arXiv subjects

B. V. Lotsch

Publications and source records attributed to B. V. Lotsch.

4 recordsLinked to original sources

arXiv2022

Origin of oscillatory structures in the magnetothermal conductivity of the putative Kitaev magnet $α$-RuCl$_3$

The layered honeycomb magnet $α$-RuCl$_3$ has been suggested to exhibit a field-induced quantum spin liquid state, in which the reported large thermal Hall effect close to the half-quantized value still remains a subject of debate. Recently, oscillatory structures of the magnetothermal conductivity were reported and interpreted as quantum oscillations of charge-neutral particles. To investigate the origin of these oscillatory structures, we performed a comprehensive measurement of the in-plane magnetothermal conductivity $κ(H)$ down to low temperature (100 mK), as well as magnetization $M$, for single crystals grown by two different techniques: Bridgman and chemical vapor transport. The results show a series of dips in $κ(H)$ and peaks in the field derivative of $M$ located at the same fields independent of the growth method. We argue that these structures originate from field-induced phase transitions rather than quantum oscillations. The positions of several of these features are temperature-dependent and connected to the magnetic phase transitions in zero field: the main transition at 7 K and weaker additional transitions which likely arise from secondary phases at 10 K and 13 K. In contrast to what is expected for quantum oscillations, the magnitude of the structure in $κ(H)$ is smaller for the higher conductivity crystal and decreases rapidly upon cooling below 1 K.

cond-mat.str-el↗

arXiv2020

Proximate ferromagnetic state in the Kitaev model material $α$-RuCl$_{3}$

$α$-RuCl$_{3}$ is a major candidate for the realization of the Kitaev quantum spin liquid, but its zigzag antiferromagnetic order at low temperatures indicates deviations from the Kitaev model. We have quantified the spin Hamiltonian of $α$-RuCl$_{3}$ by a resonant inelastic x-ray scattering study at the Ru $L_{3}$ absorption edge. In the paramagnetic state, the quasi-elastic intensity of magnetic excitations has a broad maximum around the zone center without any local maxima at the zigzag magnetic Bragg wavevectors. This finding implies that the zigzag order is fragile and readily destabilized by competing ferromagnetic correlations. The classical ground state of the experimentally determined Hamiltonian is actually ferromagnetic. The zigzag state is stabilized via a quantum "order by disorder" mechanism, leaving ferromagnetism -- along with the Kitaev spin liquid -- as energetically proximate metastable states. The three closely competing states and their collective excitations hold the key to the theoretical understanding of the unusual properties of $α$-RuCl$_{3}$ in magnetic fields.

cond-mat.str-el↗

arXiv2019

Magneto-optical probe of the fully gapped Dirac band in ZrSiS

We present a far-infrared magneto-optical study of the gapped nodal-line semimetal ZrSiS in magnetic fields $B$ up to 7 T. The observed field-dependent features, which represent intra- (cyclotron resonance) and interband transitions, develop as $\sqrt{B}$ in increasing field and can be consistently explained within a simple 2D Dirac band model with a gap of 26 meV and an averaged Fermi velocity of $3\times10^{5}$ m/s. This indicates a rather narrow distribution of these parameters along the in-plane portions of the nodal line in the Brillouin zone. A field-induced feature with an energy position that does not depend on $B$ is also detected in the spectra. Possible origins of this feature are discussed.

cond-mat.mes-hall↗

arXiv2017

Flat optical conductivity in ZrSiS due to two-dimensional Dirac bands

ZrSiS exhibits a frequency-independent interband conductivity $σ(ω) = \rm{const}(ω) \equiv σ_{\rm{flat}}$ in a broad range from 250 to 2500 cm$^{-1}$ (30 - 300 meV). This makes ZrSiS similar to (quasi)two-dimensional Dirac electron systems, such as graphite and graphene. We assign the flat optical conductivity to the transitions between quasi-two-dimensional Dirac bands near the Fermi level. In contrast to graphene, $σ_{\rm{flat}}$ is not supposed to be universal but related to the length of the nodal line in the reciprocal space, $k_{0}$. When $σ_{\rm{flat}}$ and $k_{0}$ are connected by a simple model, we find good agreement between experiment and theory. Due to the spin-orbit coupling, the discussed Dirac bands in ZrSiS possess a small gap $Δ$, for which we determine an upper bound max($Δ$) = 30 meV from our optical measurements. At low temperatures the momentum-relaxation rate collapses, and the characteristic length scale of momentum relaxation is of the order of microns below 50 K.

cond-mat.mes-hall↗