arXiv subjects

Gia-Wei Chern

Publications and source records attributed to Gia-Wei Chern.

At least 145 records · Page 8Linked to original sources

arXiv2013

Helimagnons in a chiral ground state of the pyrochlore antiferromagnets

The Goldstone mode in a helical magnetic phase, also known as the helimagnon, is a propagating mode with a highly anisotropic dispersion relation. Here we study theoretically the helimagnon excitations in a complex chiral ground state of pyrochlore antiferromagnets such as spinel CdCr2O4 and itinerant magnet YMn2. We show that the effective theory of the soft modes in the helical state possesses a symmetry similar to that of smectic liquid crystals. We compute the low-energy spin-wave spectrum based on a microscopic spin Hamiltonian and find a dispersion relation characteristic of the helimagnons. By performing dynamics simulations with realistic model parameters, we also obtain an overall agreement between experiment and the numerical spin-wave spectrum. Our work thus also clarifies the mechanisms that relive the magnetic frustration in these compounds.

cond-mat.str-el↗

arXiv2013

Dipolar order by disorder in the classical Heisenberg antiferromagnet on the kagome lattice

Ever since the experiments which founded the field of highly frustrated magnetism, the kagome Heisenberg antiferromagnet has been the archetypical setting for the study of fluctuation induced exotic ordering. To this day the nature of its classical low-temperature state has remained a mystery: the non-linear nature of the fluctuations around the exponentially numerous harmonically degenerate ground states has not permitted a controlled theory, while its complex energy landscape has precluded numerical simulations at low temperature. Here we present an efficient Monte Carlo algorithm which removes the latter obstacle. Our simulations detect a low-temperature regime in which correlations saturate at a remarkably small value. Feeding these results into an effective model and analyzing the results in the framework of an appropriate field theory implies the presence of long-range dipolar spin order with a tripled unit cell.

cond-mat.str-el↗

arXiv2012

Electronic transport in the Coulomb phase of the pyrochlore spin ice

We investigate the transport properties of itinerant electrons interacting with a background of localized spins in a correlated paramagnetic phase of the pyrochlore lattice. We find a residual resistivity at zero temperature due to the scattering of electrons by the static dipolar spin-spin correlation that characterizes the metallic Coulomb phase. As temperature increases, thermally excited topological defects, also known as magnetic monopoles, reduce the spin correlation, hence suppressing electron scattering. Combined with the usual scattering processes in metals at higher temperatures, this mechanism yields a non-monotonic resistivity, displaying a minimum at temperature scales associated with the magnetic monopole excitation energy. Our calculations agree quantitatively with resistivity measurements in Nd2Ir2O7, shedding light on the origin of the resistivity minimum observed in metallic spin-ice compounds.

cond-mat.str-el↗

arXiv2012

Spontaneous quantum Hall effect via thermally induced quadratic Fermi point

Gapless electronic systems containing topologically nontrivial Fermi points are sources of various topological insulators. Whereas most of these special band-crossing points are built in the electronic structure of the non-interacting lattice models, we show that a quadratic Fermi point characterized by a non-zero winding number emerges with a collinear triple-Q spin-density-wave state that arises from a perfectly nested but topologically trivial Fermi surface. We obtain a universal low-energy Hamiltonian for the quadratic Fermi point and show that such collinear orderings are unstable against the onset of scalar spin chirality that opens a gap and induces a spontaneous quantum Hall insulator as the temperature tends to zero.

cond-mat.str-el↗

arXiv2012

Broken translational symmetry in an emergent paramagnetic phase of graphene

We show that the spin-density wave state on the partially filled honeycomb and triangular lattices is preempted by a paramagnetic phase that breaks an emergent Z_4 symmetry of the system, associated with the four inequivalent arrangements of spins in the quadrupled unit cell. Unlike other emergent paramagnetic phases in itinerant and localized-spin systems, this state preserves the rotational symmetry of the lattice but breaks its translational symmetry, giving rise to a super-lattice structure that can be detected by scanning tunneling microscopy. This emergent phase also has distinctive signatures in the magnetic spectrum that can be probed experimentally.

cond-mat.str-el↗

arXiv2012

Orbital disorder induced by charge fluctuations in vanadium spinels

Motivated by recent experiments on vanadium spinels, $A$V$_2$O$_4$, that show an increasing degree of electronic delocalization for smaller cation sizes, we study the evolution of orbital ordering (OO) between the strong and intermediate-coupling regimes of a multi-orbital Hubbard Hamiltonian. The underlying magnetic ordering of the Mott insulating state leads to a rapid suppression of OO due to enhanced charge fluctuations along ferromagnetic bonds. Orbital double-occupancy is rather low at the transition point indicating that the system is in the crossover region between strong and intermediate-coupling regimes when the orbital degrees of freedom become disordered.

cond-mat.str-el↗

arXiv2012

Magnetic charge and ordering in kagome spin ice

We present a numerical study of magnetic ordering in spin ice on kagome, a two-dimensional lattice of corner-sharing triangles. The magnet has six ground states and the ordering occurs in two stages, as one might expect for a six-state clock model. In spin ice with short-range interactions up to second neighbors, there is an intermediate critical phase separated from the paramagnetic and ordered phases by Kosterlitz-Thouless transitions. In dipolar spin ice, the intermediate phase has long-range order of staggered magnetic charges. The high and low-temperature phase transitions are of the Ising and 3-state Potts universality classes, respectively. Freeze-out of defects in the charge order produces a very large spin correlation length in the intermediate phase. As a result of that, the lower-temperature transition appears to be of the Kosterlitz-Thouless type.

cond-mat.str-el↗

arXiv2012

Itinerant half-metal spin-density-wave state on the hexagonal lattice

We consider electrons on a honeycomb or triangular lattice doped to the saddle point of the bandstructure. We assume system parameters are such that spin density wave (SDW) order emerges below a temperature $T_N$ and investigate the nature of the SDW phase. We argue that at $T < T_N$ the system develops a uniaxial SDW phase whose ordering pattern breaks $O(3) \times Z_4$ symmetry and corresponds to an eight site unit cell with non-uniform spin moments on different sites. This state is a half-metal -- it preserves full original Fermi surface, but has gapless charged excitations in one spin branch only. It allows for electrical control of spin currents and is desirable for nano-science.

cond-mat.str-el↗

arXiv2011

Orbital ice: an exact Coulomb phase on the diamond lattice

We demonstrate the existence of orbital Coulomb phase as the exact ground state of p-orbital exchange Hamiltonian on the diamond lattice. The Coulomb phase is an emergent state characterized by algebraic dipolar correlations and a gauge structure resulting from local constraints (ice rules) of the underlying lattice models. For most ice models on the pyrochlore lattice, these local constraints are a direct consequence of minimizing the energy of each individual tetrahedron. On the contrary, the orbital ice rules are emergent phenomena resulting from the quantum orbital dynamics. We show that the orbital ice model exhibits an emergent geometrical frustration by mapping the degenerate quantum orbital ground states to the spin-ice states obeying the 2-in-2-out constraints on the pyrochlore lattice. We also discuss possible realization of the orbital ice model in optical lattices with p-band fermionic cold atoms.

cond-mat.str-el↗

arXiv2011

Spin superstructure and noncoplanar ordering in metallic pyrochlore magnets with degenerate orbitals

We study double-exchange models with itinerant t2g electrons in spinel and pyrochlore crystals. In both cases the localized spins form a network of corner-sharing tetrahedra. We show that the strong directional dependence of t2g orbitals leads to unusual Fermi surfaces that induce spin superstructures and noncoplanar orderings for a weak coupling between itinerant electrons and localized spins. Implications of our results to ZnV2O4 and Cd2Os2O7 are also discussed.

cond-mat.str-el↗

arXiv2011

Topology of Entanglement Evolution of Two Qubits

The dynamics of a two-qubit system is considered with the aim of a general categorization of the different ways in which entanglement can disappear in the course of the evolution, e.g., entanglement sudden death. The dynamics is described by the function ~n(t), where ~n is the 15-dimensional polarization vector. This representation is particularly useful because the components of ~n are direct physical observables, there is a meaningful notion of orthogonality, and the concurrence C can be computed for any point in the space. We analyze the topology of the space S of separable states (those having C = 0), and the often lower-dimensional linear dynamical subspace D that is characteristic of a specific physical model. This allows us to give a rigorous characterization of the four possible kinds of entanglement evolution. Which evolution is realized depends on the dimensionality of D and of D \cap S, the position of the asymptotic point of the evolution, and whether or not the evolution is "distance-Markovian", a notion we define. We give several examples to illustrate the general principles, and to give a method to compute critical points. We construct a model that shows all four behaviors.

quant-ph↗

arXiv2011

Quantum phase transitions in a strongly entangled spin-orbital chain: A field-theoretical approach

Motivated by recent experiments on quasi-1D vanadium oxides, we study quantum phase transitions in a one-dimensional spin-orbital model describing a Haldane chain and a classical Ising chain locally coupled by the relativistic spin-orbit interaction. By employing a field-theoretical approach, we analyze the topology of the ground-state phase diagram and identify the nature of the phase transitions. In the strong coupling limit, a long-range Néel order of entangled spin and orbital angular momentum appears in the ground state. We find that, depending on the relative scales of the spin and orbital gaps, the linear chain follows two distinct routes to reach the Néel state. First, when the orbital exchange is the dominating energy scale, a two-stage ordering takes place in which the magnetic transition is followed by melting of the orbital Ising order; both transitions belong to the two-dimensional Ising universality class. In the opposite limit, the low-energy orbital modes undergo a continuous reordering transition which represents a line of Gaussian critical points. On this line the orbital degrees of freedom form a Tomonaga-Luttinger liquid. We argue that the emergence of the Gaussian criticality results from merging of the two Ising transitions in the strong hybridization region where the characteristic spin and orbital energy scales become comparable. Finally, we show that, due to the spin-orbit coupling, an external magnetic field acting on the spins can induce an orbital Ising transition.

cond-mat.str-el↗

arXiv2011

Two-stage ordering of spins in a dipolar spin ice on the kagome lattice

Spin ice, a peculiar thermal state of a frustrated ferromagnet on the pyrochlore lattice, has a finite entropy density and excitations carrying magnetic charge. By combining analytical arguments and Monte Carlo simulations, we show that spin ice on the two-dimensional kagome lattice orders in two stages. The intermediate phase has ordered magnetic charges and is separated from the paramagnetic phase by an Ising transition. The transition to the low-temperature phase is of the three-state Potts or Kosterlitz-Thouless type, depending on the presence of defects in charge order.

cond-mat.str-el↗

arXiv2011

Quantum criticality of vanadium chains with strong relativistic spin-orbit interaction

We study quantum phase transitions induced by the on-site spin-orbit interaction lambda(L.S) in a toy model of vanadium chains. In the lambda->0 limit, the decoupled spin and orbital sectors are described by a Haldane and an Ising chain, respectively. The gapped ground state is composed of a ferro-orbital order and a spin liquid with finite correlation lengths. In the opposite limit, strong spin-orbital entanglement results in a simultaneous spin and orbital-moment ordering, which can be viewed as an orbital liquid. Using a combination of analytical arguments and density-matrix renormalization group calculation, we show that an intermediate phase, where the ferro-orbital state is accompanied by a spin Neel order, is bounded on both sides by Ising transition lines. Implications for vanadium compounds CaV2O4 and ZnV2O4 are also discussed.

cond-mat.str-el↗

arXiv2010

Pyrochlore antiferromagnet with antisymmetric exchange interactions: critical behavior and order from disorder

We investigate the nature of phase transitions induced by Dzyaloshinskii-Moriya (DM) interactions on a classical pyrochlore antiferromagnet. For both symmetry-allowed antisymmetric exchange interactions, the macroscopic degeneracy due to geometrical frustration is relieved and a long-range magnetic order appears in the ground state. We find an Ising-like phase transition in the case of direct DM interaction which selects a doubly degenerate all-in all-out magnetic order. In the presence of indirect DM coupling, the magnet undergoes an XY-like phase transition into a state with broken Z6 symmetry. We show that the critical behavior in both cases is modified due to the constrained spin fluctuations in the correlated Coulomb phase. We also demonstrate an interesting order from disorder phenomenon where the system switches between two distinct types of broken Z6 symmetry.

cond-mat.str-el↗

arXiv2010

Noncoplanar magnetic ordering driven by itinerant electrons on the pyrochlore lattice

Exchange interaction tends to favor collinear or coplanar magnetic orders in rotationally invariant spin systems. Indeed, such magnetic structures are usually selected by thermal or quantum fluctuations in highly frustrated magnets. Here we show that a complex noncoplanar magnetic order with a quadrupled unit cell is stabilized by itinerant electrons on the pyrochlore lattice. Specifically we consider the Kondo-lattice and Hubbard models at quarter filling. The electron Fermi 'surface' at this filling factor is topologically equivalent to three intersecting Fermi circles. Perfect nesting of the Fermi lines leads to magnetic ordering with multiple wavevectors and a definite handedness. The chiral order might persist without magnetic order in a chiral spin liquid at finite temperatures.

cond-mat.str-el↗

arXiv2010

Halfvortices in flat nanomagnets

We discuss a new type of topological defect in XY systems where the O(2) symmetry is broken in the presence of a boundary. Of particular interest is the appearance of such defects in nanomagnets with a planar geometry. They are manifested as kinks of magnetization along the edge and can be viewed as halfvortices with winding numbers \pm 1/2. We argue that halfvortices play a role equally important to that of ordinary vortices in the statics and dynamics of flat nanomagnets. Domain walls found in experiments and numerical simulations are composite objects containing two or more of these elementary defects. We also discuss a closely related system: the two-dimensional smectic liquid crystal films with planar boundary condition.

cond-mat.mtrl-sci↗

arXiv2010

Three-dimensional topological phases in a layered honeycomb spin-orbital model

We present an exactly solvable spin-orbital model based on the Gamma-matrix generalization of a Kitaev-type Hamiltonian. In the presence of small magnetic fields, the model exhibits a critical phase with a spectrum characterized by topologically protected Fermi points. Upon increasing the magnetic field, Fermi points carrying opposite topological charges move toward each other and annihilate at a critical field, signaling a phase transition into a gapped phase with trivial topology in three dimensions. On the other hand, by subjecting the system to a staggered magnetic field, an effective time-reversal symmetry essential to the existence of three-dimensional topological insulators is restored in the auxiliary free fermion problem. The nontrivial topology of the gapped ground state is characterized by an integer winding number and manifests itself through the appearance of gapless Majorana fermions confined to the two-dimensional surface of a finite system.

cond-mat.mes-hall↗