Orbital Antiferromagnetism Research Articles

Quantum corrections for the phase diagram of systems with competing order

We use the effective potential method of quantum field theory to obtain the quantum corrections to the zero temperature phase diagram of systems with competing order parameters. We are particularly interested in two different scenarios: regions of the phase diagram where there is a bicritical point, at which both phases vanish continuously, and the case where both phases coexist homogeneously. We consider different types of couplings between the order parameters, including a bilinear one. This kind of coupling breaks time-reversal symmetry and it is only allowed if both order parameters transform according to the same irreducible representation. This occurs in many physical systems of actual interest like competing spin density waves, different types of orbital antiferromagnetism, elastic instabilities of crystal lattices, vortices in a multigap SC and also applies to describe the unusual magnetism of the heavy fermion compound URu2Si2. Our results show that quantum corrections have an important effect on the phase diagram of systems with competing orders.

Analogy Between the "Hidden Order" and the Orbital Antiferromagnetism in URu_{2-x}Fe_{x}Si_{2}.

We study URu_{2-x}Fe_{x}Si_{2}, in which two types of staggered phases compete at low temperature as the iron concentration x is varied: the nonmagnetic "hidden order" (HO) phase below the critical concentration x_{c}, and unconventional antiferromagnetic (AFM) phase above x_{c}. By using polarization resolved Raman spectroscopy, we detect a collective mode of pseudovectorlike A_{2g} symmetry whose energy continuously evolves with increasing x; it monotonically decreases in the HO phase until it vanishes at x=x_{c}, and then reappears with increasing energy in the AFM phase. The mode's evolution provides direct evidence for a unified order parameter for both nonmagnetic and magnetic phases arising from the orbital degrees-of-freedom of the uranium-5f electrons.

Calculation for polar Kerr effect in high-temperature cuprate superconductors

A mechanism is proposed for the tantalizing evidence of polar Kerr effect in a class of high temperature superconductors--the signs of the Kerr angle from two opposite faces of the same sample are identical and magnetic field training is non-existent. The mechanism does not break global time reversal symmetry, as in an antiferromagnet, and results in zero Faraday effect. It is best understood in a phenomenological model of bilayer cuprates, such as YBCO, in which intra-bilayer tunneling nucleates a chiral d-density wave such that the individual layers have opposite chirality. Although specific to the chiral d-density wave, the mechanism may be more general to any quasi-two-dimensional orbital antiferromagnet in which time reversal symmetry is broken in each plane, but not when averaged macroscopically.

Spin-Current Order in Anisotropic Triangular Antiferromagnets

We analyze instabilities of the collinear up-up-down state of a two-dimensional quantum spin-S spatially anisotropic triangular lattice antiferromagnet in a magnetic field. We find, within the large-S approximation, that near the end point of the plateau, the collinear state becomes unstable due to the condensation of two-magnon bound pairs rather than single magnons. The two-magnon instability leads to a novel two-dimensional vector chiral phase with alternating spin currents but no magnetic order in the direction transverse to the field. This phase breaks a discrete Z(2) symmetry but preserves a continuous U(1) one of rotations about the field axis. It possesses orbital antiferromagnetism and displays a magnetoelectric effect.

Diamagnetic Susceptibility Obtained from the Six-Vertex Model and Its Implications for the High-Temperature Diamagnetic State of Cuprate Superconductors

We study the diamagnetism of the six-vertex model with the arrows as directed bond currents. To our knowledge, this is the first study of the diamagnetism of this model. A special version of this model, called the F model, describes the thermal disordering transition of an orbital antiferromagnet, known as d-density wave, a proposed state for the pseudogap phase of the high-T(c) cuprates. We find that the F model is strongly diamagnetic and the susceptibility may diverge in the high-temperature critical phase with power-law arrow correlations. These results may explain the surprising recent observation of a diverging low-field diamagnetic susceptibility seen in some optimally doped cuprates within the d-density wave model of the pseudogap phase.

Field-Induced Orbital Antiferromagnetism in Mott Insulators

We report on a new electromagnetic phenomenon that emerges in Mott insulators. The phenomenon manifests as antiferromagnetic ordering due to orbital electric currents which are spontaneously generated from the coupling between spin currents and an external homogenous magnetic field. This novel spin-charge-current effect provides the mechanism to measure the so-far elusive spin currents by means of unpolarized neutron scattering, nuclear magnetic resonance or muon spectroscopy. We illustrate this mechanism by solving a half-filled Hubbard model on a frustrated ladder.

Magnetic phases of one-dimensional lattices with 2–4 fermions per site

We study the spectral and magnetic properties of one-dimensional lattices filled with 2–4 fermions (with spin-1/2) per lattice site. We use a generalized Hubbard model that takes into account all the interactions on a lattice site, and solve the many-particle problem by exact diagonalization. We find an intriguing magnetic phase diagram which includes ferromagnetism, spin-1 Heisenberg antiferromagnetism and orbital antiferromagnetism.

Atomic matter of nonzero-momentum Bose-Einstein condensation and orbital current order

The paradigm of Bose-Einstein condensation has been associated with zero momentum to which a macroscopic fraction of bosons condense. Here we propose a new quantum state where bosonic alkali-metal atoms condense at non-zero momenta, defying the paradigm. This becomes possible when the atoms are confined in the p-orbital Bloch band of an optical lattice rather than the usual s-orbital. The new condensate simultaneously forms an order of transversely staggered orbital currents, reminiscent of orbital antiferromagnetism or d-density wave in correlated electronic systems but different in fundamental ways. We discuss several approaches of preparing atoms to the p-orbital and propose an ``energy blocking'' mechanism by Feshbach resonance to protect them from decaying to the lowest s-orbital. Such a model system seems very unique and novel to atomic gases. It suggests a new concept of quantum collective phenomena of no prior example from solid state materials.

Interplay of the superconducting state and orbital antiferromagnetic state of the high-temperature cuprate superconductors

The phase diagram of the cuprates is studied in the vicinity of the tetracritical point arising due to the interplay of superconducting (SC) and orbital antiferromagnetic (OAF) ordered states. SC pairing from repulsion results in the two-component order parameter with relative phase associated with OAF order. There are two SC phases, with and without orbital currents, inside the SC dome. The weak pseudogap is associated with an OAF ordered state whereas the strong one exhibits developed fluctuations of the order parameter and enhanced diamagnetism inside this state.

Tetracritical point and staggered vortex currents in superconducting state

A phase diagram reflecting the main features of the typical phase diagram of cuprate superconductors has been studied within the framework of the Ginzburg-Landau phenomenology in the vicinity of a tetracritical point, which appears as a result of the competition of the superconducting and insulating pairing channels. The superconducting pairing under repulsive interaction corresponds to a two-component order parameter, whose relative phase is related to the orbital antiferromagnetic insulating ordering. Under weak doping, the insulating order coexists with the superconductivity at temperatures below the superconducting phase transition temperature and is manifested as a weak pseudogap above this temperature. A part of the pseudogap region adjacent to the superconducting state corresponds to developed fluctuations of the order parameter in the form of quasi-stationary states of noncoherent superconducting pairs and can be interpreted as a strong pseudogap. As the doping level is increased, the system exhibits a phase transition from the region of coexistence of the superconductivity and the orbital antiferromagnetism to the usual superconducting state. In this state, a region of developed fluctuations of the order parameter in the form of quasi-stationary states of uncorrelated orbital circular currents exists near the phase transition line.

Itinerancy and hidden order in URu2Si2

We argue that key characteristics of the enigmatic transition atT0 = 17.5 K inURu2Si2 indicate that the hidden order is a density wave formed within a band of compositequasiparticles, whose detailed structure is determined by local physics. We expand on ourproposal (with Mydosh) of the hidden order as incommensurate orbital antiferromagnetismand present experimental predictions to test our ideas. We then turn towards a microscopicdescription of orbital antiferromagnetism, exploring possible particle–hole pairings withinthe context of a simple one-band model. We end with a discussion of recent high-field andthermal transport experiments, and discuss their implications for the nature of the hiddenorder.

Topology of the Fermi surface and the coexistence of orbital antiferromagnetism and superconductivity in cuprates

It has been shown that the strong coupling model taking into account a rise in the spin antiferromagnetic insulating state explains the doping dependence of the topology and shape of the Fermi contour of superconducting cuprates. Hole pockets with shadow bands in the second Brillouin zone form the Fermi contour with perfect ordinary and mirror nesting, which ensures the coexistence of orbital antiferromagnetism and superconductivity with a large pair momentum for T < TC. The weak pseudogap region (T* < T < T*) corresponds to the orbital antiferromagnetic ordering, which coexists with the incoherent state of superconducting pairs with large momenta in the strong pseudogap region (TC < T < T*).

Local enhancement of spin and orbital antiferromagnetism around a single vortex in high-temperature superconductors

In a simple model we investigate three competing orders: d-wave superconductivity, spin and orbital antiferromagnetism (AF) in the vicinity of a single vortex in a cuprate superconductor. We find that when the potential for the orbital AF $V_{d}$ is comparatively small, the spin-density-wave (SDW) order has an enhancement at the vortex core center, and the ''d-density-wave'' (DDW) order exhibits a rather weak behavior around the core and vanishing small away from the core. However, when $V_{d}$ becomes large, globally, the SDW order decreases and the DDW order increases; locally, not only the peak of the SDW order around the core still exists, though relatively suppressed, but also a local peak for the DDW order finally appears. Similar effects are also revealed for the features when varying doping. Comparisons with experiments are discussed.

Orbital antiferromagnetism in coupled planar systems

A realistic model for the electronic structure of ${\mathrm{SrRuO}}_{3}$ is examined to determine the possibility of orbital antiferromagnetic order in this material. By calculating the susceptibility to orbital and spin antiferromagnetic order, it is shown that the band structure of ${\mathrm{SrRuO}}_{3}$ serves to destabilize the N\'eel ordered state and that the susceptibility to orbital antiferromagnetic order is larger over a range of doping. The resultant orbital antiferromagnetic state consists of coupled two-dimensional planes. The effect of the coupling on the planar system is calculated; the energy shifts of different configurations depend only on the total current in the sample and are linear in this quantity. An orbital antiferromagnetic ground state is found which has no net current flowing along the bonds.

Ambient-pressure NMR in URu2Si2: internal-field anisotropy

Abstract Ambient-pressure 99 Ru NMR spectra and lineshape parameters in URu 2 Si 2 as functions of temperature and magnetic field orientation reveal local internal field distributions at the 99 Ru sites below the hidden-order transition temperature T 0 ∼17 K . The fields are anisotropic and larger in the ab -plane. We discuss our experiments within the framework of orbital antiferromagnetism, which predicts anisotropic field distributions at the Ru sites.

Effect of ring exchange on an orbital antiferromagnet

We study the effect of four-particle ring exchange process on orbital antiferromagnetic state that occurs in some correlated electron systems in two dimensions. The primary question is whether the ring exchange process enhances or suppresses the orbital antiferromagnetic ordering. Using the fact that the orbital antiferromagnetic state arises in the large-N limit of the SU(N) generalization of the t-J model, we consider the large-N limit of the t-J-$K$ model where $K$ represents the four-particle ring exchange term. The phase diagrams in the large-N mean field theory are obtained for the half-filling and finite hole concentrations at zero temperature. It is found that the ring exchange in general favors dimerized states or the inhomogeneous orbital antiferromagnetic state, and suppresses the homogeneous orbital antiferromagnetic state. We compare our results with other related models of strongly correlated systems with ring exchange processes.

Hidden orbital order in the heavy fermion metal URu(2)Si(2).

When matter is cooled from high temperatures, collective instabilities develop among its constituent particles that lead to new kinds of order. An anomaly in the specific heat is a classic signature of this phenomenon. Usually the associated order is easily identified, but sometimes its nature remains elusive. The heavy fermion metal URu(2)Si(2) is one such example, where the order responsible for the sharp specific heat anomaly at T(0) = 17 K has remained unidentified despite more than seventeen years of effort. In URu(2)Si(2), the coexistence of large electron electron repulsion and antiferromagnetic fluctuations leads to an almost incompressible heavy electron fluid, where anisotropically paired quasiparticle states are energetically favoured. Here we develop a proposal for the nature of the hidden order in URu(2)Si(2). We show that incommensurate orbital antiferromagnetism, associated with circulating currents between the uranium ions, can account for the local fields and entropy loss observed at the 17 K transition. We make detailed predictions for the outcome of neutron scattering measurements based on this proposal, so that it can be tested experimentally.

Pressure-induced magnetism and hidden order in URu2Si2

We discuss the discovery of pressure-induced antiferromagnetism in URu2Si2, in the context of neutron, NMR and μSR results. The identification of a critical pressure separating mean-field and Ising phase transitions leads us to propose that the system lies close to a bicritical point associated with magnetic and (non-magnetic) hidden order. We note that the recent observation of an isotropic, field-independent component in the silicon NMR line-width implies that the hidden order parameter breaks time-reversal invariance. Relying on simple estimates for the observed local fields, we conclude that URu2Si2 is a strong candidate for an incommensurate orbital antiferromagnet.

Reentrant behavior of the charge and orbital ordering and antiferromagnetism inLaSr2Mn2O7

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Effects of disorder on two strongly correlated coupled chains

We study the effects of disorder on a system of two coupled chain of strongly correlated fermions (ladder system), using renormalization group. The stability of the phases of the pure system is investigated as a function of interactions both for fermions with spin and spinless fermions. For spinless fermions the repulsive side is strongly localized whereas the system with attractive interactions is stable with respect to disorder, at variance with the single chain case. For fermions with spins, the repulsive side is also localized, and in particular the d-wave superconducting phase found for the pure system is totally destroyed by an arbitrarily small amount of disorder. On the other hand the attractive side is again remarkably stable with respect to localization. We have also computed the charge stiffness, the localization length and the temperature dependence of the conductivity for the various phases. In the range of parameter where d-wave superconductivity would occur for the pure system the conductivity is found to decrease monotonically with temperature, even at high temperature, and we discuss this surprising result. For a model with one site repulsion and nearest neighbor attraction, the most stable phase is an orbital antiferromagnet . Although this phase has no divergent superconducting fluctuation it can have a divergent conductivity at low temperature. We argue based on our results that the superconductivity observed in some two chain compounds cannot be a simple stabilization of the d-wave phase found for a pure single ladder. Applications to quantum wires are discussed.