Papers by Oleg Tchernyshyov
We argue that effective 1D models of stripes in the cuprate superconductors can be constructed by... more We argue that effective 1D models of stripes in the cuprate superconductors can be constructed by studying ground states and elementary excitations of domain walls in 2D model antiferromagnets. This method, applied to the t-J model with Ising anisotropy, yields two such limiting cases: an ordinary 1D electron gas and a 1D gas of holons strongly coupled to transversal fluctuations of the stripe.
Aps Meeting Abstracts, Mar 1, 2008
The interplay of orbital, lattice, and spin degrees of freedom in vanadium spinels has attracted ... more The interplay of orbital, lattice, and spin degrees of freedom in vanadium spinels has attracted much interest among researchers. The V^3+ ion has two electrons occupying three degenerate t2g orbitals and is thus Jahn-Teller active. It also has a total spin S=1 in accordance with Hund's rules. Moreover, the V^3+ ions sitting on the B-site of spinel form a pyrochlore lattice, the interactions between these localized spin and orbital degrees of freedom are thus geometrically frustrated [1]. Here we present a theoretical model for the ground states of vanadium spinels. We view all of the vanadates (Cd, Zn, Mg on the one hand and Mn on the other) within the same model in which the influence of Mn is simulated by a magnetic field. In the case of MnV2O4, our calculation yields a ground state with antiferro-orbital ordering accompanied by a tetragonal structural distortion with lattice constants a=b>c. In addition, the V spins develop an orthogonal antiferromagnetic order in the ab plane on top of a ferromagnetic moment along the c axis. The results are consistent with a recent experimental characterization of MnV2O4 [2]. In particular, we will discuss the important role played by cooperative Jahn-Teller interaction and spin-orbital coupling in stabilizing the orthogonal spin configuration. [1] O. Tchernyshyov, Phys. Rev. Lett., 93, 157206 (2004). [2] V. O. Garlea et al., cond-mat/0711.1844. ,1NSF Grant No. DMR-0348679
Bulletin of the American Physical Society, Mar 5, 2015
We revisit the description of the low-energy singlet sector of the spin-1/2 Heisenberg antiferrom... more We revisit the description of the low-energy singlet sector of the spin-1/2 Heisenberg antiferromagnet on kagome in terms of an effective quantum dimer model. With the help of exact diagonalizations of appropriate finite-size clusters, we show that the embedding of a given process in its kagome environment leads to dramatic modifications of the amplitudes of the elementary loop processes, an effect not accessible to the standard approach based on the truncation of the Hamiltonian to the nearest-neighbour valence-bond basis. The resulting parameters are consistent with a Z2 spin liquid rather than with a valence-bond crystal, in agreement with the last density matrix renormalization group results.
The ground state of spin 1/2 anti-ferromagnet on kagome lattice can be viewed as a collection of ... more The ground state of spin 1/2 anti-ferromagnet on kagome lattice can be viewed as a collection of small and heavy bound pairs of fermionic spinons [1]. Magnetic excitations of the model correspond to breaking such pairs into their constituents. In the current work, the dynamical structure factor is calcualted for low energy magnetic excitations just above the spin gap. It is observed that the structure factor is similar to the one of an isolated dimer due to the tightness of the bound state. The result is consistent with experiments done in Helton etal [2] and M.A. de Vries etal [3]. [4pt] [1] Zhihao Hao and Oleg Tchernyshyov, Phys. Rev. Lett. 103, 187203.[0pt] [2] J. S. Helton etal, Phys. Rev. Lett. 98, 107204.[0pt] [3] M. A. de Vries etal, arXiv:0902.3194.
Physical Review B, 1997
The high-Tc cuprates are possible candidates for d-wave superconductivity, with the Cooper pair w... more The high-Tc cuprates are possible candidates for d-wave superconductivity, with the Cooper pair wave function belonging to a nontrivial irreducible representation of the lattice point group. We argue that this d-wave symmetry is related to a special form of the fermionic kinetic energy and does not require any novel pairing mechanism. In this context, we present a detailed study of
Physical Review B, 2001
... 26 JF Zasadzinski, L. Ozyuzer, N. Miyakawa, KE Gray, DG Hinks, and C. Kendziora unpublished. ... more ... 26 JF Zasadzinski, L. Ozyuzer, N. Miyakawa, KE Gray, DG Hinks, and C. Kendziora unpublished. 27 H. Ding, MR Norman, JC Campuzano, M. Randeria, AF Bellman, T. Yokoya, T. Takahashi, T. Mochiku, and K. Kadowaki, Phys. Rev. B 54, R9678 1996. ...
Physical Review B, 2003
We study magnetic order in the Heisenberg antiferromagnet on the checkerboard lattice, a twodimen... more We study magnetic order in the Heisenberg antiferromagnet on the checkerboard lattice, a twodimensional version of the pyrochlore network with strong geometric frustration. By employing the semiclassical (1/S) expansion we find that quantum fluctuations of spins induce a long-range order that breaks the four-fold rotational symmetry of the lattice. The ordered phase is a valence-bond crystal. We discuss similarities and differences with the extreme quantum case S = 1/2 and find a useful phenomenology to describe the bond-ordered phases. J 1 2 J / J 1 2
Physical Review B, 2002
In the highly frustrated pyrochlore magnet spins form a lattice of corner-sharing tetrahedra. We ... more In the highly frustrated pyrochlore magnet spins form a lattice of corner-sharing tetrahedra. We show that the tetrahedral "molecule" at the heart of this structure undergoes a Jahn-Teller distortion when lattice motion is coupled to the antiferromagnetism. We extend this analysis to the full pyrochlore lattice by means of Landau theory and argue that it should exhibit "spin-Peierls" phases with bond order but no spin order. We find a range of Néel phases, with collinear, coplanar and noncoplanar order. While collinear Néel phases are easiest to generate microscopically, we also exhibit an interaction that gives rise to a coplanar state instead.
Physical Review B, 2000
We attempt to access the regime of strong coupling between charge carriers and transverse dynamic... more We attempt to access the regime of strong coupling between charge carriers and transverse dynamics of an isolated conducting "stripe", such as those found in cuprate superconductors. A stripe is modeled as a partially doped domain wall in an antiferromagnet (AF), introduced in the context of two different models: the t-J model with strong Ising anisotropy, and the Hubbard model in the Hartree-Fock approximation. The domain walls with a given linear charge density are supported artificially by boundary conditions. In both models we find a regime of parameters where doped holes lose their spin and become holons (charge Q = 1, spin S3 = 0), which can move along the stripe without frustrating AF environment. One aspect in which the holons on the AF domain wall differ from those in an ordinary one-dimensional electron gas is their transverse degree of freedom: a mobile holon always resides on a transverse kink (or antikink) of the domain wall. This gives rise to two holon flavors and to a strong coupling between doped charges and transverse fluctuations of a stripe.
Physical Review B, 2014
We revisit the description of the low-energy singlet sector of the spin-1/2 Heisenberg antiferrom... more We revisit the description of the low-energy singlet sector of the spin-1/2 Heisenberg antiferromagnet on kagome in terms of an effective quantum dimer model. With the help of exact diagonalizations of appropriate finite-size clusters, we show that the embedding of a given process in its kagome environment leads to dramatic modifications of the amplitudes of the elementary loop processes, an effect not accessible to the standard approach based on the truncation of the Hamiltonian to the nearest-neighbour valence-bond basis. The resulting parameters are consistent with a Z2 spin liquid rather than with a valence-bond crystal, in agreement with the last density matrix renormalization group results.
Springer Series in Solid-State Sciences, 2010
ABSTRACT We review the mechanism of spin-lattice coupling in relieving the geometrical frustratio... more ABSTRACT We review the mechanism of spin-lattice coupling in relieving the geometrical frustration of pyrochlore antiferromagnets, in particular spinel oxides. The tetrahedral unit, which is the building block of the pyrochlore lattice, undergoes a spin-driven Jahn–Teller instability when lattice degrees of freedom are coupled to the antiferromagnetism. By restricting our considerations to distortions which preserve the translational symmetries of the lattice, we present a general theory of the collective spin–Jahn–Teller effect in the pyrochlore lattice. One of the predicted lattice distortions breaks the inversion symmetry and gives rise to a chiral pyrochlore lattice, in which frustrated bonds form helices with a definite handedness. The chirality is transferred to the spin system through spin-orbit coupling, resulting in a long-period spiral state, as observed in spinel CdCr2O4. We discuss explicit models of spin-lattice coupling using local phonon modes, and their applications in other frustrated magnets.
Physical Review B, 1998
... 4 GM Luke, K. Kojima, M. Larkin, J. Merrin, B. Nachumi, YJ Uemura, Y. Nakamura, S. Uchida, an... more ... 4 GM Luke, K. Kojima, M. Larkin, J. Merrin, B. Nachumi, YJ Uemura, Y. Nakamura, S. Uchida, and M. Crawford, Hyperfine Interact. 105, 113 1997 . 5 F. Borsa, P. Carretta, JN Cho, FC Chou, Q. Nu, DC Johnston, A. Lascialfari, DR Torgeson, RJ Gooding, NM Salem, and KJE Vos ...
We study magnetization dynamics in an artificial kagome spin ice realized as a honeycomb network ... more We study magnetization dynamics in an artificial kagome spin ice realized as a honeycomb network of connected ferromagnetic nanowires [1]. Our model is focused on magnetic charges defined as the flux of magnetization into a network site. In this system, the allowed values of magnetic charge are ±1 and ±3, while in the original square ice [2] they are 0,
The motion of a vortex domain wall in a ferromagnetic nanowire under the influence of an applied ... more The motion of a vortex domain wall in a ferromagnetic nanowire under the influence of an applied magnetic field has been recently cast in the language of collective coordinates [1]. The theory, taking into account the two softest modes of the domain wall, works well below and immediately above Walker's breakdown [2] and can be extended to include the influence of spin current. Here we examine the motion of a vortex domain wall in a wire with rough edges. Integrating out the transverse coordinate yields an effective one-dimensional problem of a massive particle moving in a viscous medium. The edge roughness translates into a combination of a random pinning potential and a random Zeeman force. We calculate the average velocity of the domain wall and the probability of passing a wire of specified length as a function of the applied magnetic field. [1] O. A. Tretiakov et al., Phys. Rev. Lett. 100, 127204 (2008). [2] D. J. Clarke et al., Phys. Rev. B 78, 134412 (2008). The work was s...
Dipolar interactions in spin ice are described most effectively in terms of magnetic charges resi... more Dipolar interactions in spin ice are described most effectively in terms of magnetic charges residing on the dual lattice [1]. While spin ice on the pyrochlore lattice contains no magnetic monopoles at low temperatures, spin ice on kagome [2] contains a unit magnetic charge (±1) on every triangle. With the aid of Monte-Carlo simulations, we show that long-range Coulomb interaction between the monopoles lifts the degeneracy of the spin-ice states and induces a phase transition into a state with ordered magnetic charges but no spin order. The residual entropy is reduced from the spin-ice value but remains extensive. The phase transition is continuous with critical exponents close to the two-dimensional Ising universality class. [1] C. Castelnovo, R. Moessner, and S. L. Sondhi, Nature 451, 42 (2008). [2] A. S. Wills, R. Ballou, and C. Lacroix, Phys. Rev. B 66, 144407 (2002).
Nanorings can acquire the vortex state with flux closure and zero stray field. We have developed ... more Nanorings can acquire the vortex state with flux closure and zero stray field. We have developed a method for fabricating a large number of nanorings over a macroscopic area. However, for 100-nm nanorings, the nanorings can acquire both the rotating onion state and the vortex state during magnetic reversal with comparable probability. In this work, we report the fabrication and properties of asymmetric nanorings, whose thickness and width vary along the circumference. In contrast to symmetric nanorings, the percentage of vortex formation in asymmetric nanorings can be controlled by the direction of the magnetic field. When the field is along the asymmetry axis, nearly every nanoring can acquire the vortex state. The introduction of asymmetry in the nanorings allows full vortex formation without losing the virtue of small dimension, high stability and high areal density. We have also developed a theoretical model to calculate the dependence of domain wall energy on the local width an...
Physical Review B, 2015
ABSTRACT Low-energy states of quantum spin liquids are thought to involve partons living in a gau... more ABSTRACT Low-energy states of quantum spin liquids are thought to involve partons living in a gauge-field background. We study the spectrum of Majorana fermions of Kitaev's honeycomb model on spherical clusters. The gauge field endows the partons with half-integer orbital angular momenta. As a consequence, the multiplicities reflect not the point-group symmetries of the cluster, but rather its projective symmetries, operations combining physical and gauge transformations. The projective symmetry group of the ground state is the double cover of the point group.
Physical Review B, 2014
ABSTRACT We study the gapped phase of Kitaev's honeycomb model (a $Z_2$ spin liquid) on a... more ABSTRACT We study the gapped phase of Kitaev's honeycomb model (a $Z_2$ spin liquid) on a lattice with topological defects. We find that some dislocations and string defects carry unpaired Majorana fermions. Physical excitations associated with these defects are (complex) fermion modes made out of two (real) Majorana fermions connected by a $Z_2$ gauge string. The quantum state of these modes is robust against local noise and can be changed by winding a $Z_2$ vortex around one of the dislocations. The exact solution respects gauge invariance and reveals a crucial role of the gauge field in the physics of Majorana modes. To facilitate these theoretical developments, we recast the degenerate perturbation theory for spins in the language of Majorana fermions.
ABSTRACT String-like excitations in quantum spin-ice are a fascinating manifestation of quantum f... more ABSTRACT String-like excitations in quantum spin-ice are a fascinating manifestation of quantum fluctuations and may be observable in materials such as Yb2Ti2O7 and Pr2Zr2O7. We study quantum spin-ice under external magnetic fields on both the checkerboard and pyrochlore lattice for experimentally relevant conditions. We show that excitations in quantum spin ice may be string-like, and that stronger quantum fluctuations reduce string tension and lead to deconfined monopoles. Additionally, we discuss the crossover from strings to magnons in the high-field regime. We provide predictions for observing strings via inelastic neutron scattering and THz spectroscopy.
ABSTRACT Topological defects play an important role in nanoscale ferromagnets. We have previously... more ABSTRACT Topological defects play an important role in nanoscale ferromagnets. We have previously demonstrated that domain walls in thin strips and rings are composite objects made of bulk vortices (winding numbers n = ±1) and edge defects (fractional winding numbers n = ±1/2) and given analytical solutions in the exchange limit [1]. Experimentally accessible systems are in the opposite regime where the dipolar interaction dominate. In this limit the vortex solution remains unchanged, the antivortex and antihalfvortex are deformed but survive, whereas the halfvortex acquires a high magnetostatic energy and becomes unstable. Accordingly, domain walls in this limit consist of two antihalfvortices and a vortex between them. We present a model of the domain wall in the magnetostatic limit in which the location of the vortex core is a variational parameter. As the width and thickness of a strip change, the global and local minima of the total magnetic energy yield the familiar transverse and vortex walls, as well as more exotic configurations such as the ``diagonal wall'' with a vortex hanging close to an edge. [1] O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. 95, 197204 (2005).
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Papers by Oleg Tchernyshyov