arXiv subjects

L. Isaev

Publications and source records attributed to L. Isaev.

15 recordsLinked to original sources

arXiv2017

Topological superfluidity with repulsive alkaline-earth atoms in optical lattices

Topological superfluids are of technological relevance since they are believed to host Majorana bound states, a powerful resource for quantum computation and memory. Here we propose to realize topological superfluidity with fermionic atoms in an optical lattice. We consider a situation where atoms in two internal states experience different lattice potentials: one species is localized and the other itinerant, and show how quantum fluctuations of the localized fermions give rise to an attraction and strong spin-orbit coupling in the itinerant band. At low temperature, these effects stabilize a topological superfluid of mobile atoms even if their bare interactions are repulsive. This emergent state can be engineered with ${}^{87}$Sr atoms in a superlattice with a dimerized unit cell. To probe its unique properties we describe protocols that use high spectral resolution and controllability of the Sr clock transition, such as momentum-resolved spectroscopy and supercurrent response to a synthetic (laser-induced) magnetic field.

cond-mat.quant-gas↗

arXiv2017

Spin mixing and protection of ferromagnetism in a spinor dipolar condensate

We study spin mixing dynamics in a chromium dipolar Bose-Einstein Condensate, after tilting the atomic spins by an angle $\theta$ with respect to the magnetic field. Spin mixing is triggered by dipolar coupling, but, once dynamics has started, it is mostly driven by contact interactions. For the particular case $\theta=\pi/2$, an external spin-orbit coupling term induced by a magnetic gradient is required to enable the dynamics. Then the initial ferromagnetic character of the gas is locally preserved, an unexpected feature that we attribute to large spin-dependent contact interactions.

cond-mat.quant-gas↗

arXiv2016

Spin-orbit coupled correlated metal phase in Kondo lattices: an implementation with alkaline-earth atoms

We show that an interplay between quantum effects, strong on-site ferromagnetic exchange interaction and antiferromagnetic correlations in Kondo lattices can give rise to an exotic spin-orbit coupled metallic state in regimes where classical treatments predict a trivial insulating behavior. This phenomenon can be simulated with ultracold alkaline-earth fermionic atoms subject to a laser-induced magnetic field by observing dynamics of spin-charge excitations in quench experiments.

cond-mat.quant-gas↗

arXiv2015

Tunable unconventional Kondo effect on topological insulator surfaces

We study Kondo physics of a spin-$\frac{1}{2}$ impurity in electronic matter with strong spin-orbit interaction, which can be realized by depositing magnetic adatoms on the surface of a three-dimensional topological insulator. We show that magnetic properties of topological surface states and the very existence of Kondo screening strongly depend on details of the bulk material, and specifics of surface preparation encoded in time-reversal preserving boundary conditions for electronic wavefunctions. When this tunable Kondo effect occurs, the impurity spin is screened by purely orbital motion of surface electrons. This mechanism gives rise to a transverse magnetic response of the surface metal, and spin textures that can be used to experimentally probe signatures of a Kondo resonance. Our predictions are particularly relevant for STM measurements in ${\rm Pb Te}$-class crystalline topological insulators, but we also discuss implications for other classes of topological materials.

cond-mat.str-el↗

arXiv2015

Heavy-fermion valence-bond liquids in ultracold atoms: Cooperation of Kondo effect and geometric frustration

We analyze a microscopic mechanism behind coexistence of a heavy Fermi liquid and geometric frustration in Kondo lattices. We consider a geometrically frustrated periodic Anderson model and demonstrate how orbital fluctuations lead to a Kondo-screened phase in the limit of extreme strong frustration when only local {\it singlet} states participate in the low-energy physics. We also propose a setup to realize and study this exotic state with $SU (3)$-symmetric alkaline-earth cold atoms.

cond-mat.quant-gas↗

arXiv2013

Orbital order and Hund's rule frustration in Kondo lattices

We analyze a microscopic origin of the Kondo effect-assisted orbital order in heavy-fermion materials. By studying the periodic two-orbital Anderson model with two local electrons, we show that frustration of Hund's rule coupling due to the Kondo effect leads to an incommensurate spiral orbital and magnetic order, which exists only inside the Kondo screened (heavy-electron) phase. This spiral state can be observed in neutron and resonant X-ray scattering measurements in ${\rm U}$- and ${\rm Pr}$-based heavy-fermion compounds, and realized in cold atomic gases, e.g. fermionic ${}^{173}{\rm Yb}$.

cond-mat.str-el↗

arXiv2012

Heavy antiferromagnetic phases in Kondo lattices

We propose a microscopic physical mechanism that stabilizes 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 or parameters, and discuss transitions between different phases. We also address connection to experiments on heavy fermion compounds.

cond-mat.str-el↗

arXiv2012

Frustrated magnets and quantum paramagnetic phases at finite temperature

We develop a general framework, which combines exact diagonalization in small clusters with a density matrix variational principle, to study frustrated magnets at finite temperature. This thermodynamic hierarchical mean-field technique is used to determine the phase diagram and magnetization process of the three-dimensional spin-1/2 $J_1$-$J_2$ antiferromagnet on a stacked square lattice. Its non-magnetic phase exhibits a thermal crossover from a quantum to a classical paramagnet at a temperature $T=T_0$ which can be extracted from thermodynamic measurements. At low temperature an applied magnetic field stabilizes, through order-by-disorder, a variety of phases with non-trivial spin textures and a magnetization plateau at half-saturation which continuously disappears at $T\sim T_0$. Our results are relevant for frustrated vanadium oxides.

cond-mat.str-el↗

arXiv2011

Kondo effect in the presence of spin-orbit coupling

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.

cond-mat.str-el↗

arXiv2011

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 $\z$ definition of the bulk-boundary correspondence, compatible with the $\z$ classification of topological insulators.

cond-mat.mes-hall↗

arXiv2010

Comment on "Quantum phase transition in the four-spin exchange antiferromagnet" by V. N. Kotov, D.-X. Yao, A. H. Castro-Neto, and D. K. Campbell

In a recent paper [Phys. Rev. {\bf B80}, 174403 (2009)] Kotov {\it et al.} studied the paramagnetic-to-antiferromagnetic transition in the $J$-$Q$ model. Their findings were claimed to be in "fairly good agreement" with previous quantum Monte-Carlo (QMC) results. In this Comment we show that the above claim is misleading and in reality their phase transition point is not only far from the corresponding QMC value but also lies in a region of parameter space not yet explored in the literature. We also show that their reference dimer state is unstable against formation of a plaquette condensate, which could in part explain the large fluctuations they found.

cond-mat.str-el↗

arXiv2010

Superconductivity in strongly repulsive fermions: the role of kinetic-energy frustration

We discuss a physical mechanism of a non-BCS nature which can stabilize a superconducting state in a {\it strongly repulsive} electronic system. By considering the two-dimensional Hubbard model with spatially modulated electron hoppings, we demonstrate how kinetic-energy frustration can lead to robust d-wave superconductivity at {\it arbitrarily} large on-site repulsion. This phenomenon should be observable in experiments using fermionic atoms, e.g. ${}^{40}K$, in specially prepared optical lattices.

cond-mat.supr-con↗

arXiv2009

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 {\it spin pattern}, satisfying {\it 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 $\scbo$.

cond-mat.str-el↗

arXiv2009

Phase diagram of the Heisenberg antiferromagnet with four-spin interactions

We study the quantum phase diagram of the Heisenberg planar antiferromagnet with a subset of four-spin ring exchange interactions, using the recently proposed heirarchical mean-field approach. By identifying relevant degrees of freedom, we are able to use a single variational anzatz 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 N\'eel 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.

cond-mat.str-el↗

arXiv2009

Hierarchical mean-field approach to the $J_1$-$J_2$ Heisenberg model on a square lattice

We study the quantum phase diagram and excitation spectrum of the frustrated $J_1$-$J_2$ spin-1/2 Heisenberg Hamiltonian. A hierarchical mean-field approach, at the heart of which lies the idea of identifying {\it relevant} degrees of freedom, is developed. Thus, by performing educated, manifestly symmetry preserving mean-field approximations, we unveil fundamental properties of the system. We then compare various coverings of the square lattice with plaquettes, dimers and other degrees of freedom, and show that only the {\it symmetric plaquette} covering, which reproduces the original Bravais lattice, leads to the known phase diagram. The intermediate quantum paramagnetic phase is shown to be a (singlet) {\it plaquette crystal}, connected with the neighboring Néel phase by a continuous phase transition. We also introduce fluctuations around the hierarchical mean-field solutions, and demonstrate that in the paramagnetic phase the ground and first excited states are separated by a finite gap, which closes in the Néel and columnar phases. Our results suggest that the quantum phase transition between Néel and paramagnetic phases can be properly described within the Ginzburg-Landau-Wilson paradigm.

cond-mat.str-el↗