Department of Physics PapersCopyright (c) 2015 University of Pennsylvania All rights reserved.
http://repository.upenn.edu/physics_papers
Recent documents in Department of Physics Papersen-usSun, 23 Aug 2015 01:34:48 PDT3600Possible Orientationally Ordered States of Bucky-Cubane
http://repository.upenn.edu/physics_papers/488
http://repository.upenn.edu/physics_papers/488Fri, 21 Aug 2015 11:47:21 PDT
For temperatures above T^{∗ }= 130K C_{60}⋅C_{8}H_{8} forms a cubic crystal consisting of two interpenetrating fcc sublattices, one of freely rotating Buckys C_{60} and the other of orientationally ordered cubane C_{8}H_{8}. The crystal structure below a discontinuous transition is found to be orthorhombic, but the nature of the ordering of the Buckys has not yet been determined. Here possible orderings of the Buckys consistent with the size and symmetry of the orthorhombic unit cell are analyzed. Most likely inversion symmetry is preserved at the transition, in which case the small number of possible orderings are described. If inversion symmetry is removed, the point group can be C_{2v} which supports ferroelectricity or D_{2} which can be confirmed by proton NMR measurements.
]]>
A. Brooks HarrisPhonons and Their Coupling to Magnons in <em>n</em>=2 Ruddlesden-Popper Compounds
http://repository.upenn.edu/physics_papers/487
http://repository.upenn.edu/physics_papers/487Fri, 21 Aug 2015 11:40:37 PDT
The number of absorption and Raman optical modes for each Wyckoff orbit in the high-temperature tetragonal (I4/mmm) parent lattice of the Ruddlesden-Popper compounds Ca_{3}X_{2}O_{7}, with X=Mn or X=Ti is given. We analyze the effect of sequential perturbations which lower the symmetry to Cmcm and Cmc2_{1} and finally include magnetic ordering. We determine the power law behavior (within mean field theory) for the cross section for photon absorption and Raman scattering of modes which appear as the symmetry is successively lowered. In the Cmc2_{1} phase we give a symmetry analysis to discuss the magnon-phonon coupling which in other systems gives rise to "electromagnons." From our results we suggest several experiments to clarify the phase diagram and other properties of these systems.
]]>
A. Brooks HarrisMolecular Chirality and Chiral Parameter
http://repository.upenn.edu/physics_papers/486
http://repository.upenn.edu/physics_papers/486Fri, 21 Aug 2015 11:33:21 PDT
The fundamental issues of symmetry related to chirality are discussed and applied to simple situations relevant to liquid crystals. The authors show that any chiral measure of a geometric object is a pseudoscalar (invariant under proper rotations but changing sign under improper rotations) and must involve three-point correlations that only come into play when the molecule has at least four atoms. In general, a molecule is characterized by an infinite set of chiral parameters. The authors illustrate the fact that these parameters can have differing signs and can vanish at different points as a molecule is continuously deformed into its mirror image. From this it is concluded that handedness is not an absolute concept but depends on the property being observed. Within a simplified model of classical interactions, the chiral parameter of the constituent molecules that determines the macroscopic pitch of cholesterics is identified.
]]>
A. Brooks Harris et al.Resistance of Random Walks
http://repository.upenn.edu/physics_papers/485
http://repository.upenn.edu/physics_papers/485Thu, 20 Aug 2015 08:46:22 PDT
Unrestricted lattice random walks in which a unit conductor is placed along each bond traversed are considered. The mean end-to-end resistance is studied as a function of the number of steps in the walk and the spatial dimension. A critical scaling law is found whose exponent is consistently given by four different calculational schemes.
]]>
Jayanth R. Banavar et al.ε Expansion for the Conductivity of a Random Resistor Network
http://repository.upenn.edu/physics_papers/484
http://repository.upenn.edu/physics_papers/484Thu, 20 Aug 2015 08:46:21 PDT
We present a reanalysis of the renormalization-group calculation to first order in ε=6−d, where d is the spatial dimensionality, of the exponent, t, which describes the behavior of the conductivity of a percolating network at the percolation threshold. If we set t=(d−2)ν_{p}+ζ, where ν_{p} is the correlation-length exponent, then our result is ζ=1+(ε/42). This result clarifies several previously paradoxical results concerning resistor networks and shows that the Alexander-Orbach relation breaks down at order ε.
]]>
A. Brooks Harris et al.Orientational Phases of Hydrogen Molecules on a Triangular Lattice
http://repository.upenn.edu/physics_papers/483
http://repository.upenn.edu/physics_papers/483Thu, 20 Aug 2015 08:46:19 PDT
A mean-field theory for the ordering of hydrogen molecules on a triangular net is derived in terms of the quadrupole coupling constant Γ, the crystal field V_{c}, and the temperature T. The phase diagram consists of six regions, separated by first- and second-order transitions. For almost all values of V_{c}/Γ for which ordering occurs, the disordered to ordered phase transition is continuous. At T=0 K, all phases have energy gaps except for a ferrorotational phase which behaves like an XY model.
]]>
A. John Berlinsky et al.1/z Expansion for the Heisenberg Antiferromagnet at Low Temperatures
http://repository.upenn.edu/physics_papers/482
http://repository.upenn.edu/physics_papers/482Thu, 20 Aug 2015 08:46:18 PDT
It is shown that the expansion in density of spin deviations for the Heisenberg antiferromagnet at low temperatures leads to a 1/z expansion for the free energy. In contrast to the 1/S expansion, the kinematic properties of spin operators are realized within each order of the expansion. The spin-wave energy at long wavelength is also calculated.
]]>
A. Brooks HarrisLocalization and Quantum Percolation
http://repository.upenn.edu/physics_papers/481
http://repository.upenn.edu/physics_papers/481Thu, 20 Aug 2015 08:46:17 PDT
Electronic wave functions are studied on dilute lattices, at dimensionalities 1⩽d⩽8. Generalized average inverse participation ratios are expanded in powers of the bond concentration, p. Dlog Padé approximants indicate that these ratios diverge as (p_{q}−p)^{-γq}, signaling the appearance of extended states for p>p_{q}. These Anderson transitions occur above classical percolation. No divergence is detected at d=2. These results are consistent with the existence of localized states at the center of the band.
]]>
Yonathan Shapir et al.Location of the Ising Spin-Glass Multicritical Point on Nishimori's Line
http://repository.upenn.edu/physics_papers/480
http://repository.upenn.edu/physics_papers/480Thu, 20 Aug 2015 08:46:16 PDT
We present arguments, based on local gauge invariance, that the multicritical point of Ising spin-glasses should be located on a particular line of the phase diagram known as Nishimori's line [tanh(βJ)=2p−1 for the ±J distribution]. One scaling axis is along the line, and the other is along the temperature direction. This scenario is generic for any random Ising model with a Nishimori line, in any number of dimensions, if the transitions are second order. The renormalization-group fixed point located inside Nishimori's manifold is expected to control multicriticality for a wider class of models.
]]>
Pierre Le Doussal et al.Comment on "Orientational Ordering Transition in Solid C<sub>60</sub>"
http://repository.upenn.edu/physics_papers/479
http://repository.upenn.edu/physics_papers/479Thu, 20 Aug 2015 08:46:15 PDT
A Comment on the Letter by P. A. Heiney et al., Phys. Rev. Lett. 66, 2911 (1991).
]]>
Ravi Sachidanandam et al.Vibrational Excitations in Percolation: Localization and Multifractality
http://repository.upenn.edu/physics_papers/478
http://repository.upenn.edu/physics_papers/478Thu, 20 Aug 2015 08:46:14 PDT
We discuss localized excitations on the incipient infinite percolation cluster. Assuming a simple exponential decay of the amplitudes ψ_{i} in terms of the chemical (minimal) path, we show theoretically that the ψ’s are characterized by a logarithmically broad distribution, and display multifractal features as a function of the Euclidean distance. The moments of ψ_{i} exhibit novel crossover phenomena. Our numerical simulations of fractons exhibit a nontrivial distribution of localization lengths, even when the chemical distance is fixed. These results are explained via a generalization of the theory.
]]>
Armin Bunde et al.Comment on "New Orientationally Ordered Low-Temperature Superstructure in High-Purity C<sub>60</sub>"
http://repository.upenn.edu/physics_papers/477
http://repository.upenn.edu/physics_papers/477Thu, 20 Aug 2015 08:46:12 PDT
A Comment on the Letter by G. Van Tendeloo et al. Phys. Rev. Lett. 69, 1065 (1992).
]]>
A. Brooks Harris et al.Was Superlocalization Observed in Carbon-Black–Polymer Composites?
http://repository.upenn.edu/physics_papers/476
http://repository.upenn.edu/physics_papers/476Thu, 20 Aug 2015 08:46:11 PDT
A Comment on the Letter by van der Putten, Phys. Rev. Lett. 69, 494 (1992).
]]>
Amnon Aharony et al.Evidence for Two Exponent Scaling in the Random Field Ising Model
http://repository.upenn.edu/physics_papers/475
http://repository.upenn.edu/physics_papers/475Thu, 20 Aug 2015 08:36:40 PDT
Novel methods were used to generate and analyze new 15 term high temperature series for both the (connected) susceptibility χ and the structure factor (disconnected susceptibility) χ_{d} for the random field Ising model with dimensionless coupling K=J/kT, in general dimension d. For both the bimodal and the Gaussian field distributions, with mean square field J^{2}g, we find that (χ_{d}-χ)/K^{2}gχ^{2}=1 as T→T_{c}(g), for a range of [h^{2}]=J^{2}g and d=3,4,5. This confirms the exponent relation γ^{¯}=2γ (where χ_{d}~t^{−γ¯}, χ~t^{−γ}, t=T-T_{c}) providing that random field exponents are determined by two (and not three) independent exponents. We also present new accurate values for γ.
]]>
Michael Gofman et al.Orientational Phase Transition in NaxC60 (1 < x < 3)
http://repository.upenn.edu/physics_papers/474
http://repository.upenn.edu/physics_papers/474Thu, 20 Aug 2015 08:36:38 PDT
X-ray diffraction and calorimetry data on cubic Na_{x}C_{60}(1<xT_{m} above that of pure C_{60}, e.g., T_{m}(x=1.3)=325 K. The ordered phases are the same as in pure C_{60}: simple cubic, space group Pa3^{¯}, but the orientations in the disordered phase are more restricted. We explain how Na stabilizes the ordered phase to rather high T, while K and Rb do not, in terms of Coulomb interactions between C_{60} molecules and and Na ions which we calculate from the local density approximation charge density of C_{60}.
]]>
Taner Yildirim et al.Spin Structures of Tetragonal Lamellar Copper Oxides
http://repository.upenn.edu/physics_papers/473
http://repository.upenn.edu/physics_papers/473Thu, 20 Aug 2015 08:36:37 PDT
The spin Hamiltonian of tetragonal lamellar antiferromagnets is shown to contain several novel anisotropies. Symmetry allows bond-dependent anisotropic exchange interactions, which lead to (a) interplane mean-field coupling and (b) an in-plane anisotropy which vanishes classically but arises from quantum zero point energy (QZPE). A similar QZPE involving the interplane isotropic interaction prefers collinear spins. Adding also diploar anisotropy, the competition between all these effects explains for the first time the spin structures of many cuprates.
]]>
Taner Yildirim et al.Symmetry, Spin-Orbit Interactions, and Spin Anisotropies
http://repository.upenn.edu/physics_papers/472
http://repository.upenn.edu/physics_papers/472Thu, 20 Aug 2015 08:36:35 PDT
The origins of anisotropy in the effective spin Hamiltonian, describing the ground manifold of Hubbard models with spin-orbit interactions, are critically discussed. For tetragonal symmetry, we show exactly that spin anisotropy can arise only if one includes both spin-orbit and Coulomb exchange interactions. For lower symmetries, additional anisotropies arise from terms which were hitherto neglected. Our analytic results are supported by numerical solutions for single bond clusters. These results can explain the easy plane anisotropy in the antiferromagnetic cuprates.
]]>
Taner Yildirim et al.Localization Length Exponent in Quantum Percolation
http://repository.upenn.edu/physics_papers/471
http://repository.upenn.edu/physics_papers/471Thu, 20 Aug 2015 08:36:34 PDT
Connecting perfect one-dimensional leads to sites i and j on the quantum percolation (QP) model, we calculate the transmission coefficient T_{ij}(E) at an energy E near the band center and the averages of Σ_{ij}T_{ij}, Σ_{ij}r^{2}_{ij}T_{ij}, and Σ_{ij}r^{4}_{ij}T_{ij}to tenth order in the concentration p. In three dimensions, all three series diverge at p_{q}=0.36^{+0.01}_{−0.02}, with exponents γ=0.82^{+0.10}_{−0.15}, γ+2ν, and γ+4ν. We find ν=0.38±0.07, differing from “usual” Anderson localization and violating the bound ν≥2/d of Chayes et al. [Phys. Rev. Lett. 57, 2999 (1986)]. Thus, QP belongs to a new universality class.
]]>
Iksoo Chang et al.Quantum Fluctuations in the Axial Next-Nearest-Neighbor Ising Model
http://repository.upenn.edu/physics_papers/470
http://repository.upenn.edu/physics_papers/470Thu, 20 Aug 2015 08:36:33 PDT
We obtain the axial next-nearest-neighbor Ising model from a Heisenberg model with large single-ion anisotropy energy, D, as might be relevant for helical spin systems. We treat quantum fluctuations to lowest order in 1/S at zero temperature within an expansion in J/D, where J is an exchange energy. The transition from the state with periodicity p=4 to the uniform state (p=∞) occurs via a sequence of first order transitions in which p increases monotonically.
]]>
A. Brooks Harris et al.Yildirim <em>et al.</em> Reply
http://repository.upenn.edu/physics_papers/469
http://repository.upenn.edu/physics_papers/469Thu, 20 Aug 2015 08:36:31 PDT
A Reply to the Comment by S. Skanthakumar, J. W. Lynn, and I. W. Sumarlin, Phys. Rev. Lett. 74, 2842 (1995).
]]>
Taner Yildirim et al.Absence of Self-Averaging and Universal Fluctuations in Random Systems Near Critical Points
http://repository.upenn.edu/physics_papers/468
http://repository.upenn.edu/physics_papers/468Thu, 20 Aug 2015 08:36:30 PDT
The distributions P(X) of singular thermodynamic quantities, on an ensemble of d-dimensional quenched random samples of linear size L near a critical point, are analyzed using the renormalization group. For L much larger than the correlation length ξ, we recover strong self-averaging (SA): P(X) approaches a Gaussian with relative squared width R_{X}~(L/ξ)^{−d}. For L≪ξ we show weak SA (R_{X}decays with a small power of L) or no SA [P(X) approaches a non-Gaussian, with universal L-independent relative cumulants], when the randomness is irrelevant or relevant, respectively.
]]>
Amnon Aharony et al.Microscopic Origin of Cholesteric Pitch
http://repository.upenn.edu/physics_papers/467
http://repository.upenn.edu/physics_papers/467Thu, 20 Aug 2015 08:36:28 PDT
We present a microscopic analysis of the instability of the nematic phase to chirality when molecular chirality is introduced perturbatively. We show that for central-force interactions the previously neglected short–range biaxial correlations play a crucial role in determining the cholesteric pitch. We propose a pseudoscalar strength which quantifies the chirality of a molecule.
]]>
A. Brooks Harris et al.Ferromagnetic Moment and Spin Rotation Transitions in Tetragonal Antiferromagnetic Sr<sub>2</sub>Cu<sub>3</sub>O<sub>4</sub>Cl<sub>2</sub>
http://repository.upenn.edu/physics_papers/466
http://repository.upenn.edu/physics_papers/466Thu, 20 Aug 2015 08:36:27 PDT
Sr_{2}Cu_{3}O_{4}Cl_{2} is a variant of the lamellar copper oxides, containing an extra Cu^{2+} ion in the center of every second plaquette of the square CuO_{2} lattice. The two types of Cu form interpenetrating Heisenberg antiferromagnets, which order at 380 and 40 K. Magnetization measurements yield a small spontaneous ferromagnetic moment below 380 K and two spin rotation transitions. The results are explained in detail by a pseudodipolar coupling between the two Cu lattices. A quantitative analysis of the data yields several previously unknown microscopic coupling constants, relevant to other lamellar, chain, and ladder copper oxides.
]]>
Fangcheng Chou et al.Critical Disordered Systems With Constraints and the Inequality ν > 2/d
http://repository.upenn.edu/physics_papers/465
http://repository.upenn.edu/physics_papers/465Thu, 20 Aug 2015 08:21:17 PDT
The renormalization group approach is used to study the effects of a “canonical” constraint (e.g., a fixed number of occupied bonds) on critical quenched disordered systems. The constraint is found to be always irrelevant, even near the “random” fixed point. This proves that α<0, or that ν>2/d. “Canonical” and “grand canonical” averages thus belong to the same universality class. Related predictions concerning the universality of non-self-averaging distributions are tested by Monte Carlo simulations of the site-diluted Ising model on the cubic lattice. In this case, the approach to the asymptotic distribution for “canonical” averaging is slow, resulting in effectively smaller fluctuations.
]]>
Amnon Aharony et al.Ordering Due to Quantum Fluctuations in Sr<sub>2</sub>Cu<sub>3</sub>O<sub>4</sub>Cl<sub>2</sub>
http://repository.upenn.edu/physics_papers/464
http://repository.upenn.edu/physics_papers/464Thu, 20 Aug 2015 08:21:16 PDT
Sr_{2}Cu_{3}O_{4}Cl_{2} has Cu_{I} and Cu_{II} subsystems, forming interpenetrating S=1/2 square lattice Heisenberg antiferromagnets. The classical ground state is degenerate, due to frustration of the intersubsystem interactions. Magnetic neutron scattering experiments show that quantum fluctuations cause a two dimensional Ising ordering of the Cu_{II}'s, lifting the degeneracy, and a dramatic increase of the Cu_{I} out-of-plane spin-wave gap, unique for order out of disorder. The spin-wave energies are quantitatively predicted by calculations which include quantum fluctuations.
]]>
Youngjune Kim et al.Two-Stage Rotational Disordering of a Molecular Crystal Surface: C<sub>60</sub>
http://repository.upenn.edu/physics_papers/463
http://repository.upenn.edu/physics_papers/463Thu, 20 Aug 2015 08:21:15 PDT
We propose a two-stage mechanism for the rotational surface disordering phase transition of a molecular crystal, as realized in C_{60} fullerite. Our study, based on Monte Carlo simulations, uncovers the existence of a new intermediate regime, between a low-temperature ordered (2×2) state, and a high-temperature (1×1) disordered phase. In the intermediate regime there is partial disorder, strongest for a subset of particularly frustrated surface molecules. These concepts and calculations provide a coherent understanding of experimental observations, with possible extension to other molecular crystal surfaces.
]]>
C. Laforge et al.Unusual Symmetries in the Kugel-Khomskii Hamiltonian
http://repository.upenn.edu/physics_papers/462
http://repository.upenn.edu/physics_papers/462Thu, 20 Aug 2015 08:11:32 PDT
The Kugel-Khomskii Hamiltonian for cubic titanates describes spin and orbital superexchange interactions between d^{1} ions having threefold degenerate t_{2g}orbitals. Since orbitals do not couple along “inactive” axes, perpendicular to the orbital planes, the total number of electrons in |α⟩ orbitals in any such plane and the corresponding total spin are both conserved. A Mermin-Wagner construction shows that there is no long-range spin ordering at nonzero temperatures. Inclusion of spin-orbit coupling allows such ordering, but even then the excitation spectrum is gapless due to a continuous symmetry. Thus, the observed order and gap require more symmetry breaking terms.
]]>
A. Brooks Harris et al.Competing Magnetic Phases on a Kagomé Staircase
http://repository.upenn.edu/physics_papers/461
http://repository.upenn.edu/physics_papers/461Thu, 20 Aug 2015 08:11:30 PDT
We present thermodynamic and neutron data on Ni_{3}V_{2}O_{8}, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T—one amplitude modulated, the other helical—plus a commensurate canted antiferromagnet for T→0. The H−T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni_{3}V_{2}O_{8} thus provides an elegant example of order from subleading interactions in a highly frustrated system.
]]>
Gavin J. Lawes et al.Magnetically Driven Ferroelectric Order in Ni<sub>3</sub>V<sub>2</sub>O<sub>8</sub>
http://repository.upenn.edu/physics_papers/460
http://repository.upenn.edu/physics_papers/460Thu, 20 Aug 2015 08:11:28 PDT
We show that long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition in Ni_{3}V_{2}O_{8}. The temperature and magnetic-field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry using Landau theory for continuous phase transitions, which shows that the spin structure alone can break spatial inversion symmetry leading to ferroelectric order. This phenomenological theory explains our experimental observation that the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.
]]>
Gavin J. Lawes et al.Magnetic Inversion Symmetry Breaking and Ferroelectricity in TbMnO<sub>3</sub>
http://repository.upenn.edu/physics_papers/459
http://repository.upenn.edu/physics_papers/459Thu, 20 Aug 2015 08:11:27 PDT
TbMnO_{3} is an orthorhombic insulator where incommensurate spin order for temperature T_{N}<41 K is accompanied by ferroelectric order for T<28 K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO_{3}.
]]>
Michel Kenzelmann et al.1 ∕ <em>d</em> Expansion for <em>k</em>-Core Percolation
http://repository.upenn.edu/physics_papers/458
http://repository.upenn.edu/physics_papers/458Thu, 20 Aug 2015 08:11:25 PDT
The physics of k-core percolation pertains to those systems whose constituents require a minimum number of k connections to each other in order to participate in any clustering phenomenon. Examples of such a phenomenon range from orientational ordering in solid ortho-para H_{2} mixtures to the onset of rigidity in bar-joint networks to dynamical arrest in glass-forming liquids. Unlike ordinary (k=1) and biconnected (k=2) percolation, the mean field k⩾3-core percolation transition is both continuous and discontinuous, i.e., there is a jump in the order parameter accompanied with a diverging length scale. To determine whether or not this hybrid transition survives in finite dimensions, we present a 1∕d expansion for k-core percolation on the d-dimensional hypercubic lattice. We show that to order 1/d^{3} the singularity in the order parameter and in the susceptibility occur at the same value of the occupation probability. This result suggests that the unusual hybrid nature of the mean field k-core transition survives in high dimensions.
]]>
A. Brooks Harris et al.Spin Waves in the Frustrated Kagomé Lattice Antiferromagnet KFe<sub>3</sub>(OH)<sub>6</sub>(SO<sub>4</sub>)<sub>2</sub>
http://repository.upenn.edu/physics_papers/457
http://repository.upenn.edu/physics_papers/457Thu, 20 Aug 2015 08:11:24 PDT
The spin wave excitations of the S=5/2 kagomé lattice antiferromagnet KFe_{3}(OH)_{6}(SO_{4})_{2} have been measured using high-resolution inelastic neutron scattering. We directly observe a flat mode which corresponds to a lifted “zero energy mode,” verifying a fundamental prediction for the kagomé lattice. A simple Heisenberg spin Hamiltonian provides an excellent fit to our spin wave data. The antisymmetric Dzyaloshinskii-Moriya interaction is the primary source of anisotropy and explains the low-temperature magnetization and spin structure.
]]>
Kittiwit Matan et al.Direct Transition From a Disordered to a Multiferroic Phase on a Triangular Lattice
http://repository.upenn.edu/physics_papers/456
http://repository.upenn.edu/physics_papers/456Thu, 20 Aug 2015 08:01:49 PDT
We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO_{4})_{2}. Ferroelectricity is observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.
]]>
Michel Kenzelmann et al.Comment on "Ferroelectricity in Spiral Magnets"
http://repository.upenn.edu/physics_papers/455
http://repository.upenn.edu/physics_papers/455Thu, 20 Aug 2015 08:01:47 PDT
A Comment on the Letter by Maxim Mostovoy, [Phys. Rev. Lett. 96, 067601 (2006)]. The author of the Letter offers a Reply.
]]>
Michel Kenzelmann et al.Order Parameters and Phase Diagram of Multiferroic <em>R</em>Mn<sub>2</sub>O<sub>5</sub>
http://repository.upenn.edu/physics_papers/454
http://repository.upenn.edu/physics_papers/454Thu, 20 Aug 2015 08:01:45 PDT
The generic magnetic phase diagram of multiferroic RMn_{2}O_{5} (with R=Y, Ho, Tb, Er, Tm), which allows different sequences of ordered magnetic structures for different R’s and different control parameters, is described using order parameters which explicitly incorporate the magnetic symmetry. A phenomenological magnetoelectric coupling is used to explain why some of these magnetic phases are also ferroelectric. Several new experiments, which can test this theory, are proposed.
]]>
A. Brooks Harris et al.Nuclear Magnetic Resonance and Relaxation in Solid Hydrogen
http://repository.upenn.edu/physics_papers/453
http://repository.upenn.edu/physics_papers/453Thu, 20 Aug 2015 08:01:44 PDT
[No abstract included]
]]>
A. Brooks Harris et al.Energy Width of Spin Waves in the Heisenberg Ferromagnet
http://repository.upenn.edu/physics_papers/452
http://repository.upenn.edu/physics_papers/452Thu, 20 Aug 2015 08:01:43 PDT
A calculation of the energy width of spin waves whose energy is small compared to kT in a Heisenberg ferromagnet is presented. Full account is taken of all two-particle processes. For 2JS/kT≫1 and 2JSa^{2}λ^{2}/kT≪1, the energy width Δ_{ωλ} of a spin wave of wave number λ is Δ_{ωλ}~λ^{4}T^{2}[A+Bln(kT/2JSa^{2}λ^{2})+Cln^{2}(kT/2JSa^{2}λ^{2})]. The disagreement with some previous treatments is explained physically by their neglect of two many-body effects.
]]>
A. Brooks HarrisSpiral Spin Structures and Origin of the Magnetoelectric Coupling in YMn<sub>2</sub>O<sub>5</sub>
http://repository.upenn.edu/physics_papers/451
http://repository.upenn.edu/physics_papers/451Thu, 20 Aug 2015 08:01:41 PDT
By combining neutron four-circle diffraction and polarized neutron-diffraction techniques we have determined the complex spin structures of a multiferroic YMn_{2}O_{5} that exhibits two ferroelectric phases at low temperatures. The obtained magnetic structure has spiral components in both the low-temperature ferroelectric phases that are magnetically commensurate and incommensurate, respectively. Among proposed microscopic theories for the magnetoelectric coupling, our results are consistent with both the spin-current mechanism and the magnetostriction mechanism. Our results also explain why the electric polarization changes at the low-temperature commensurate-to-incommensurate phase transition.
]]>
JungHwa Kim et al.Replica-Symmetry Breaking in the Critical Behaviour of the Random Ferromagnet
http://repository.upenn.edu/physics_papers/450
http://repository.upenn.edu/physics_papers/450Thu, 20 Aug 2015 08:01:40 PDT
We study the critical properties of the weakly disordered p-component random Heisenberg ferromagnet. It is shown that if the specific-heat critical exponent of the pure system is positive, the traditional renormalization group (RG) flows at dimensions D=4-ε, which are usually considered as describing the disorder-induced universal critical behaviour, are unstable with respect to replica-symmetry breaking (RSB) potentials such as those found in spin glasses. It is demonstrated that the RG flows involving RSB potentials lead to fixed points which have a structure known as the one-step RSB, and there exists a whole spectrum of such fixed points. It is argued that spontaneous RSB can occur due to the interactions of the fluctuating fields with the local non-perturbative degrees of freedom coming from the multiple local minima solutions of the mean-field equations. However, it is not clear whether or not RSB occurs for infinitesimally weak disorder. Physical consequences of these conclusions are discussed.
]]>
Viktor Dotsenko et al.Direct Observation of the Quantum Energy Gap in S = ½ Tetragonal Cuprate Antiferromagnets
http://repository.upenn.edu/physics_papers/449
http://repository.upenn.edu/physics_papers/449Thu, 20 Aug 2015 08:01:38 PDT
Using an electron spin resonance spectrometer covering a wide range of frequency and magnetic field, we have measured the low-energy excitations of the S = ½ tetragonal antiferromagnets, Sr_{2}CuO_{2}Cl_{2} and Sr_{2}Cu_{3}O_{4}Cl_{2}. Our observations of in-plane energy gaps of order 0.1 meV at zero external magnetic field are consistent with a spin-wave calculation, which includes several kinds of quantum fluctuations that remove frustration. Such gaps vanish for classical spins, and were too small to be observed by other techniques. Results agree with other experiments and with exchange anisotropy parameters determined from a five-band Hubbard model.
]]>
Koichi Katsumata et al.Electric-Field Control of a Magnetic Phase Transition in Ni<sub>3</sub>V<sub>2</sub>O<sub>8</sub>
http://repository.upenn.edu/physics_papers/448
http://repository.upenn.edu/physics_papers/448Thu, 20 Aug 2015 08:01:37 PDT
We report on the electric-field tuning of a magnetic phase transition temperature (T_{L}) in multiferroic Ni_{3}V_{2}O_{8} thin films. The simultaneous magnetic and ferroelectric transition in Ni_{3}V_{2}O_{8} exhibits a clear dielectric anomaly; we monitored T_{L} under applied electric and magnetic fields using dielectric measurements. The transition temperature increases by 0.2 K±0.05 K when the sample is biased approximately 25 MV/m compared to zero bias. This electric-field control of the magnetic transition can be qualitatively understood using a mean-field model incorporating a tri-linear coupling between the magnetic order parameters and spontaneous polarization. The shape of the electric field-temperature phase boundary is consistent with the proper order parameter for the multiferroic phase in Ni_{3}V_{2}O_{8} being a linear combination of the magnetic and ferroelectric correlation functions.
]]>
Parashu R. Kharel et al.A Complete Devil's Staircase in the Falicov - Kimball Model
http://repository.upenn.edu/physics_papers/447
http://repository.upenn.edu/physics_papers/447Thu, 20 Aug 2015 08:01:36 PDT
We consider the neutral, one-dimensional Falicov - Kimball model at zero temperature in the limit of a large electron - ion attractive potential, U. By calculating the general n-ion interaction terms to leading order in 1/U we argue that the ground state of the model exhibits the behaviour of a complete devil's staircase.
]]>
Cristian Micheletti et al.Order Parameters and Phase Diagrams of Multiferroics
http://repository.upenn.edu/physics_papers/446
http://repository.upenn.edu/physics_papers/446Thu, 20 Aug 2015 08:01:34 PDT
The symmetry properties, order parameters, and magnetoelectric phase diagrams of multiferroics are discussed. After brief reviews of Ni_{3}V_{2}O_{8}, TbMnO_{3}, and RbFe(MoO_{4})_{2}, we present a detailed analysis of RMn_{2}O_{5} (with R = Y, Ho, Dy, Er, Tb, Tm).
]]>
A. Brooks Harris et al.Evidence for Large Electric Polarization From Collinear Magnetism in TmMnO<sub>3</sub>
http://repository.upenn.edu/physics_papers/445
http://repository.upenn.edu/physics_papers/445Thu, 20 Aug 2015 08:01:33 PDT
There has been tremendous research activity in the field of magneto-electric (ME) multiferroics after Kimura et al (2003 Nature426 55) showed that antiferromagnetic and ferroelectric orders coexist in orthorhombically distorted perovskite TbMnO_{3} and are strongly coupled. It is now generally accepted that ferroelectricity in TbMnO_{3} is induced by magnetic long-range order that breaks the symmetry of the crystal and creates a polar axis (Kenzelmann et al 2005 Phys. Rev. Lett.95 087206). One remaining key question is whether magnetic order can induce ferroelectric polarization that is as large as that of technologically useful materials. We show that ferroelectricity in orthorhombic (o) TmMnO_{3} is induced by collinear magnetic order, and that the lower limit for its electric polarization is larger than in previously investigated orthorhombic heavy rare-earth manganites. The temperature dependence of the lattice constants provides further evidence of large spin–lattice coupling effects. Our experiments suggest that the ferroelectric polarization in the orthorhombic perovskites with commensurate magnetic ground states could pass the 1 μC cm^{-2} threshold, as predicted by theory (Sergienko et al 2006 Phys. Rev. Lett.97 227204; Picozzi et al 2007 Phys. Rev. Lett.99 227201).
]]>
Vladimir Y. Pomjakushin et al.Chirality in Liquid Crystals: From Microscopic Origins to Macroscopic Structure
http://repository.upenn.edu/physics_papers/444
http://repository.upenn.edu/physics_papers/444Thu, 20 Aug 2015 08:01:31 PDT
Molecular chirality leads to a wonderful variety of equilibrium structures, from the simple cholesteric phase to the twist-grain-boundary phases, and it is responsible for interesting and technologically important materials like ferroelectric liquid crystals. This paper will review some recent advances in our understanding of the connection between the chiral geometry of individual molecules and the important phenomenological parameters that determine macroscopic chiral structure. It will then consider chiral structure in columnar systems and propose a new equilibrium phase consisting of a regular lattice of twisted ropes.
]]>
Tom C. Lubensky et al.Complete Wetting in the Three-Dimensional Transverse Ising Model
http://repository.upenn.edu/physics_papers/443
http://repository.upenn.edu/physics_papers/443Thu, 20 Aug 2015 08:01:30 PDT
We consider a three-dimensional Ising model in a transverse magnetic field h and a bulk field H. An interface is introduced by an appropriate choice of boundary conditions. At the point (H=0,h=0) spin configurations corresponding to different positions of the interface are degenerate. By studying the phase diagram near this multiphase point using quantum mechanical perturbation theory, we show that the quantum fluctuations, controlled by h, split the multiphase degeneracy giving rise to an infinite sequence of layering transitions.
]]>
A. Brooks Harris et al.Renormalization-Group Approach to the Critical Behavior of Random-Spin Models
http://repository.upenn.edu/physics_papers/442
http://repository.upenn.edu/physics_papers/442Thu, 20 Aug 2015 08:01:28 PDT
A renormalization-group technique is used to study the critical behavior of spin models in which each interaction has a small independent random width about its average value. The cluster approximation of Niemeyer and Van Leeuwen indicates that the two-dimensional Ising model has the same critical behavior as the homogeneous system. The ε expansion for n-component continuous spins shows that this behavior holds to first order in ε for n>4. For n<4, there is a new stable fixed point with 2ν=1+[3n/16(n−1)]ε.
]]>
A. Brooks Harris et al.