Charge Frustration Research Articles

Superconducting-insulating transition in quantum three-dimensional Josephson junction arrays with magnetic and charge frustration

We study the effect of the competition between Josephson and charging energies on the phase coherence transition in three-dimensional anisotropic network of Josephson-coupled junctions. in the presence of external magnetic flux and charge frustration. To capture the effects of quantum and spatial fluctuations we employ a quantum spherical approximation for the Josephson-junction array Hamiltonian. We examine the T=0 and finite temperature superconductor-insulator phase transitions as functions of a number of control parameters which include self-capitance and junction capacitance, rational magnetic flux, and the charge frustration parameter as well as the lattice anisotropy.

Structure Transition and Charge Competition on YFe 2 O 4

We report the low temperature charge ordered structure and magnetic relaxation in nearly stoichiometric YFe 2 O 4 in which the charge frustration effect is considered to be dominant. The superlattice reflections have revealed the presence of four different phases with a triple, double, quadruple and septuplex charge-superstructure in the temperature range from room temperature to 130 K. The observed successive transitions have been proposed by a previous X-ray powder diffraction study. A slow relaxation of the superstructure was found at 130 K. Time dependence of the magnetic moment was observed at about 240 K to suggest a similar relaxation.

New Phenomena Arising from Charge and Geometrical Frustration

New Phenomena Arising from Charge and Geometrical Frustration

Charge Ordering in the Geometrically Frustrated Spinel AlV2O4

We have found that spinel AlV 2 O 4 shows a phase transition at ∼700 K with anomalies of transport and magnetic properties. It is clarified that this corresponds to a charge-ordering transition, where the V 2.5+ ions are separated into three V 2.5-δ ions and one V 2.5+3δ ion. This three-one type of charge ordering, which is coupled with rhombohedral distortion along the (111) axis, reduces the high degeneracy of ground states inherent in this spinel structure, and thus can be understood in terms of “charge frustration.”

Superconducting phase transition in quantum three-dimensional Josephson junction arrays:c-axis anisotropy and charge frustration effects

We study the quantum phase transition in three-dimensional network of Josephson-coupled junctions including anisotropy and capacitance-matrix effects. The quantum effects are due to the charging energy ${E}_{C}$ which is related to interplay of self-${(C}_{0})$ and mutual (or junction ${C}_{1})$ capacitances. The anisotropy is defined as the ratio of the in-plane Josephson energy ${E}_{J\ensuremath{\Vert}}$ to the interlayer one ${E}_{J\ensuremath{\perp}}.$ The external charge frustration effects are also considered at both zero and finite temperatures. To capture the effects of quantum and spatial fluctuations we go beyond the mean-field level description. Noting that the invariant of phase fluctuation algebra between number and phase operators (given by the Euclidean group ${E}_{2})$ is related to the closure relation of the corresponding solvable quantum spherical model we map the quantum Hamiltonian of the array onto an effective action of a generalized spherical model in the path integral formalism. Subsequently, we examine the $T=0$ and finite-temperature superconductor-paracoherent phase boundary as a function of various control parameters.

Charge Frustration and Dielectric Dispersion in LuFe2O4

We report the dielectric anomaries at the phase transitions of Lu(Fe 2+ Fe 3+ )O 4 , which is considered as a charge and spin frustrated system. Dielectric constant shows a hump, just above the transition from the high temperature two-dimensional to the low temperature three-dimensional charge ordering. A few glassy behaviors were observed at this transition. The presence of spontaneous polarization in the low temperature phase was confirmed by the measurement of pyroelectricity. A step-wise increase of the dielectric constant with increasing temperature was observed between 150 K and 250 K as in other RFe 2 O 4 . These phenomena are explained by the presence of the polar domains with the ordering of Fe 2+ and Fe 3+ and the motions of the antiphase boundaries between them.

Mean-field theory for Josephson junction arrays with charge frustration

Using the path integral approach, we provide an explicit derivation of the equation for the phase boundary for quantum Josephson junction arrays with offset charges and non-diagonal capacitance matrix. For the model with nearest neighbor capacitance matrix and uniform offset charge $q/2e=1/2$, we determine, in the low critical temperature expansion, the most relevant contributions to the equation for the phase boundary. We explicitly construct the charge distributions on the lattice corresponding to the lowest energies. We find a reentrant behavior even with a short ranged interaction. A merit of the path integral approach is that it allows to provide an elegant derivation of the Ginzburg-Landau free energy for a general model with charge frustration and non-diagonal capacitance matrix. The partition function factorizes as a product of a topological term, depending only on a set of integers, and a non-topological one, which is explicitly evaluated.

Charge frustration effects in capacitively coupled two-dimensional Josephson-junction arrays

We investigate the quantum phase transitions in two capacitively coupled two-dimensional Josephson-junction arrays with charge frustration. The system is mapped onto the S=1 and $S=1/2$ anisotropic Heisenberg antiferromagnets near the particle-hole symmetry line and near the maximal-frustration line, respectively, which are in turn argued to be effectively described by a single quantum phase model. Based on the resulting model, it is suggested that near the maximal frustration line the system may undergo a quantum phase transition from the charge-density wave to the super-solid phase, which displays both diagonal and off- diagonal long-range order.

Cotunneling Transport and Quantum Phase Transitions in Coupled Josephson-Junction Chains with Charge Frustration

We investigate the quantum phase transitions in two capacitively coupled chains of ultrasmall Josephson junctions, where the particle-hole symmetry is broken by the gate voltage applied to each superconducting island. Near the maximal-frustration line, cotunneling of the particles along the two chains is shown to play a major role in the transport and to drive a quantum phase transition out of the charge-density-wave insulator, as the Josephson-coupling energy is increased. We also argue briefly that slightly off the symmetry line the universality class of the transition remains the same as that right on the line, being driven by the particle-hole pairs.

Incommensurate Charge Ordering in Charge-Frustrated LuFe2O4System

The charge ordering process of a mixed valence LuFe 2 O 4 system has been studied, focusing attention on the frustration effect associated with the charge configuration. It is revealed that the system undergoes successive phase transitions following the sequence of disorder→2D CDW→3D CDW. In the 2D-CDW state, the charge density wave defined by the wave vector (1/3, 1/3), is formed in the hexagonal layer. On the nodal lines of the CDW, the charge order is still undetermined due to the charge frustration. From a dielectric viewpoint, this phase is viewed as random stacking of 2D ferroelectric layers. In the 3D-CDW state, antiphase stacking of the 2D ordered layer develops. At the same time, the in-plane charge order exhibits a doubly modulated long-period structure. From a dielectric viewpoint, this phase is characterized by an incommensurate antiferroelectric phase. Relevance of the charge ordering mechanism to the observed anomalously large dielectric constant in LuFe 2 O 4 is discussed.

Incommensurate charge ordering in mixed valence system LuFe 2O 4

Charge ordering of a mixed valence system LuFe 2O 4 has been studied by neutron scattering technique, focusing attention on the charge frustration effect on triangular lattice. It is revealed that the system undergoes successive phase transitions following the sequence of disorder → two-dimensional charge density wave state (2D-CDW) → three-dimensional charged density wave state (3D-CDW). In 2D-CDW state, the charge density wave defined by the wave vector ( 1 3 1 3 ) is formed in the hexagonal layer.

Charge frustration in complex fluids and in electronic systems

The idea of charge frustration is applied to describe the properties of such diverse physical systems as oil-water-surfactant mixtures and metal-ammonia solutions. The minimalist charge-frustrated model possesses one energy scale and two length scales. For oil-water-surfactant mixtures, these parameters have been determined starting from the microscopic properties of the physical systems under study. Thus, microscopic properties are successfully related to the observed mesoscopic structure.

Supersolid phase in fully frustrated Josephson-junction arrays

We study the phase diagram and the excitation spectra of an array of small Josephson junctions at $f=1/2$ and arbitrary charge frustration. We find that the supersolid region in the phase diagram is larger than the correspondent region at $f=0$ and it includes two different phases.In the chiral supersolid (SS) charges and vortices are arranged in a checkerboard pattern on a $2\:\times \:2$ supercell analogously to the unfrustrated case. We find a new phase, which we term {\em non-chiral supersolid} (NCSS), which has no corresponding phase at $f=0$. The excitation spectra in the supersolid regions show particle like dispersions which is related to the defectons. The defecton condensation leads to superfluidity in the presence of charge ordered background.

Charge representation of a small two-dimensional Josephson-junction array in the quantum regime.

Using the charge representation, we calculate the ground state energy and the critical current of a small two-dimensional Josephson junction array subject to both charge and magnetic frustration. In the quantum regime the ground state of the array is a superposition of charge states, allowing a supercurrent to flow through the circuit. Both the ground state energy and the critical current can be tuned by the two frustrations. We show that the notion of a vortex is compatible with a charge representation of the array. \textcopyright{} 1996 The American Physical Society.

Supersolid in fully frustrated Josephson junction array

We study the phase diagram of a fully frustrated quantum Josephson junction array in the presence of charge frustration. In addition to the superconducting and the Mott phases, there are regions in the phase diagram with supersolid order. One of the supersolid phases shows a vortex current distribution which differs from the classical one. We also study the excitaton spectrum of the low lying excitations in the various regions of the phase diagram.

Assembly of extended interfaces and micelles: charge frustrated models of amphiphilic mixtures

This paper describes aspects of self-assembly in complex fluids in terms of the concept of charge frustration. This concept follows from the analogy between electrostatic energy and the entropic constraints of amphiphilic stoichiometry. It provides a minimalist molecular description of self-assembling systems. The concept is illustrated in this paper with two types of applications. The first concerns the theory of extended interfaces i.e. bilayers in water–surfactant mixtures. The second concerns the theory of micelles. The conditions under which these structures assemble in terms of molecular quantities such as the chain length of the surfactant molecules are predicted, and comparison with experiment is discussed.

Energetics and gap engineering in alternating layer and intralayer substituted boron-nitrogen-carbon compounds

Using ab initio quantum chemical methods, the electronic structures of both alternating layer (BNC 2) and intralayer substituted hybrids (BNC 2, BNC 3.33, BNC 6 and BNC 14) of graphite and hexagonal boron nitride are investigated. While total energy considerations under inclusion of interlayer interactions are emphasized for the former, the magnitude of the fundamental gap is of central interest for the latter. Our main results are: (1) alternating layer hybrids are not less stable than intralayer substituted species; (2) all compounds are found to be semiconductors; (3) the semiconducting behaviour is a consequence of the structural stabilization of charge density waves within the carbon sublattices; (4) at least in the case of intralayer substituted BNC compounds, gaps can theoretically be ‘engineered’ by varying the ratio of BN to C and by using the ‘concept of charge frustration’.

Quantum phase transitions and commensurability in frustrated Josephson junction arrays

The superconductor-insulator transition of an array of Josephson junctions is studied. In junctions with small capacitance the interaction of charges introduces quantum fluctuations and shifts the phase transition. An external voltage controls the total charge and creates "charge frustration". Commensurability effects lead to similar structure in the phase diagram as had been found for magnetic frustration. The model used to describe a nonclassical junction array is similar to a Bose-Hubbard model. We study and compare the phase diagrams obtained from both models. If the interaction in the Bose model has a finite range we predict the commensurability to lead to a richly structured dependence on the chemical potential.

On ammonium and the solubility of metals without chemical action

The idea of charge frustration is applied to describe the properties of such diverse physical systems as oil-water-surfactant mixtures and metal-ammonia solutions. The minimalist charge-frustrated model possesses one energy scale and two length scales. For oil-water-surfactant mixtures, these parameters have been determined starting from the microscopic properties of the physical systems under study. Thus, microscopic properties are successfully related to the observed mesoscopic structure.