semiclassical picture.

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PubMed2002

Very-low-frequency excitations in frustrated two-dimensional S = 1/2 Heisenberg antiferromagnets.

Muon spin resonance and 7Li NMR relaxation measurements in frustrated two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice are presented. It is found that, in both Li2VOSiO4 and Li2VOGeO4, spin dynamics at frequencies orders of magnitude below the Heisenberg exchange frequency are present. These dynamics are associated with the motions of walls separating coexisting collinear domains with a magnetic wave vector rotated by 90 degrees.

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PubMed2002

Chiral Kosterlitz-Thouless transition in the frustrated Heisenberg antiferromagnet on a pyrochlore slab.

Ordering of the geometrically frustrated two-dimensional Heisenberg antiferromagnet on a pyrochlore slab is studied by Monte Carlo simulations. In contrast to the kagomé Heisenberg antiferromagnet, the model exhibits locally noncoplanar spin structures at low temperatures, bearing nontrivial chiral degrees of freedom. Under certain conditions, the model exhibits a novel Kosterlitz-Thouless-type transition at a finite temperature associated with these chiral degrees of freedom.

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PubMed2002

Kink pairs unbinding on domain walls and the sequence of phase transitions in fully frustrated XY models.

The unbinding of kink pairs on domain walls in the fully frustrated XY model (on square or triangular lattices) is shown to induce the vanishing of phase coupling across the walls. This forces the phase transition, associated with unbinding of vortex pairs, to take place at a lower temperature than the other phase transition, associated with proliferation of the Ising-type domain walls. The results are applicable for a description of superconducting junction arrays and wire networks in a perpendicular magnetic field, as well as of planar antiferromagnets with a triangular lattice.

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PubMed2002

Macroscopic magnetization jumps due to independent magnons in frustrated quantum spin lattices.

For a class of frustrated spin lattices including the Kagomé lattice we construct exact eigenstates consisting of several independent, localized one-magnon states and argue that they are ground states for high magnetic fields. If the maximal number of local magnons scales with the number of spins in the system, which is the case for the Kagomé lattice, the effect persists in the thermodynamic limit and gives rise to a macroscopic jump in the zero-temperature magnetization curve just below the saturation field. The effect decreases with increasing spin quantum number and vanishes in the classical limit. Thus it is a true macroscopic quantum effect.

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PubMed2002

Peierls-like transition induced by frustration in a two-dimensional antiferromagnet.

We show that the introduction of frustration into the spin- 1/2 two-dimensional (2D) antiferromagnetic Heisenberg model on a square lattice via a next-nearest-neighbor exchange interaction can lead to a Peierls-like transition, from a tetragonal to an orthorhombic phase, when the spins are coupled to adiabatic phonons. The two different orthorhombic ground states define an Ising order parameter, which is expected to lead to a finite temperature transition. Implications for Li(2)VOSiO(4), the first realization of that model, will be discussed.

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PubMed2002

Multiple field-induced phase transitions in the geometrically frustrated dipolar magnet: Gd(2) Ti(2)O(7).

Field-driven phase transitions generally arise from competition between Zeeman energy and exchange or crystal-field anisotropy. Here we present the phase diagram of a frustrated pyrochlore magnet Gd(2)Ti(2)O(7), where crystal-field splitting is small compared to the dipolar energy. We find good agreement between zero-temperature critical fields and those obtained from a mean-field model. Here, dipolar interactions couple real space and spin space, so the transitions in Gd(2)Ti(2)O(7) arise from field-induced "cooperative anisotropy," reflecting the broken spatial symmetries of the pyrochlore lattice.

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PubMed2002

Frustrated soft modes and negative thermal expansion in ZrW2O8.

Negative thermal expansion (NTE) in cubic ZrW2O8 has generated much interest due to its large, isotropic, and temperature independent behavior. Here, x-ray absorption fine structure data are presented for various atom pairs, providing evidence that the low-energy modes causing NTE correspond to the correlated vibrations of a WO4 tetrahedron and its three nearest ZrO6 octahedra. This involves translations of the WO4 as a rigid unit along each of the four <111> axes. The interconnectivity of these modes prevents an anisotropic soft mode from developing, a new geometrical phenomenon that we call the "frustrated soft mode."

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PubMed2002

Geometrical-frustration-induced (Semi)metal-to-insulator transition.

We study the low-energy properties of the geometrically frustrated Hubbard model on a three-dimensional pyrochlore lattice and a two-dimensional checkerboard lattice on the basis of the renormalization group method and mean field analysis. It is found that, in the half-filling case, a (semi)metal to insulator transition (MIT) occurs. Also, in the insulating phase, which has a spin gap, the spin rotational symmetry is not broken, while charge ordering exists. The results are applied to the description of the MIT observed in the pyrochlore system Tl2Ru2O7.

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PubMed2002

Orbital ordering in frustrated Jahn-Teller systems with 90 degrees exchange.

We show that superexchange interactions in frustrated Jahn-Teller systems with transition metal ions connected by the 90 degrees metal-oxygen-metal bonds (e.g., NaNiO2, LiNiO2, and ZnMn2O4) are much different from those in materials with the 180 degrees bonds. In the 90 degrees -exchange systems spins and orbitals are decoupled: the spin exchange is much weaker than the orbital one and it is ferromagnetic for all orbital states. Though the mean-field orbital ground state is strongly degenerate, quantum orbital fluctuations select particular ferro-orbital states. We explain the orbital and magnetic ordering observed in NaNiO2 and show that LiNiO2 is not a spin-orbital liquid.

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PubMed2002

LiV2O4: frustration induced heavy fermion metal.

We propose a two-stage spin-quenching scenario for the unusual heavy fermion state realized in the mixed valent metal LiV2O4. In this theory, local valence fluctuations are responsible for the formation of partially quenched, spin-1 / 2 moments below room temperature. Frustration of the intersite spin couplings then drives the system to produce the heavy Fermi liquid seen at low temperatures. The anomalous resistivity and the sign change of the Hall constant can be understood naturally within this model, which also predicts a unique symmetry for the heavy quasiparticle bands that may be observed in de Haas-van Alphen experiments.

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PubMed2003

Frustrated spin model as a hard-sphere liquid.

We show that one-dimensional topological objects (kinks) are natural degrees of freedom for an antiferromagnetic Ising model on a triangular lattice. Its ground states and the coexistence of spin ordering with an extensive zero-temperature entropy can easily be understood in terms of kinks forming a hard-sphere liquid. Using this picture we explain effects of quantum spin dynamics on that frustrated model, which we also study numerically.

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PubMed2003

Doped coupled frustrated spin-1/2 chains with four-spin exchange.

The role of various magnetic interchain couplings is investigated by numerical methods in doped frustrated quantum spin chains. A nonmagnetic dopant introduced in a gapped spin chain releases a free spin-1/2 soliton. The formation of a local magnetic moment is analyzed in terms of soliton confinement. A four-spin coupling which might originate from cyclic exchange is shown to produce such a confinement. Dopants on different chains experience an effective space-extended nonfrustrating pairwise spin interaction.

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PubMed2003

Vortex glass transition in a frustrated 3D XY model with disorder.

The anisotropic frustrated three-dimensional (3D) XY model with disorder in the coupling constants is simulated as a model of a point disordered superconductor in an applied magnetic field. A finite size scaling analysis of the helicity modulus gives strong evidence for a finite temperature transition with isotropic scaling and the correlation length exponent nu=1.5+/-0.3, consistent with 3D gauge glass universality.

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PubMed2003

Quenched disorder and the critical behavior of a partially frustrated system.

We report the direct observation of the effects of quenched disorder on the critical behavior of partially frustrated amorphous FeMnZr alloys by the systematic analysis of high-precision ac susceptibility data and dc magnetization data. Interestingly, the analysis reveals that the presence of short-range quenched disorder does not alter the actual critical behavior. However, it does affect quantities such as the Curie temperature, the peak value of effective exponent gamma, width of the peak, and crossover temperatures. The observed temperature dependence of the effective critical exponent can be understood in terms of the field-theoretical renormalization group approach. Also, the present results would help in identifying the main source of the spread in the exponent values reported in the literature.

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PubMed2004

Null orbital frustration at the pseudogap boundary in a layered cuprate superconductor.

We assess the relative importance of orbital frustration at the pseudogap closing field H(pg). Using interlayer tunneling transport in pulsed magnetic fields nearly up to 60 T, we track the field-temperature (H-T) phase diagram for fields parallel ( parallel ab) and normal ( parallel c) to the layered structure of Bi(2)Sr(2)CaCu(2)O(8+y). In contrast to large orientational anisotropy of the superconducting state related to the orbital motion of Cooper pairs, we find anisotropy of H(pg) temperature independent and small, due solely to the g factor. The obtained Zeeman relation with the pseudogap temperature T small star, filled, g( parallel c)micro(B)H( parallel c)(pg)=g( parallel ab)micro(B)H( parallel ab)(pg) approximately k(B)T small star, filled, is fully consistent with the correlations only in the spin channel.

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PubMed2004

Spin-charge separation in two-dimensional frustrated quantum magnets.

The dynamics of a mobile hole in two-dimensional frustrated quantum magnets is investigated by exact diagonalization techniques. Our results provide evidence for spin-charge separation upon doping the kagome lattice, a prototype of a spin liquid. In contrast, in the checkerboard lattice, a symmetry broken valence bond crystal, a small quasiparticle peak is seen for some crystal momenta, a finding interpreted as a restoration of weak holon-spinon confinement.

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PubMed2004

Fluctuations and vortex-pattern ordering in the fully frustrated XY model on a honeycomb lattice.

The accidental degeneracy of various ground states of a fully frustrated XY model with a honeycomb lattice is shown to survive even when the free energy of the harmonic fluctuations is taken into account. The reason for that consists in the existence of a hidden gauge symmetry between the Hamiltonians describing the harmonic fluctuations in all these ground states. A particular vortex pattern is selected only when anharmonic fluctuations are taken into account. However, the observation of vortex ordering requires relatively large system size L>>Lc > or approximately equal to 10(5).

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