arXiv subjects

Leon Balents

Publications and source records attributed to Leon Balents.

At least 181 records · Page 10Linked to original sources

arXiv2006

Mechanisms of degeneracy breaking in pyrochlore antiferromagnets

Motivated by the low temperature magnetization curves of several spinel chromites, we theoretically study classical mechanisms of degeneracy lifting in pyrochlore antiferromagnets. Our main focus is on the coupling of spin exchange to lattice distortions. Prior work by Penc et al. (Phys. Rev. Lett. 93, 197203 (2004)) has demonstrated that such coupling leads to a robust magnetization plateau at half the saturation moment per spin, in agreement with experiment. We show that a simple Einstein model incorporating local site distortions generates a ``universal'' magnetic order on the plateau, and highlight the distinct predictions of this model from that in Penc et al. (Phys. Rev. Lett. 93, 197203 (2004)). We also consider the complementary degeneracy-lifting effects of further neighbor exchange interactions. We discuss the implications for transitions off the plateau at both the high field and low field end, as well as at fields close to the saturation value. We predict that under certain circumstances there is spontaneous {\sl uniform} XY magnetization (transverse to the field) for field values just above the plateau. These features may be tested in experiments. While selecting a unique magnetic order in the half magnetization plateau, at zero magnetic field the Einstein model retains an extensive degeneracy, though significantly reduced compared with the pure Heisenberg antiferromagnet.

cond-mat.str-el↗

arXiv2006

Detecting the quantum zero-point motion of vortices in the cuprate superconductors

We explore the experimental implications of a recent theory of the quantum dynamics of vortices in two-dimensional superfluids proximate to Mott insulators. The theory predicts modulations in the local density of states in the regions over which the vortices execute their quantum zero point motion. We use the spatial extent of such modulations in scanning tunnelling microscopy measurements on the vortex lattice of BSCCO to estimate the inertial mass of a point vortex. We discuss other, more direct, experimental signatures of the vortex dynamics.

cond-mat.supr-con↗

arXiv2006

Estimating the mass of vortices in the cuprate superconductors

We explore the experimental implications of a recent theory of the quantum dynamics of vortices in superfluids proximate to Mott insulators (cond-mat/0408329 and cond-mat/0409470). The theory predicts modulations in the local density of states in the regions over which the vortices execute their quantum zero point motion. We use the spatial extent of such modulations in scanning tunnelling microscopy measurements (Hoffman et al., cond-mat/0201348) on the vortex lattice of BSCCO to estimate the inertial mass of a point vortex. We discuss other, more direct, experimental signatures of the vortex dynamics.

cond-mat.str-el↗

arXiv2005

Coulomb drag between two spin incoherent Luttinger liquids

In a one dimensional electron gas at low enough density, the magnetic (spin) exchange energy $J$ between neighboring electrons is exponentially suppressed relative to the characteristic charge energy, the Fermi energy $E_F$. At non-zero temperature $T$, the energy hierarchy $J \ll T \ll E_F$ can be reached, and we refer to this as the spin incoherent Lutinger liquid state. We discuss the Coulomb drag between two parallel quantum wires in the spin incoherent regime, as well as the crossover to this state from the low temperature regime by using a model of a fluctuating Wigner solid. As the temperature increases from zero to above $J$ for a fixed electron density, the $2k_F$ oscillations in the density-density correlations are lost. As a result, the temperature dependence of the Coulomb drag is dramatically altered and non-monotonic dependence may result. Drag between wires of equal and unequal density are discussed, as well as the effects of weak disorder in the wires. We speculate that weak disorder may play an important role in extracting information about quantum wires in real drag experiments.

cond-mat.mes-hall↗

arXiv2005

Ordering in a frustrated pyrochlore antiferromagnet proximate to a spin liquid

We perform a general study of spin ordering on the pyrochlore lattice with a 3:1 proportionality of two spin polarizations. Equivalently, this describes valence bond solid conformations of a quantum dimer model on the diamond lattice. We determine the set of likely low temperature ordered phases, on the assumption that the ordering is weak, i.e the system is close to a ``U(1)'' quantum spin liquid in which the 3:1 proportionality is maintained but the spins are strongly fluctuating. The nature of the 9 ordered states we find is determined by a ``projective symmetry'' analysis. All the phases exhibit translational and rotational symmetry breaking, with an enlarged unit cell containing 4 to 64 primitive cells of the underlying pyrochlore. The simplest of the 9 phases is the same ``R'' state found earlier in a theoretical study of the ordering on the magnetization plateau in the $S=3/2$ materials \cdaf and \hgaf. We suggest that the spin/dimer model proposed therein undergoes a direct transition from the spin liquid to the R state, and describe a field theory for the universal properties of this critical point, at zero and non-zero temperatures.

cond-mat.str-el↗

arXiv2005

Quantum effects in a half-polarized pyrochlore antiferromagnet

We study quantum effects in a spin-3/2 antiferromagnet on the pyrochlore lattice in an external magnetic field, focusing on the vicinity of a plateau in the magnetization at half the saturation value, observed in CdCr$_2$O$_4$, and HgCr$_2$O$_4$. Our theory, based on quantum fluctuations, predicts the existence of a symmetry-broken state on the plateau, even with only nearest-neighbor microscopic exchange. This symmetry broken state consists of a particular arrangement of spins polarized parallel and antiparallel to the field in a 3:1 ratio on each tetrahedron. It quadruples the lattice unit cell, and reduces the space group from $Fd\bar{3}m$ to $P4_332$. We also predict that for fields just above the plateau, the low temperature phase has transverse spin order, describable as a Bose-Einstein condensate of magnons. Other comparisons to and suggestions for experiments are discussed.

cond-mat.str-el↗

arXiv2005

Transport in a spin incoherent Luttinger liquid

We theoretically investigate transport in a spin incoherent one dimensional electron system, which may be realized in quantum wires at low electron density and finite temperature. Both the pure and disordered cases are considered, both in finite wires and in the thermodynamic limit. The effect of Fermi liquid leads attached to the finite length system is also addressed. In the infinite system, we find a phase diagram identical to that obtained for a spinless Luttinger Liquid, provided we make the identification $g=2g_c$, where $g$ is the interaction parameter in a spinless Luttinger Liquid and $g_c$ is the interaction parameter of the charge sector of a Luttinger Liquid theory for electrons with spin. For a finite length wire attached to Fermi liquid leads the transport depends on details of the disorder in the wire. A simple picture for the cross-over from the spin incoherent regime to the spin coherent regime as the temperature is varied is also discussed, as well as some physical implications.

cond-mat.mes-hall↗

arXiv2005

Superfluid-Insulator Transitions on the Triangular Lattice

We report on a phenomenological study of superfluid to Mott insulator transitions of bosons on the triangular lattice, focusing primarily on the interplay between Mott localization and geometrical charge frustration at 1/2-filling. A general dual vortex field theory is developed for arbitrary rational filling factors f, based on the appropriate projective symmetry group. At the simple non-frustrated density f=1/3, we uncover an example of a deconfined quantum critical point very similar to that found on the half-filled square lattice. Turning to f=1/2, the behavior is quite different. Here, we find that the low-energy action describing the Mott transition has an emergent nonabelian SU(2)\times U(1) symmetry, not present at the microscopic level. This large nonabelian symmetry is directly related to the frustration-induced quasi-degeneracy between many charge-ordered states not related by microscopic symmetries. Through this ``pseudospin'' SU(2)symmetry, the charged excitations in the insulator close to the Mott transition develop a skyrmion-like character. This leads to an understanding of the recently discovered supersolid phase of the triangular lattice XXZ model (cond-mat/0505258, cond-mat/0505257, cond-mat/0505298) as a ``partially melted'' Mott insulator. The latter picture naturally explains a number of puzzling numerical observations of the properties of this supersolid. Moreover, we predict that the nearby quantum phase transition from this supersolid to the Mott insulator is in the recently-discovered non-compact CP^1 critical universality class (PRB 70, 075104 (2004)). A description of a broad range of other Mott and supersolid states, and a diverse set of quantum critical points between them, is also provided.

cond-mat.str-el↗

arXiv2005

Anisotropic pyrochlores and the global phase diagram of the checkerboard antiferromagnet

We study the phase diagram of two models of spin-1/2 antiferromagnets composed of corner-sharing tetrahedra, the basis of the pyrochlore structure. Primarily, we focus on the Heisenberg antiferromaget on the checkerboard lattice (also called the planar pyrochlore and crossed-chains model). This model has an anisotropic limit, when the dimensionless ratio of two exchange constants, J_\times/J << 1, in which it consists of one-dimensional spin chains coupled weakly together in a frustrated fashion. Using recently developed techniques combining renormalization group ideas and one-dimensional bosonization and current algebra methods, we show that in this limit the model enters a crossed dimer state with two-fold spontaneous symmetry breaking but no magnetic order. We complement this result by an approximate ``quadrumer triplet boson'' calculation, which qualitatively captures the physics of the ``plaquette valence bond solid'' state believed to obtain for J_\times/J = 1. Using these known points in parameter space, the instabilities pointed to by the quadrumer boson calculation, and the simple limit J_\times/J >> 1, we construct a few candidate global phase diagrams for the model, and discuss the nature of the quantum phase transitions contained therein. Finally, we apply our quasi-one-dimensional techniques to an anisotropic limit of the three-dimensional pyrochlore antiferromagnet, an approximate model for magnetism in GeCu2O4. A crossed dimer state is predicted here as well.

cond-mat.str-el↗

arXiv2005

Competing Orders and non-Landau-Ginzburg-Wilson Criticality in (Bose) Mott transitions

This paper reviews a recent non-Landau-Ginzburg-Wilson (LGW) approach to superfluid to Mott insulator transitions in two dimensional bosonic lattice systems, using a dual vortex field theory (cond-mat/0408329). The physical interpretation of conventional LGW theory of quantum criticality is re-examined and similarities and differences with the vortex picture are discussed. The ``unification'' of various competing (insulating) orders, and the coincidence of these orders with the Mott transition are readily understood in this formulation. Some aspects of the recent theory of ``deconfined'' quantum criticality, which are to an extent subsumed in this approach, are discussed. A pedagogical presentation of the ``nuts and bolts'' of boson-vortex duality at the hamiltonian level is included, tailored to a condensed matter audience.

cond-mat.str-el↗

arXiv2005

Putting competing orders in their place near the Mott transition

We describe the localization transition of superfluids on two-dimensional lattices into commensurate Mott insulators with average particle density p/q (p, q relatively prime integers) per lattice site. For bosons on the square lattice, we argue that the superfluid has at least q degenerate species of vortices which transform under a projective representation of the square lattice space group (a PSG). The formation of a single vortex condensate produces the Mott insulator, which is required by the PSG to have density wave order at wavelengths of q/n lattice sites (n integer) along the principle axes; such a second-order transition is forbidden in the Landau-Ginzburg-Wilson framework. We also discuss the superfluid-insulator transition in the direct boson representation, and find that an interpretation of the quantum criticality in terms of deconfined fractionalized bosons is only permitted at special values of q for which a permutative representation of the PSG exists. We argue (and demonstrate in detail in a companion paper: L. Balents et al., cond-mat/0409470) that our results apply essentially unchanged to electronic systems with short-range pairing, with the PSG determined by the particle density of Cooper pairs. We also describe the effect of static impurities in the superfluid: the impurities locally break the degeneracy between the q vortex species, and this induces density wave order near each vortex. We suggest that such a theory offers an appealing rationale for the local density of states modulations observed by Hoffman et al. (cond-mat/0201348) in STM studies of the vortex lattice of BSCCO, and allows a unified description of the nucleation of density wave order in zero and finite magnetic fields. We note signatures of our theory that may be tested by future STM experiments.

cond-mat.str-el↗

arXiv2005

Competing orders II: the doped quantum dimer model

We study the phases of doped spin S=1/2 quantum antiferromagnets on the square lattice, as they evolve from paramagnetic Mott insulators with valence bond solid (VBS) order at zero doping, to superconductors at moderate doping. The interplay between density wave/VBS order and superconductivity is efficiently described by the quantum dimer model, which acts as an effective theory for the total spin S=0 sector. We extend the dimer model to include fermionic S=1/2 excitations, and show that its mean-field, static gauge field saddle points have projective symmetries (PSGs) similar to those of `slave' particle U(1) and SU(2) gauge theories. We account for the non-perturbative effects of gauge fluctuations by a duality mapping of the S=0 dimer model. The dual theory of vortices has a PSG identical to that found in a previous paper (L. Balents et al., cond-mat/0408329) by a duality analysis of bosons on the square lattice. The previous theory therefore also describes fluctuations across superconducting, supersolid and Mott insulating phases of the present electronic model. Finally, with the aim of describing neutron scattering experiments, we present a phenomenological model for collective S=1 excitations and their coupling to superflow and density wave fluctuations.

cond-mat.str-el↗

arXiv2004

Green's function for magnetically incoherent interacting electrons in one dimension

Using a path integral approach and bosonization, we calculate the low energy asymptotics of the one particle Green's function for a ``magnetically incoherent'' one dimensional strongly interacting electron gas at temperatures much greater than the typical exchange energy but much lower than the Fermi energy. The Green's function exhibits features reminiscent of spin-charge separation, with exponential spatial decay and scaling behavior with interaction dependent anomalous exponents inconsistent with any unitary conformal field theory. We compute the tunneling density of states at low energies and find that it is a power law in energy with exponent $1/(4g)-1$, where $g$ is the Luttinger interaction parameter in the charge sector. The underlying physics is made transparent by the simplicity of the approach. Our results generalize those of Cheianov and Zvonarev [Phys. Rev. Lett. {\bf 92}, 176401 (2004)].

cond-mat.mes-hall↗

arXiv2004

Deconfined criticality critically defined

We describe characteristic physical properties of the recently introduced class of deconfined quantum critical points. Using some simple models, we highlight observables which clearly distinguish such critical points from those described by the conventional Landau-Ginzburg-Wilson framework: such a distinction can be made quite precisely even though both classes of critical points are strongly coupled, and neither has sharp quasiparticle excitations. We also contrast our classification from proposals by Bernevig et al. (cond-mat/0004291) and Yoshioka et al. (cond-mat/0404427).

cond-mat.str-el↗

arXiv2004

Numerical study of fractionalization in an Easy-axis Kagome antiferromagnet

Based on exact numerical calculations, we show that the generalized Kagome spin model in the easy axis limit exhibits a spin liquid, topologically degenerate ground state over a broad range of phase space. We present an (to our knowledge the first) explicit calculation of the gap (and dispersion) of ``vison'' excitations, and exponentially decaying spin and vison 2-point correlators, hallmarks of deconfined, fractionalized and gapped spinons. The region of the spin liquid phase includes a point at which the model is equivalent to a Heisenberg model with purely two-spin interactions. Beyond this range, a negative ``potential'' term tunes a first order transition to a magnetic ordered state. The nature of the phase transition is also discussed in light of the low energy spectrum. These results greatly expand the results and range of a previous study of this model in the vicinity of an exactly soluble point.

cond-mat.mes-hall↗

arXiv2004

Thermal fluctuations in pinned elastic systems: field theory of rare events and droplets

Using the functional renormalization group (FRG) we study the thermal fluctuations of elastic objects, described by a displacement field u and internal dimension d, pinned by a random potential at low temperature T, as prototypes for glasses. A challenge is how the field theory can describe both typical (minimum energy T=0) configurations, as well as thermal averages which, at any non-zero T as in the phenomenological droplet picture, are dominated by rare degeneracies between low lying minima. We show that this occurs through an essentially non-perturbative *thermal boundary layer* (TBL) in the (running) effective action Gamma[u] at T>0 for which we find a consistent scaling ansatz to all orders. The TBL resolves the singularities of the T=0 theory and contains rare droplet physics. The formal structure of this TBL is explored around d=4 using a one loop Wilson RG. A more systematic Exact RG (ERG) method is employed and tested on d=0 models. There we obtain precise relations between TBL quantities and droplet probabilities which are checked against exact results. We illustrate how the TBL scaling remains consistent to all orders in higher d using the ERG and how droplet picture results can be retrieved. Finally, we solve for d=0,N=1 the formidable "matching problem" of how this T>0 TBL recovers a critical T=0 field theory. We thereby obtain the beta-function at T=0, *all ambiguities removed*, displayed here up to four loops. A discussion of d>4 case and an exact solution at large d are also provided.

cond-mat.dis-nn↗

arXiv2004

Spin pumping and magnetization dynamics in ferromagnet-Luttinger liquid junctions

We study spin transport between a ferromagnet with time-dependent magnetization and a conducting carbon nanotube or quantum wire, modeled as a Luttinger liquid. The precession of the magnetization vector of the ferromagnet due for instance to an outside applied magnetic field causes spin pumping into an adjacent conductor. Conversely, the spin injection causes increased magnetization damping in the ferromagnet. We find that, if the conductor adjacent to the ferromagnet is a Luttinger liquid, spin pumping/damping is suppressed by interactions, and the suppression has clear Luttinger liquid power law temperature dependence. We apply our result to a few particular setups. First we study the effective Landau-Lifshitz-Gilbert (LLG) coupled equations for the magnetization vectors of the two ferromagnets in a FM-LL-FM junction. Also, we compute the Gilbert damping for a FM-LL and a FM-LL-metal junction.

cond-mat.str-el↗

arXiv2004

Dimerized phase and transitions in a spatially anisotropic square lattice antiferromagnet

We investigate the spatially anisotropic square lattice quantum antiferromagnet. The model describes isotropic spin-1/2 Heisenberg chains (exchange constant J) coupled antiferromagnetically in the transverse (J_\perp) and diagonal (J_\times), with respect to the chain, directions. Classically, the model admits two ordered ground states -- with antiferromagnetic and ferromagnetic inter-chain spin correlations -- separated by a first order phase transition at J_\perp=2J_\times. We show that in the quantum model this transition splits into two, revealing an intermediate quantum-disordered columnar dimer phase, both in two dimensions and in a simpler two-leg ladder version. We describe quantum-critical points separating this spontaneously dimerized phase from classical ones.

cond-mat.str-el↗