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arXiv2013

Frustrating antiferromagnetic exchange interactions enhance specific valence-bond-pair motifs

We present variational results for the ground state of the antiferromagnetic quantum Heisenberg model with frustrating next-nearest-neighbour interactions. The trial wave functions employed are of resonating-valence-bond type, elaborated to account for various geometric motifs of adjacent bond pairs. The calculation is specialized to a square-lattice cluster consisting of just sixteen sites, large enough that the system can accommodate nontrivial singlet dimer correlations but small enough that exhaustive enumeration of states in the total spin zero sector is still feasible. A symbolic computation approach allows us to generate an algebraic expression for the expectation value of any observable and hence to carry out the energy optimization exactly. While we have no measurements that could unambiguously identify a spin liquid state in the controversial region at intermediate frustration, we can say that the bond-bond correlation factors that emerge do not appear to be consistent with the existence of a columnar valence bond crystal. Furthermore, our results suggest that the magnetically disordered region may accommodate two distinct phases.

cond-mat.str-el↗

arXiv2013

Hamiltonian Theory of Anisotropic Fractional Quantum Hall States

Rotationally invariant fractional quantum Hall (FQH) states have long been understood in terms of composite bosons or composite fermions. Recent investigations of both incompressible and compressible states in highly tilted fields, which renders them anisotropic, have revealed puzzling features which have so far defied quantitative explanation. The author's work with R. Shankar in constructing and analyzing an operator-based theory in the rotationally invariant FQHE is generalized here to the anisotropic case. We compute the effective anisotropies of many principal fraction states in the lowest and the first Landau levels and find good agreement with previous theoretical results. We compare the effective anisotropy in a model potential with finite sample thickness and find good agreement with experimental results.

cond-mat.str-el↗

arXiv2013

The Integrable Bootstrap Program at Large N and its Applications in Gauge Theory

We present results for the large-$N$ limit of the (1+1)-dimensional principal chiral sigma model. This is an asymptotically-free $N\times N$ matrix-valued field with massive excitations. All the form factors and the exact correlation functions of the Noether-current operator and the energy-momentum tensor are found, from Smirnov's form-factor axioms. We consider (2+1)-dimensional $SU(\infty)$ Yang-Mills theory as an array of principal chiral models with a current-current interaction. We discuss how to use our new form factors to calculate physical quantities in this gauge theory.

hep-lat↗

arXiv2013

Minority-spin $t_{2g}$ states and the degree of spin polarization in ferromagnetic metallic La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ ($x=0.38$)

Using angle-resolved photoemission spectroscopy (ARPES), we investigate the electronic band structure and Fermi surface of ferromagnetic La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ ($x=0.38$). Besides the expected two hole pockets and one electron pocket of majority-spin $e_g$ electrons, we show an extra electron pocket around the $Γ$ point. A comparison with first-principles spin-polarized band-structure calculations shows that the extra electron pocket arises from $t_{2g}$ electrons of minority-spin character, indicating this compound is not a complete half-metallic ferromagnet, with similar expectations for lightly-doped cubic manganites. However, our data suggest that a complete half-metallic state is likely to be reached as long as the bandwidth is mildly reduced. Moreover, the band-resolved capability of ARPES enables us to investigate the band structure effects on spin polarization for different experimental conditions.

cond-mat.str-el↗

arXiv2013

Temperature dependence of crystal field excitations in CuO

We report a study on the temperature dependence of the charge-neutral crystal field (dd) excitations in cupric oxide, using nonresonant inelastic x-ray scattering (IXS) spectroscopy. Thanks to a very high energy resolution (60 meV), we observe thermal effects on the dd excitation spectrum fine structure between temperatures of 10-320 K. With an increasing temperature, the spectra broaden considerably. We assign the temperature dependence of the dd excitations to the relatively large electron-phonon coupling.

cond-mat.str-el↗

arXiv2013

Low-energy theory of a two-dimensional itinerant chiral magnet

Effective actions, such as nonlinear sigma models, are important tools in studying low-energy fluctuations of magnetically ordered states. Here we derive an effective action for the smooth order-parameter distortions of noncoplanar magnetic textures [with an SO(3) order parameter, as opposed to the SO(3)/SO(2)=S^2 of collinear magnets] coupled to itinerant electrons. Noncoplanar magnetic textures commonly arise in the presence of electron-mediated long-range spin exchange interactions, giving rise to the emergence of rich electronic phenomena such as anomalous quantum Hall effect. We parameterize the smooth order-parameter distortions of such a magnetic texture by certain non-Abelian fields, and derive the action in terms of these fields by integrating out the itinerant electrons. As a concrete example, we perform explicit calculations for a triangular-lattice model with tetrahedral magnetic ordering. The action encodes the charge and spin quantum-Hall responses as well as the energetics of twisting the magnetic texture.

cond-mat.str-el↗

arXiv2013

Local increase of symmetry on cooling in KNi$_2$Se$_2

KNi$_2$Se$_2$ exhibits an increase of symmetry on cooling below $T\le50$ K, as observed by Raman spectroscopy and synchrotron x-ray diffraction. X-ray absorption spectroscopy confirms that the symmetry increase is due to changes in nickel-nickel interactions and suppression of charge density wave fluctuations. Density functional theory calculations reveal a zone- boundary lattice instability that provides a model of the room-temperature x-ray pair distribution function data, but fails to describe the higher local symmetry observed for $T\le50$K. Together, these results support many- body correlation effects as drivers for the unusual heavy fermion electronic ground state in KNi$_2$Se$_2$.

cond-mat.str-el↗

arXiv2013

Density Functional Theory and Generalized Tight-Binding combined method for Hubbard fermion-phonon coupling study in strongly correlated LSCO-system

We present ab initio results for the electron-phonon interaction of the Γ-point phonons in the tetragonal high-temperature phase of La2CuO4. Eigenfrequencies and eigenvectors for the symmetry-allowed phonon modes are calculated with the full-potential augmented plane wave+local orbitals method using the frozen phonon approach. It is found that the Γ-point phonons with the strongest electron-phonon interaction are the A{2u} modes with 236 cm^{-1}, 131 cm^{-1} and 476 cm^{-1}. To take effect of strong electron on-site interaction into account we use generalized tight-binding method that results in the interaction of phonons with Hubbard fermions forming quasiparticle's band structure. Finally, the matrix elements of Hubbard fermion-phonon interaction and their reduction due to strong electron correlation are obtained.

cond-mat.str-el↗

arXiv2013

Fundamental limitations of half-metallicicity in spintronic materials

Zero-point spin fluctuations are shown to strongly influence the ground state of ferromagnetic metals and to impose limitations for the fully spin polarized state assumed in half-metallic ferromagnets, which may influence their applications in spintronics. This phenomenon leads to the low-frequency Stoner excitations and cause strong damping and softening of magnons in magnetoresistive manganites observed experimentally.

cond-mat.str-el↗

arXiv2013

Effects of quantum zero-point spin fluctuations on the ground state of plutonium

The magnetic stability of δ-plutonium is analyzed taking into account zero-point spin fluctuations. Within the generalized theory of spin fluctuations described within a simple phenomenological model neglecting its spatial dispersion we show that zero-point local magnetic moments are giant (at.Pu) and suppress the magnetic order predicted by ab initio calculations resulting in the observed paramagnetic state of δ-plutonium.

cond-mat.str-el↗

arXiv2013

Electronic Structure Reconstruction across the Antiferromagnetic Transition in TaFe$_{1.23}$Te$_3$ Spin Ladder

With angle-resolved photoemission spectroscopy, we studied the electronic structure of TaFe$_{1.23}$Te$_3$, which is a two-leg spin ladder compound with a novel antiferromagnetic ground state. Quasi-two-dimensional Fermi surface is observed, indicating sizable inter-ladder hopping, which would facilitate the in-plane ferromagnetic ordering through double exchange interactions. Moreover, an energy gap is not observed at the Fermi surface in the antiferromagnetic state. Instead, the shifts of various bands have been observed. Combining these observations with density-functional-theory calculations, we propose that the large scale reconstruction of the electronic structure, caused by the interactions between the coexisting itinerant electrons and local moments, is most likely the driving force behind the magnetic transition. TaFe$_{1.23}$Te$_3$ thus provides a simpler system that contains similar ingredients as the parent compounds of iron-based superconductors, which yet could be readily modeled and understood.

cond-mat.str-el↗

arXiv2013

UV-IR transmutation for hybrid realizations of Z_k parafermion systems

Recent experiments brought Majorana particles closer to reality and raised interest in realizing $\mathbb{Z}_k$ parafermions for general value of $k$ in hybrid structures. Of particular interest is the prospect of realizing a two dimensional system with the edge states described by $\mathbb{Z}_k$ parafermion conformal field theory (CFT) by coupling one dimensional hybrid systems. However in order to understand the effective perturbation due to local inter-wire coupling, it is crucial to relate short distance ultra-violet (UV) degrees of freedom to primary fields of the CFT describing long distance infra-red (IR) physics. Here we consider two recent proposals for realizing a one dimensional system described by $\mathbb{Z}_k$ parafermion CFT upon tuning to quantum critical points: a parafermion chain and a pair of $ν=2/k$ quantum Hall chiral edge states under uniform backscattering and pairing, where the latter is represented by self-dual sine-Gordon model. We explicitly derive the relation between distinct UV degrees of freedom relevant for each proposals and the primary fields of $\mathbb{Z}_3$ parafermion CFT. Our results point to a marked difference in the effect of local coupling between wires represented by each proposals, regarding robustness of the edge states in the resulting two dimensional systems.

cond-mat.str-el↗

arXiv2013

Pressure effect on the energy structure and superexchange interaction of the undoped orthorhombic La2CuO4: beyond the low-energy approximation

Using LDA+GTB multi-band approach, we studied the compression dependence of the electronic structure and in-plane superexchange interaction J(P) in the antiferromagnetic La214 at the 0% and 3% - hydrostatic and unaxial (along c axial) compression. We obtained the superexchange interaction J(P=0)~0.15eV is enhanced by ~20% under the 3% - hydrostatic compression and vice versa the J(P) is decreased slightly by ~5,7% under the 3% - uniaxial compression. In both cases the J(P) correlates with the in-plane hopping parameters and dd-excitation energy δ_s=e(^3B_{1})-e(A_{1})$ involving the the two-hole states: Zhang-Rice singlet and ^3{B_{1}} triplet states. The spectral density of the first removal states is a combined singlet-triplet character and a sign of changes in the one with the pressure clearly reproduces the \vec{k}-distribution of quasiparticle states with a different a_1- and b_1-symmetry over the Brillouin zone as a whole.

cond-mat.str-el↗

arXiv2013

Ground state of S=1 zigzag spin-orbital chain

We investigate ground-state properties of a $t_{\rm 2g}$-orbital Hubbard model on a zigzag chain relevant for CaV$_{2}$O$_{4}$, by exploiting numerical techniques such as Lanczos diagonalization and density-matrix renormalization group. Assuming a V$^{3+}$ ion, a local spin $S$=$1$ state is formed by two electrons in the $t_{\rm 2g}$ orbitals. That is, the system is a Haldane system with active $t_{\rm 2g}$-orbital degrees of freedom. We observe orbital-state transitions, yielding a distinct spin system under the orbital-ordered background. We also discuss the orbital structure induced by open edges, originating in the spatial anisotropy of the $t_{\rm 2g}$ orbitals.

cond-mat.str-el↗

arXiv2013

Bootstrapping the $\cN=1$ SCFT in three dimensions

We suggest a way to implement conformal bootstrap program for the case of the ${\cal N}=1$ SCFT in three dimensions using the previous analysis of the Ising model in \cite{CB}. We find approximate values for the conformal dimensions of several operators and the central charge $C_T$, the coefficient in the two-point function of the stress-tensor. Bootstrapping this particular (minimal) SCFT is of special interest as it was suggested in \cite{CM} that it may realize supersymmetry in 2+1 dimensions in experiment. The values are in a good agreement with the previous estimate in \cite{CM}.

hep-th↗

arXiv2013

Comment on "Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet"

In a recent publication [M. B. Stone et al., New Journal of Physics 9, 31 (2007)] a "Renormalized Classical 2D" (RC) phase has been reported in a quasi-two-dimensional quantum antiferromagnet PHCC. Its key signature is a sharp cusp-like feature in the magnetic susceptibility which appears below the critical field of magnetic ordering indicated by specific heat anomaly and emergence of a Bragg peak. Here we present experimental data which shows that regardless of experimental geometry, the specific heat and susceptibility anomalies in PHCC both coincide with the onset of true long range order. This leaves no room for any additional intermediate "RC" phase.

cond-mat.str-el↗

arXiv2013

Multiagent's model of stock market with p-adic description of prices

A new multiagent model of the stock market is formulated that contains four states in which the agents may be located. Next, the model is reformulated in the language of the functional integral containing fluctuations of prices and quantities of cash flows. It is shown that in the functional integral of that type description of the prices is given not by the real numbers as is made in many papers but the p-adic numbers. It is shown in the following simple examples extracted from the proposed theory that the p-adic description of prices gives good description of fractal behavior of the trends. The formula is given for the p-adic mapping of prices. Using this formula we obtain the main p-adic patterns which are the same as the patterns of Elliott wave theory, this fact allows us to give a rigorous mathematical proof of this theory. Our model is the only model that gives a strict point like description of the fractal behavior of prices. The developed approach opens the possibility to give the formulation of p-adic technical analysis of the stock market.

q-fin.TR↗

arXiv2013

Novel dielectric anomalies due to spin-chains above and below Neel temperature in Ca3Co2O6

We bring out novel dielectric behavior of a spin-chain compound, Ca3Co2O6, undergoing Neel order at (T_N=) 24 K. It is found that the virgin curve in the plot of isothermal dielectric constant versus magnetic-field lies outside the 'butterfly-shaped' envelope curve well below T_N (e.g., 2.6 K), with a signature of a partial arrest of the high-field magnetoelectric ME) phase in zero-field after travelling through magnetic-field-induced magnetic transitions. This behavior is in contrast to that observed in the isothermal magnetization data. Thus, this work brings out a novel case for 'phase-coexistence phenomenon' due to ME coupling. Another strange finding is that there is a weak, but a broad, peak in dielectric constant around 85 - 115 K well above T_N, attributable to incipient spin-chain magnetic ordering. This finding should inspire further work to study ME coupling on artificial assemblies of magnetic chains, also keeping in mind miniaturization required for possible applications.

cond-mat.mtrl-sci↗