Staggered Order Parameter Research Articles

Giant magnetoanisotropy in the Josephson effect and switching of staggered order in antiferromagnets

We predict that the amplitude of the Josephson current through an antiferromagnetic weak link changes by several orders of magnitude upon rotation of the N\'eel order parameter characterizing the staggered magnetic order. This occurs due to the presence of spin-orbit coupling arising from structural inversion asymmetry which makes the band gap in the antiferromagnet highly sensitive to the staggered order parameter direction. We also demonstrate that when phase-biasing the junction, magnetization dynamics is induced which switches the direction of the N\'eel vector in the antiferromagnet. These results reveal an interesting versatility of antiferromagnetic Josephson junctions as they offer both a large tunability of the supercurrent magnitude via the staggered magnetization and phase-coherent control over the N\'eel order parameter.

Pairing transition in a double layer with interlayer Coulomb repulsion

We study the effect of interlayer Coulomb interaction in an electronic double layer. Assuming that each of the layers consists of a bipartite lattice, a sufficiently strong interlayer interaction leads to an interlayer pairing of electrons with a staggered order parameter. We show that the correlated pairing state is dual to the excitonic pairing state with uniform order parameter in an electron-hole double layer. The interlayer pairing of electrons leads to strong current-current correlations between the layers. We also analyze the interlayer conductivity and the fluctuations of the order parameter, which consists of a gapped and a gapless mode.

Giant magnetic response of a two-dimensional antiferromagnet

A fundamental difference between antiferromagnets and ferromagnets is the lack of linear coupling to a uniform magnetic field due to the staggered order parameter. Such coupling is possible via the Dzyaloshinskii-Moriya (DM) interaction but at the expense of reduced antiferromagnetic (AFM) susceptibility due to the canting-induced spin anisotropy. We solve this long-standing problem with a top-down approach that utilizes spin-orbit coupling in the presence of a hidden SU(2) symmetry. We demonstrate giant AFM responses to sub-Tesla external fields by exploiting the extremely strong two-dimensional critical fluctuations preserved under a symmetry-invariant exchange anisotropy, which is built into a square-lattice artificially synthesized as a superlattice of SrIrO3 and SrTiO3. The observed field-induced logarithmic increase of the ordering temperature enables highly efficient control of the AFM order. As antiferromagnets promise to afford switching speed and storage security far beyond ferromagnets, our symmetry-invariant approach unleashes the great potential of functional antiferromagnets.

Ising metamagnet driven by propagating magnetic field wave: Nonequilibrium phases and transitions

The nonequilibrium responses of Ising metamagnet (layered antiferromagnet) to the propagating magnetic wave are studied by Monte Carlo simulation. Here, the spatio-temporal variations of magnetic field keep the system away from equilibrium. The sublattice magnetisations show dynamical symmetry breaking in the low temperature ordered phase. The nonequilibrium phase transitions are studied from the temperature dependences of dynamic staggered order parameter, its derivative and the dynamic specific heat. The transitions are marked by the peak position of dynamic specific heat and the position of dip of the derivative of dynamic staggered order parameter. It is observed that, for lower values of the amplitudes of the propagating magnetic field, if the system is cooled from a high temperature, it undergoes a phase transition showing sharp peak of dynamic specific heat and sharp dip of the derivative of the dynamic staggered order parameter. However, for higher values of the amplitude of the propagating magnetic field, system exhibits multiple phase transitions. A comprehensive phase diagram is also plotted. The transition, for vanishingly small value of the amplitude of the propagating wave, is very close to that for equilibrium ferro–para phase transition of cubic Ising ferromagnet.

Dynamics of antiferromagnets driven by spin current

When a spin-polarized current flows through a ferromagnetic (FM) metal, angular momentum is transferred to the background magnetization via spin-transfer torques. In antiferromagnetic (AFM) materials, however, the corresponding problem is unsolved. We derive microscopically the dynamics of an AFM system driven by spin current generated by an attached FM polarizer, and find that the spin current exerts a driving force on the local staggered order parameter. The mechanism does not rely on the conservation of spin angular momentum, nor does it depend on the induced FM moments on top the AFM background. Two examples are studied: (i) A domain wall is accelerated to a terminal velocity by purely adiabatic effect where the Walker's break-down is avoided; and (ii) Spin injection modifies the AFM resonance frequency, and spin current injection triggers spin wave instability of local moments above a threshold.

STRUCTURAL DISTORTION IN LUDWIGITES

Structural distortion in a three-leg-ladder is studied in connection with Ludwigites, in particular the Fe and Co homometallic ones. Static impurities in t2g-orbitals as infinite repulsion potentials randomly located in the three-leg-ladder and a Su–Schrieffer–Heeger like tight-binding Hamiltonian are proposed and discussed. It is found that such potentials block itinerant electrons and diminish a structural staggered order parameter, related with structural distortion, as 3-M being M the number of impurities. This diminution is in detriment of Peierls like distortion that occurs in these ladders as in the case of Fe -Ludwigite. On the other hand, this diminution could explain the lack of structural distortion as in the case of Co -Ludwigite.

Geometric fluctuations in a two-dimensional quantum antiferromagnet

The paper considers the effects of random fluctuations of the local spin connectivities (fluctuations of the geometry) on ground state properties of a two-dimensional quantum antiferromagnet. We analyse the behavior of spins described by the Heisenberg model as a function of what we call phason flip disorder, following a terminology used for aperiodic systems. The calculations were carried out both within linear spin wave theory and using quantum Monte Carlo simulations. An "order by disorder" phenomenon is observed in this model, wherein antiferromagnetism is found to be enhanced by phason disorder. The value of the staggered order parameter increases with the number of defects, accompanied by an increase in the ground state energy of the system.

Variational ground states of the two-dimensional Hubbard model

Recent refinements of analytical and numerical methods have improved our understanding of the ground-state phase diagram of the two-dimensional (2D) Hubbard model. Here, we focus on variational approaches, but comparisons with both quantum cluster and Gaussian Monte Carlo methods are also made. Our own ansatz leads to an antiferromagnetic ground state at half filling with a slightly reduced staggered order parameter (as compared to simple mean-field theory). Away from half filling, we find d-wave superconductivity, but confined to densities where the Fermi surface passes through the antiferromagnetic zone boundary (if hopping between both nearest-neighbour and next-nearest-neighbour sites is considered). Our results agree surprisingly well with recent numerical studies using the quantum cluster method. An interesting trend is found by comparing gap parameters Δ (antiferromagnetic or superconducting) obtained with different variational wave functions. Δ varies by an order of magnitude and thus cannot be taken as a characteristic energy scale. In contrast, the order parameter is much less sensitive to the degree of sophistication of the variational schemes, at least at and near half filling.

Weakening of antiferromagnetism due to frustration on inhomogeneous 2D lattices

I consider the ground state of quantum spins interacting via Heisenberg antiferromagnetic exchange, in lattices having several types of local environment. In unfrustrated cases, when a Néel type order exists, it is known that quantum fluctuations are site-dependent, and lead to an inhomogeneous local staggered order parameter. This paper discusses some effects of adding frustration to such a system, by including next nearest neighbor antiferromagnetic interactions. Within linear spinwave theory, it is found that as the frustration parameter increases, the local order parameters decrease at a rate that depends on the type of sites involved in the frustrated loops. The cases of a decorated square lattice system, and of a quasiperiodic tiling are considered to study difierent ways in which antiferromagnetism is suppressed in these inhomogeneous systems.

Derivation of the generalized non-linear sigma model in the presence of the Dzyaloshinskii–Moriya interaction

We derive the long-wavelength non-linear sigma model for a two-dimensional Heisenberg system in the presence of the Dzyaloshinskii–Moriya and pseudodipolar interactions. We show that the system is a non-conventional easy-axis antiferromagnet, displaying an anomalous coupling between the magnetic field and the staggered order parameter. Our results are in good agreement with recent experimental data for undoped La 2 CuO 4 compounds.

Theory of thed-density wave from a vertex model and its implications

The thermal disordering of the $d$-density wave, proposed to be the origin of the pseudogap state of high temperature superconductors, is suggested to be the same as that of the statistical mechanical model known as the 6-vertex model. The low temperature phase consists of a staggered order parameter of circulating currents, while the disordered high temperature phase is a power-law phase with no order. A special feature of this transition is the complete lack of an observable specific heat anomaly at the transition. There is also a transition at a even higher temperature at which the magnitude of the order parameter collapses. These results are due to classical thermal fluctuations and are entirely unrelated to a quantum critical point in the ground state. The quantum mechanical ground state can be explored by incorporating processes that causes transitions between the vertices, allowing us to discuss quantum phase transition in the ground state as well as the effect of quantum criticality at a finite temperature as distinct from the power-law fluctuations in the classical regime. A generalization of the model on a triangular lattice that leads to a 20-vertex model may shed light on the Wigner glass picture of the metal-insulator transition in two-dimensional electron gas. The power-law ordered high temperature phase may be generic to a class of constrained systems and its relation to recent advances in the quantum dimer models is noted.

Effect of environmental spins on quantum coherence in antiferromagnet clusters

The two-dimensional Heisenberg antiferromagnet with an additional isotropic coupling to ``environmental'' spin degrees of freedom is studied by exact diagonalization of small clusters to examine the effect of decoherence from first principles. The dynamical correlation function for the staggered order parameter $c(\ensuremath{\omega})$ is calculated for two systems containing $10$ and $18$ electronic spins coupled to two environmental spins. The evolution of spectral weight in the main macroscopic quantum coherence resonance is followed as a function of environmental coupling and spin anisotropy and is discussed in connection with theoretical work on decoherence.

Anomalous peak effect in heavy‐fermion, intermediate‐valence and A15 superconductors: Evidence for a Fulde‐Ferrell‐Larkin‐Ovchinnikov state?

AbstractAn experimental study based upon magnetic and dilatometric measurements is presented for the clean high‐κ superconductors UPd2Al3, CeRu2 and V3Si. All three compounds show an enhanced spin susceptibility. Their superconducting state is strongly Pauli limited, and an anomalous peak effect is observed at T<(0.8‐0.9)Tc, slightly below Hc2(T). This phenomenon appears to be qualitatively consistent with a first‐order transition between weak and collective pinning, caused by the formation of a staggered order parameter in a generalized Fulde‐Ferrell‐Larkin‐Ovchinnikov phase (M. Tachiki et al., Z. Phys. B, in press).

Experimental evidence for a generalized FFLO state in clean type-II superconductors with short coherence length and enhanced Pauli susceptibility

We report an investigation of the magnetic and dilatometric properties of single crystals of the superconductors UPd 2Al 3 and CeRu 2, both compounds exhibiting enhanced spin susceptibilities. Our results suggest for both systems a first-order transition between weak and strong pinning at T < 0.9 T c, somewhat below H c2( T). We argue that these observations are compatible with a staggered order parameter due to the formation of a “generalized Fulde-Ferrell-Larkin-Ovchinnikov state”.

The staggered charge-order phase of the extended Hubbard model in the atomic limit

We study the phase diagram of the extended Hubbard model in the atomic limit. At zero temperature, the phase diagram decomposes into six regions: three with homogeneous phases (characterized by particle densities , 1 and 2 and staggered charge density ) and three with staggered phases (characterized by the densities , 1 and and staggered densities , 1, and ). Here we use Pirogov - Sinai theory to analyse the details of the phase diagram of this model at low temperatures. In particular, we show that for any sufficiently low non-zero temperature the three staggered regions merge into one staggered region S, without any phase transitions (analytic free energy and staggered order parameter ) within S.

First-order transition between weak and strong pinning in clean superconductors with enhanced spin susceptibility.

Dilatometric and magnetic experiments on high-quality single crystals of the superconducting compounds UP${\mathrm{d}}_{2}$A${\mathrm{l}}_{3}$ and CeR${\mathrm{u}}_{2}$, which have enhanced spin susceptibilities, demonstrate a first-order transition between weak and strong pinning at $Tl{0.9T}_{c}$ ( $Hg10$ kOe). This is compatible with a staggered order parameter due to the formation of a ``Fulde-Ferrell-Larkin-Ovchinnikov'' state.

Multilayer high-temperature superconductors

Abstract Using the BCS approach and a tightbinding description of the band structure, we explore some superconducting properties of layered superconductors. Particular emphasis is devoted to the interlayer pairing interaction g 3 . Negative values of g 3 lead to a staggered order parameter, having alternating signs from layer to layer, while g 3 positive leads to one sign only. Adopting a superconducting loop, interrupted by a multilayer, it is shown that the sign of g 3 can be determined experimentally. The same holds for a DC SQUID configuration, consisting of two multilayer Josephson junctions in a loop, and for an AC SQUID.

Cluster study of the t-J model: Chiral fluctuations.

Uniform and staggered chiral order are investigated in the two-dimensional {ital t}-{ital J} model at arbitrary doping fractions. Equal-time and dynamical correlations are calculated on a 4{times}4 cluster by using a Lanczos algorithm. We found that static uniform correlations are suppressed with increasing doping and {ital t}/{ital J} while static staggered correlations seem enhanced, in agreement with previous mean-field studies. The plaquette-plaquette correlation function falls off rapidly with distance, and, thus, we obtain an upper bound on the plaquette order parameter, which is very small. However, our numerical results show that doping for {ital t}/{ital J}{ge}1 induces large chiral fluctuations at low energy, which may become important even in the absence of long-range order. The staggered order parameter exhibits low-energy fluctuations of energy {similar to}2{ital J}, while for the uniform case fluctuations also occur at a higher energy {similar to}2{ital t}.