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PubMed2000

Noncausal time response in frustrated total internal reflection?

Tunneling of photons in frustrated total internal reflection has been studied in the time domain with single-cycle femtosecond pulses. It is seen that both the phase and energy of the pulse travel faster than the speed of light in vacuum. Theoretical analysis of the experiments shows that the time-response function for electromagnetic waves propagating in the air gap is noncausal. However, it is found that superluminal signal propagation is not possible in this case because of the inevitable diffractive spreading of the signal beam.

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PubMed2000

Local spin resonance and spin-peierls-like phase transition in a geometrically frustrated antiferromagnet

Inelastic magnetic neutron scattering reveals a localized spin resonance at 4.5 meV in the ordered phase of the geometrically frustrated cubic antiferromagnet ZnCr2O4. The resonance develops abruptly from quantum critical fluctuations upon cooling through a first order transition to a co-planar antiferromagnet at T(c) = 12. 5(5) K. We argue that this transition is a three dimensional analog of the spin-Peierls transition.

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PubMed2000

Two-dimensional periodic frustrated ising models in a transverse field

We investigate the interplay of classical degeneracy and quantum dynamics in a range of periodic frustrated transverse field Ising systems at zero temperature. We find that such dynamics can lead to unusual ordered phases and phase transitions or to a quantum spin liquid (cooperative paramagnetic) phase as in the triangular and kagome lattice antiferromagnets, respectively. For the latter, we further predict passage to a bond-ordered phase followed by a critical phase as the field is tilted. These systems also provide exact realizations of quantum dimer models introduced in studies of high temperature superconductivity.

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PubMed2000

LiV2O4 spinel as a heavy-mass fermi liquid: anomalous transport and role of geometrical frustration

Transport and specific heat measurements on hydrothermally grown single crystals reveal the formation of a heavy-mass Fermi liquid in the LiV2O4 spinel, below a coherence temperature of T* = 20-30 K. A few observations which illustrate the uniqueness of this spinel are discussed in connection with the origin of the heavy mass, such as the anomalous absence of resistivity saturation above T* and the close proximity to a spin glass phase where the influence of the magnetic frustration is evident.

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PubMed2000

Li2VO(Si,Ge)O4, a prototype of a two-dimensional frustrated quantum heisenberg antiferromagnet

NMR and magnetization measurements in Li2VOSiO4 and Li2VOGeO4 are reported. The analysis of the susceptibility shows that both compounds are two-dimensional S = 1/2 Heisenberg antiferromagnets on a square lattice with a sizable frustration induced by the competition between the superexchange couplings J1 along the sides of the square and J2 along the diagonal. Li2VOSiO4 undergoes a low-temperature phase transition to a collinear order, as theoretically predicted for J2/J1>0.5. Just above the magnetic transition the degeneracy between the two collinear ground states is lifted by the onset of a structural distortion.

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PubMed2000

Intrinsic finite-size effects in the two-dimensional XY model with irrational frustration

This study investigates in detail the finite-size scaling of the two-dimensional irrationally frustrated XY model. By means of Monte Carlo simulations with entropic sampling, we examine the size dependence of the specific heat, and find remarkable deviation from the conventional finite-size scaling theory, which reveals novel intrinsic finite-size effects. Relaxation dynamics of the system is also considered, and, correspondingly, finite-size scaling of the relaxation time is examined, again giving evidence for the intrinsic finite-size effects and suggesting a zero-temperature glass transition.

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PubMed2000

Low temperature spin dynamics of the geometrically frustrated antiferromagnetic garnet Gd3Ga5O12

The low temperature spin dynamics of the geometrically frustrated antiferromagnet Gd 3Ga 5O (12) (GGG) have been investigated using muon spin relaxation. No evidence for static order is seen down to a temperature of 25 mK or a few percent of the Curie-Weiss temperature. Instead there is a linear decrease in the Gd spin fluctuation rate below 1 K which extrapolates to a small but finite value of 2 GHz at zero temperature. In terms of the spin fluctuations the system appears essentially to remain dynamic at low temperatures (T>0.02 K) and magnetic fields up to 1.8 T.

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PubMed2000

Topology-induced spin frustrations at the Cr(001) surface studied by spin-polarized scanning tunneling spectroscopy

The magnetic structure of the Cr(001) surface was investigated by spin-polarized scanning tunneling spectroscopy by making use of the spin-polarized surface state located close to the Fermi level. Periodic alternations of the intensity of the surface state peak in local tunneling spectra measured above different ferromagnetic terraces separated by monatomic steps confirm the topological antiferromagnetic order of the Cr(001) surface. Screw dislocations cause topology-induced spin frustration, leading to the formation of domain walls with a width of about 120 nm.

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PubMed2000

Frustrations in polymer conformation in gels and their minimization through molecular imprinting.

We report an experimental realization of a gel system in which frustrations exist and can be minimized, thus meeting two crucial criteria predicted to enable memory of conformations in polymers. The gels consist of a thermosensitive major monomer component and two minor components. One minor component is positively charged and will form complexes around negatively charged target molecules placed in solution. The complexes can be imprinted into the gel by then cross-linking the second minor component, which will form cross-links additional to those in the major polymer matrix. The complexes are destroyed and reformed upon swelling and reshrinking of the gels, showing that memorization has been achieved.

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PubMed2000

Stripe glasses: self-generated randomness in a uniformly frustrated system

We show that a system with competing interactions on different length scales, relevant to the formation of stripes in doped Mott insulators, undergoes a self-generated glass transition which is caused by the frustrated nature of the interactions and not related to the presence of quenched disorder. An exponentially large number of metastable configurations is found, leading to a slow, landscape dominated long time relaxation and a breakup of the system into a disordered inhomogeneous state.

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PubMed2001

Entropy-vanishing transition and glassy dynamics in frustrated spins.

In an effort to understand the glass transition, the dynamics of a nonrandomly frustrated spin model has been analyzed. The phenomenology of the spin model is similar to that of a supercooled liquid undergoing the glass transition. The slow dynamics can be associated with the presence of extended stringlike structures which demarcate regions of fast spin flips. An entropy-vanishing transition, with the string density as the order parameter, is related to the observed glass transition in the spin model.

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PubMed2001

Quantum frustration in the "spin liquid" phase of two-dimensional 3He.

We have measured the ultralow temperature and low field magnetic susceptibility of the 4/7 phase of two-dimensional 3He adsorbed on graphite preplated by one layer of 4He. The experiments are performed by progressively adding 4He to the system, thus suppressing in a controlled way the 3He atoms trapped in substrate heterogeneities. This procedure enables us to determine the intrinsic properties of this spin 1/2 model magnet in the zero field limit. The results show quantitatively that the system is strongly frustrated by multiple spin exchange interactions. A characteristic gapped spin liquid behavior is observed at ultralow temperature.

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PubMed2001

Orbital frustration at the origin of the magnetic behavior in LiNiO2.

We report on the ESR, magnetization, and magnetic susceptibility measurements performed over a large temperature range, from 1.5 to 750 K, on high-quality stoichiometric LiNiO2. We find that this compound displays two distinct temperature regions where its magnetic behavior is anomalous. With the help of a statistical model based on the Kugel'-Khomskii Hamiltonian, we show that below T(of) approximately 400 K, an orbitally frustrated state characteristic of the triangular lattice is established. This then gives a solution to the long-standing controversial problem of the magnetic behavior in LiNiO2.

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PubMed2001

Spin dynamics in a frustrated magnet.

180 degrees spin flips have been identified as the dominant fluctuation mechanism at the transverse spin freezing transition in partially frustrated a-Fe92Zr8. The form of the selective excitation double Mössbauer spectra, coupled with the perfect agreement with zero-field muon spin relaxation data, eliminates other relaxation forms.

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PubMed2001

Spin fluctuations in a magnetically frustrated metal LiV(2)O(4).

Inelastic neutron scattering is used to characterize spin fluctuations in the d-electron heavy fermion spinel LiV(2)O(4). The spin-relaxation rate, gamma(Q), for Q = 0.6 A(-1) is 1.4(2) meV at low temperatures and increases linearly with temperature at a rate of 0.46(8)k(B). There is antiferromagnetic short-range order at low temperatures with a characteristic wave vector Q(c) = 0.64(2) A(-1) and a correlation length of 6(1) A. While warming shifts intensity towards lower Q, the staggered susceptibility peaks at a finite wave vector for T < 80 K. The data are compared with conventional heavy fermion systems, geometrically frustrated insulating magnets, and recent theories for LiV(2)O(4).

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PubMed2001

Probable Langevin-like director reorientation in an interface-induced disordered SmC*-like state of liquid crystals characterized by frustration between ferro- and antiferroelectricity.

To clarify the thresholdless, hysteresis free V-shaped switching due to frustration between ferro- and antiferroelectricity, we have studied a prototype binary mixture system. The apparent orientational order parameters, and , obtained from polarized Raman scattering in thin homogeneous cells indicate that substrate interfaces induce some randomization of local in-plane directors at the tip of the V. Their correlation lengths, xi( parallel) approximately 3.5 nm and xi( perpendicular) approximately 75 nm, have been estimated by assuming the Langevin-like reorientation. Because of the much shorter xi(parallel) and xi(perpendicular) than the visible light wavelength, the switching process looks uniform.

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PubMed2001

Nonequilibrium transitions in fully frustrated Josephson junction arrays.

We study the effect of thermal fluctuations in a fully frustrated Josephson junction array driven by a current I larger than the apparent critical current I(c)(T). We calculate numerically the behavior of the chiral order parameter of Z2 symmetry and the transverse helicity modulus [related to the U(1) symmetry] as a function of temperature. We find that the Z2 transition occurs at a temperature T(Z2)(I) which is lower than the temperature T(U(1))(I) for the U(1) transition. Both transitions could be observed experimentally from measurements of the longitudinal and transverse voltages.

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PubMed2001

Resonating valence bond wave functions for strongly frustrated spin systems.

The resonating-valence-bond (RVB) theory for two-dimensional quantum antiferromagnets is shown to be the correct paradigm for large enough "quantum frustration." This scenario, proposed a long time ago but never confirmed by microscopic calculations, is strongly supported by a new type of variational wave function, which is extremely close to the exact ground state of the J(1)-J(2) Heisenberg model for 0.4 less than approximately J(2)/J(1) less than approximately 0.5. This wave function is proposed to represent the generic spin-half RVB ground state in spin liquids.

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