35 research outputs found
Quadrupolar Superexchange Interactions, Multipolar Order and Magnetic Phase Transition in UO
The origin of non-collinear magnetic order in UO is studied by an ab
initio dynamical-mean-field-theory framework in conjunction with a
linear-response approach for evaluating inter-site superexchange interactions
between U 5 shells. The calculated quadrupole-quadruple superexchange
interactions are found to unambiguously resolve the frustration of
face-centered-cubic U sublattice toward stabilization of the experimentally
observed non-collinear 3k-magnetic order. Therefore, the exotic 3k
antiferromagnetic order in UO can be accounted for by a purely electronic
exchange mechanism acting in the undistorted cubic lattice structure. The
quadrupolar short-range order above magnetic ordering temperature is
found to qualitatively differ from the long-range order below .Comment: 8 pages, 4 figures. Final version published in Phys. Rev.
Non-collinear magnetism engendered by a hidden another order
Standard microscopic approach to magnetic orders is based on assuming a
Heisenberg form for inter-atomic exchange interactions. These interactions are
considered as a driving force for the ordering transition with magnetic moments
serving as the primary order parameter. Any higher-rank multipoles appearing
simultaneously with such magnetic order are typically treated as auxiliary
order parameters rather than a principal cause of the transition. In this
study, we show that these traditional assumptions are violated in the case of
PrO. Evaluating the full set of Pr-Pr superexchange interactions from a
first-principles many-body technique we find that its unusual non-collinear 2k
magnetic structure stems from high-rank multipolar interactions, and that the
corresponding contribution of the Heisenberg interactions is negligible. The
observed magnetic order in PrO is thus auxiliary to high-rank "hidden"
multipoles. Within this picture we consistently account for previously
unexplained experimental observations like the magnitude of exchange splitting
and the evolution of magnetic structure in external field. Our findings
challenge the standard paradigm of observable magnetic moments being the
driving force for magnetic transitions.Comment: 13 pages + 4 pages of Supplementary, 3 Figure
Competing magnetic interactions in spin-1/2 square lattice: hidden order in SrVO
With decreasing temperature SrVO undergoes two structural phase
transitions, tetragonal-to-orthorhombic-to-tetragonal, without long-range
magnetic order. Recent experiments suggest, that only at very low temperature
SrVO might enter some, yet unknown, phase with long-range magnetic
order, but without orthorhombic distortion. By combining relativistic density
functional theory with an extended spin-1/2 compass-Heisenberg model we find an
antiferromagnetic single-stripe ground state with highly competing exchange
interactions, involving a non negligible inter-layer coupling, which places the
system at the crossover between between the XY and Heisenberg picture. Most
strikingly, we find a strong two-site "spin-compass" exchange anisotropy which
is relieved by the orthorhombic distortion induced by the spin stripe order.
Based on these results we discuss the origin of the hidden order phase and the
possible formation of a spin-liquid at low temperatures
Ising superconductivity and magnetism in NbSe
Recent studies on superconductivity in NbSe have demonstrated a large
anisotropy in the superconducting critical field when the material is reduced
to a single monolayer. Motivated by this recent discovery, we use density
functional theory (DFT) calculations to quantitatively address the
superconducting properties of bulk and monolayer NbSe. We demonstrate that
NbSe is close to a ferromagnetic instability, and analyze our results in
the context of experimental measurements of the spin susceptibility in
NbSe. We show how this magnetic instability, which is pronounced in a
single monolayer, can enable sizeable singlet-triplet mixing of the
superconducting order parameter, contrary to contemporary considerations of the
pairing symmetry in monolayer NbSe, and discuss approaches as to how this
degree of mixing can be addressed quantitatively within our DFT framework. Our
calculations also enable a quantitative description of the large anisotropy of
the superconducting critical field, using DFT calculations of monolayer
NbSe in the normal stateComment: 13 pages, 6 figure
Lifshitz transition driven by spin fluctuations and spin-orbit renormalization in NaOsO
In systems where electrons form both dispersive bands and small local spins,
we show that changes of the spin configuration can tune the bands through a
Lifshitz transition, resulting in a continuous metal-insulator transition
associated with a progressive change of the Fermi surface topology. In contrast
to a Mott-Hubbard and Slater pictures, this spin-driven Lifshitz transition
appears in systems with small electron-electron correlation and large
hybridization. We show that this situation is realized in 5 distorted
perovskites with an half-filled bands such as NaOsO, where the
strong hybridization reduces the local moment, and spin-orbit coupling
causes a large renormalization of the electronic mobility. This weakens the
role of electronic correlations and drives the system towards an itinerant
magnetic regime which enables spin-fluctuations
Anisotropy of magnetic interactions and symmetry of the order parameter in unconventional superconductor Sr2RuO4
Sr2RuO4is the best candidate for spin-triplet superconductivity, an unusual and elusive superconducting state of fundamental importance. In the last three decades, Sr2RuO4has been very carefully studied and despite its apparent simplicity when compared with strongly correlated high-Tccuprates, for which the pairing symmetry is understood, there is no scenario that can explain all the major experimental observations, a conundrum that has generated tremendous interest. Here, we present a density-functional-based analysis of magnetic interactions in Sr2RuO4and discuss the role of magnetic anisotropy in its unconventional superconductivity. Our goal is twofold. First, we access the possibility of the superconducting order parameter rotation in an external magnetic field of 200 Oe, and conclude that the spin-orbit interaction in this material is several orders of magnitude too strong to be consistent with this hypothesis. Thus, the observed invariance of the Knight shift across Tchas no plausible explanation, and casts doubt on using the Knight shift as an ultimate litmus paper for the pairing symmetry. Second, we propose a quantitative double-exchange-like model for combining itinerant fermions with an anisotropic Heisenberg magnetic Hamiltonian. This model is complementary to the Hubbard-model-based calculations published so far, and forms an alternative framework for exploring superconducting symmetry in Sr2RuO4. As an example, we use this model to analyze the degeneracy between various p-triplet states in the simplest mean-field approximation, and show that it splits into a single and two doublets with the ground state defined by the competition between the "Ising" and "compass" anisotropic terms