arXiv subjects

Andriy H. Nevidomskyy

Publications and source records attributed to Andriy H. Nevidomskyy.

83 records · Page 5Linked to original sources

arXiv2010

Frustration and the Kondo effect in heavy fermion materials

The observation of a separation between the antiferromagnetic phase boundary and the small-large Fermi surface transition in recent experiments has led to the proposal that frustration is an important additional tuning parameter in the Kondo lattice model of heavy fermion materials. The introduction of a Kondo (K) and a frustration (Q) axis into the phase diagram permits us to discuss the physics of heavy fermion materials in a broader perspective. The current experimental situation is analysed in the context of this combined "QK" phase diagram. We discuss various theoretical models for the frustrated Kondo lattice, using general arguments to characterize the nature of the $f$-electron localization transition that occurs between the spin liquid and heavy Fermi liquid ground-states. We concentrate in particular on the Shastry--Sutherland Kondo lattice model, for which we establish the qualitative phase diagram using strong coupling arguments and the large-$N$ expansion. The paper closes with some brief remarks on promising future theoretical directions.

cond-mat.str-el↗

arXiv2010

Bulk magnetic order in a two dimensional Ni1+/Ni2+ (d9/d8) nickelate, isoelectronic with superconducting cuprates

Bulk magnetic order in two dimensional La4Ni3O8 nickelate with Ni1+/Ni2+ (d9/d8), isoelectronic with superconducting cuprates is demonstrated experimentally and theoretically. Magnetization, specific heat and 139La NMR evidence a transition at 105 K to an antiferromagnetic state. Theoretical calculations by DFT relate the transition to a nesting instability of the Fermi surface with ordering wave-vector Q = [1/3, 1/3, 0].

cond-mat.str-el↗

arXiv2009

High-pressure phase and transition phenomena in ammonia borane NH3BH3 from X-ray diffraction, Landau theory, and ab initio calculations

Structural evolution of a prospective hydrogen storage material, ammonia borane NH3BH3, has been studied at high pressures up to 12 GPa and at low temperatures by synchrotron powder diffraction. At 293 K and above 1.1 GPa a disordered I4mm structure reversibly transforms into a new ordered phase. Its Cmc21 structure was solved from the diffraction data, the positions of N and B atoms and the orientation of NH3 and BH3 groups were finally assigned with the help of density functional theory calculations. Group-theoretical analysis identifies a single two-component order parameter, combining ordering and atomic displacement mechanisms, which link an orientationally disordered parent phase I4mm with ordered distorted Cmc21, Pmn21 and P21 structures. We propose a generic phase diagram for NH3BH3, mapping three experimentally found and one predicted (P21) phases as a function of temperature and pressure, along with the evolution of the corresponding structural distortions. Ammonia borane belongs to the class of improper ferroelastics and we show that both temperature- and pressure-induced phase transitions can be driven to be of the second order. The role of N-H...H-B dihydrogen bonds and other intermolecular interactions in the stability of NH3BH3 polymorphs is examined.

cond-mat.mtrl-sci↗

arXiv2009

Kondo resonance narrowing in d- and f-electron systems

By developing a simple scaling theory for the effect of Hund's interactions on the Kondo effect, we show how an exponential narrowing of the Kondo resonance develops in magnetic ions with large Hund's interaction. Our theory predicts an exponential reduction of the Kondo temperature with spin S of the Hund's coupled moment, a little-known effect first observed in d-electron alloys in the 1960's, and more recently encountered in numerical calculations on multi-band Hubbard models with Hund's interactions. We discuss the consequences of Kondo resonance narrowing for the Mott transition in d-band materials, particularly iron pnictides, and the narrow ESR linewidth recently observed in ferromagnetically correlated f-electron materials.

cond-mat.str-el↗

arXiv2009

Layered Kondo lattice model for quantum critical beta-YbAlB4

We analyze the magnetic and electronic properties of the quantum critical heavy fermion superconductor beta-YbAlB4, calculating the Fermi surface and the angular dependence of the extremal orbits relevant to the de Haas--van Alphen measurements. Using a combination of the realistic materials modeling and single-ion crystal field analysis, we are led to propose a layered Kondo lattice model for this system, in which two dimensional boron layers are Kondo coupled via interlayer Yb moments in a $J_{z}=\pm 5/2$ state. This model fits the measured single ion magnetic susceptibility and predicts a substantial change in the electronic anisotropy as the system is pressure-tuned through the quantum critical point.

cond-mat.str-el↗

arXiv2008

Nontrivial interplay between superconductivity and spin-orbit coupling in non-centrosymmetric ferromagnets

Motivated by the recent discoveries of ferromagnetic and non-centrosymmetric superconductors, we present a mean-field theory for a superconductor that \textit{both} lacks inversion symmetry and displays ferromagnetism, a scenario which is believed to be realized in UIr. We study the interplay between the order parameters to clarify how superconductivity is affected by the presence of ferromagnetism and spin-orbit coupling. One of our key findings is that the spin-orbit coupling seems to enhance both ferromagnetism and superconductivity in all spin channels. We discuss our results in the context of the heavy fermion superconductor UIr and analyze possible symmetries of the order parameter by the group theory method.

cond-mat.supr-con↗

arXiv2007

Magnetism and d-wave superconductivity on the half-filled square lattice with frustration

The role of frustration and interaction strength on the half-filled Hubbard model is studied on the square lattice with nearest and next-nearest neighbour hoppings t and t' using the Variational Cluster Approximation (VCA). At half-filling, we find two phases with long-range antiferromagnetic (AF) order: the usual Neel phase, stable at small frustration t'/t, and the so-called collinear (or super-antiferromagnet) phase with ordering wave-vector $(π,0)$ or $(0,π)$, stable for large frustration. These are separated by a phase with no detectable long-range magnetic order. We also find the d-wave superconducting (SC) phase ($d_{x^2-y^2}$), which is favoured by frustration if it is not too large. Intriguingly, there is a broad region of coexistence where both AF and SC order parameters have non-zero values. In addition, the physics of the metal-insulator transition in the normal state is analyzed. The results obtained with the help of the VCA method are compared with the large-U expansion of the Hubbard model and known results for the frustrated J1-J2 Heisenberg model. These results are relevant for pressure studies of undoped parents of the high-temperature superconductors: we predict that an insulator to d-wave SC transition may appear under pressure.

cond-mat.str-el↗

arXiv2007

Convexity of the self-energy functional in the variational cluster approximation

In the variational cluster approximation (VCA) (or variational cluster perturbation theory), widely used to study the Hubbard model, a fundamental problem that renders variational solutions difficult in practice is its known lack of convexity at stationary points, i.e. the physical solutions can be saddle points rather than extrema of the self-energy functional. Here we suggest two different approaches to construct a convex functional of the self-energy. In the first approach, one can show analytically that in the approximation where the irreducible particle-hole vertex depends only on center of mass coordinates, the functional is convex away from phase transitions in the corresponding channel. Numerical tests on a tractable version of that functional show that convexity can be a nuisance when looking for instabilities both in the pairing and particle-hole channels. Therefore, an alternative phenomenological functional is proposed. Convexity is explicitly enforced only with respect to a restricted set of variables, such as the cluster chemical potential that is known to be otherwise problematic. Numerical tests show that our functional is convex at the physical solutions of VCA and allows second-order phase transitions in the pairing channel as well. This opens the way to the use of more efficient algorithms to find solutions of the VCA equations.

cond-mat.str-el↗

arXiv2006

Magnetically induced oscillations of the spin polarization in the Datta-Das geometry

The control of intrinsic magnetic degrees of freedom is very important as it offers a practical means to manipulate and probe electron spin transport. Tunable spin-orbit effect in quantum wires can in principle serve as a means to achieve this goal. Here, we investigate within the scattering matrix approach the effect of an in-plane magnetic field on the conductance of the quantum wire in the Datta-Das geometry and show that the interplay of the spin-orbit interaction with the magnetic field provides enhanced control over the electron spin polarization. In particular, we predict a novel effect of magnetically induced oscillations of the electron spin in a certain range of magnetic field.

cond-mat.mes-hall↗

arXiv2004

Chemically active substitutional nitrogen impurity in carbon nanotubes

We investigate the nitrogen substitutional impurity in semiconducting zigzag and metallic armchair single-wall carbon nanotubes using ab initio density functional theory. At low concentrations (less than 1 atomic %), the defect state in a semiconducting tube becomes spatially localized and develops a flat energy level in the band gap. Such a localized state makes the impurity site chemically and electronically active. We find that if two neighboring tubes have their impurities facing one another, an intertube covalent bond forms. This finding opens an intriguing possibility for tunnel junctions, as well as the functionalization of suitably doped carbon nanotubes by selectively forming chemical bonds with ligands at the impurity site. If the intertube bond density is high enough, a highly packed bundle of interlinked single-wall nanotubes can form.

cond-mat.mtrl-sci↗

arXiv2004

Coexistence of ferromagnetism and superconductivity near quantum phase transition: The Heisenberg- to Ising-type crossover

A microscopic mean-field theory of the phase coexistence between ferromagnetism and superconductivity in the weakly ferromagnetic itinerant electron system is constructed, while incorporating a realistic mechanism for superconducting pairing due to the exchange of critical spin fluctuations. The self-consistent solution of the resulting equations determines the superconducting transition temperature which is shown to depend strongly on the exchange splitting. The effect of phase crossover from isotropic (Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum phase transition is analysed and the generic phase diagram is obtained. This scenario is then applied to the case of itinerant ferromagnet ZrZn2, which sheds light on the proposed phase diagram of this compound. Possible explanation of superconductivity in UGe2 is also discussed.

cond-mat.str-el↗