Leonid Isaev

Local Physics of Magnetization Plateaux in the Shastry-Sutherland Model

We address the physical mechanism responsible for the emergence of magnetization plateaux in the Shastry-Sutherland model. By using a hierarchical mean-field approach, we demonstrate that a plateau is stabilized in a certain spin pattern, satisfying local commensurability conditions derived from our formalism. Our results provide evidence in favor of a robust local physics nature of the plateaux states and are in agreement with recent NMR experiments on SrCu2(BO3)2.

Bulk-boundary correspondence in three-dimensional topological insulators

We discuss the relation between bulk topological invariants and the spectrum of surface states in three dimensional non-interacting topological insulators. By studying particular models, and considering general boundary conditions for the electron wavefunction on the crystal surface, we demonstrate that using experimental techniques that probe surface states, only strong topological and trivial insulating phases can be distinguished; the latter state being equivalent to a weak topological insulator. In a strong topological insulator, only the {\it parity} of the number of surface states, but not the number itself, is robust against time-reversal invariant boundary perturbations. Our results suggest a

Kondo effect in the presence of spin-orbit coupling

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Heavy antiferromagnetic phases in Kondo lattices.

definition of the bulk-boundary correspondence, compatible with the

The phase diagram of the Heisenberg antiferromagnet with four-spin interactions

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Tunable unconventional Kondo effect on topological insulator surfaces

classification of topological insulators.

Superconductivity in Strongly Repulsive Fermions: The Role of Kinetic-Energy Frustration

We study the T=0 Kondo physics of a spin-1/2 impurity in a non-centrosymmetric metal with spin-orbit interaction. Within a simple variational approach we compute ground state properties of the system for an {\it arbitrary} form of spin-orbit coupling consistent with the crystal symmetry. This coupling produces an unscreened impurity magnetic moment and can lead to a significant change of the Kondo energy. We discuss implications of this finding both for dilute impurities and for heavy-fermion materials without inversion symmetry.

Orbital order and Hund's rule frustration in Kondo lattices.

We propose a microscopic physical mechanism that stabilizes the coexistence of the Kondo effect and antiferromagnetism in heavy-fermion systems. We consider a two-dimensional quantum Kondo-Heisenberg lattice model and show that long-range electron hopping leads to a robust antiferromagnetic Kondo state. By using a modified slave-boson mean-field approach we analyze the stability of the heavy antiferromagnetic phase across a range of parameters, and discuss transitions between different phases. Our results may be used to guide future experiments on heavy fermion compounds.

Hierarchical mean-field approach to the J1-J2 Heisenberg model on a square lattice

We study the quantum phase diagram of the Heisenberg planar antiferromagnet with a subset of four-spin ring exchange interactions, using the recently proposed hierarchical mean-field approach. By identifying relevant degrees of freedom, we are able to use a single variational ansatz to map the entire phase diagram of the model and uncover the nature of its various phases. It is shown that there exists a transition between a Néel state and a quantum paramagnetic phase, characterized by broken translational invariance. The non-magnetic phase preserves the lattice rotational symmetry, and has a correlated plaquette nature. Our results also suggest that this phase transition can be properly described within the Landau paradigm.

Comment on >quantum phase transition in the four-spin exchange antiferromagnet>

We study Kondo physics of a spin-