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Register a new userThermodynamic and Transport Properties in Disordered Kitaev Models
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Effects of bond randomness and site dilution are systematically investigated for the Kitaev model describing a quantum spin liquid with fractional excitations of itinerant Majorana fermions and localized fluxes. We find that, in the high-temperature region where the itinerant Majorana fermions release their entropy, both types of disorders suppress the longitudinal thermal conductivity while keeping the specific heat almost unchanged. This suggests that both disorders reduce the mean-free path of the Majorana fermions. On the other hand, in the low-temperature region, the other specific heat peak associated with the entropy release from the localized fluxes is suppressed for both cases, but it is broadened and shifted to the lower-temperature side by the bond randomness, while the position and the width are almost unchanged against the site dilution. Contrasting behavior is also found in the thermal Hall effect under a magnetic field; the half quantization of the thermal Hall conductivity is fragile against the site dilution, while it remains for the bond randomness despite the reduced onset temperature. We discuss the contrasting behavior from the stability of the topological nature by calculating flux condensation and Majorana excitation gap.
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We compute the transport and chaos properties of lattices of quantum Sachdev-Ye-Kitaev islands coupled by single fermion hopping, and with the islands coupled to a large number of local, low energy phonons. We find two distinct regimes of linear-in-temperature ($T$) resistivity, and describe the crossover between them. When the electron-phonon coupling is weak, we obtain the `incoherent metal regime, where there is near-maximal chaos with front propagation at a butterfly velocity $v_B$, and the associated diffusivity $D_{rm chaos} = v_B^2/(2 pi T)$ closely tracks the energy diffusivity. On the other hand, when the electron-phonon coupling is strong, and the linear resistivity is largely due to near-elastic scattering of electrons off nearly free phonons, we find that the chaos is far from maximal and spreads diffusively. We also describe the crossovers to low $T$ regimes where the electronic quasiparticles are well defined.
Descriptors that characterize the geometry and topology of the pore space of porous media are intimately linked to their transport properties. We quantify such descriptors, including pore-size functions and the critical pore radius $delta_c$, for four different models: maximally random jammed sphere packings, overlapping spheres, equilibrium hard spheres, and inherent structures of the quantizer energy. For precise estimates of the percolation thresholds, we use a strict relation of the void percolation around sphere configurations to weighted bond percolation on the corresponding Voronoi networks. We use the Newman-Ziff algorithm to determine the percolation threshold using universal properties of the cluster size distribution. Often, $delta_c$ is used as the key characteristic length scale that determines the fluid permeability $k$. A recent study [Torquato. Adv. Wat. Resour. 140, 103565 (2020)] suggested for porous media with a well-connected pore space an alternative estimate of $k$ based on the second moment of the pore size $langledelta^2rangle$. Here, we confirm that, for all porosities and all models considered, $delta_c^2$ is to a good approximation proportional to $langledelta^2rangle$. However, unlike $langledelta^2rangle$, the permeability estimate based on $delta_c^2$ does not predict the correct ranking of $k$ for our models. Thus, we confirm $langledelta^2rangle$ to be a promising candidate for convenient and reliable estimates of $k$ for porous media with a well-connected pore space. Moreover, we compare the fluid permeability of our models with varying degrees of order, as measured by the $tau$ order metric. We find that (effectively) hyperuniform models tend to have lower values of $k$ than their nonhyperuniform counterparts. Our findings could facilitate the design of porous media with desirable transport properties via targeted pore statistics.
The exactly-solvable Kitaev model of two-dimensional honeycome magnet leads to a quantum spin liquid (QSL) characterized by Majorana fermions, relevant for fault-tolerant topological quantum computations.In the high-field paramagnetic state of $alpha$-RuCl$_3$, half-integer quantization of thermal Hall conductivity has been reported as a signature of Majorana fermions, but the bulk nature of this state remains elusive.Here, from high-resolution heat capacity measurements under in-plane field rotation, we find strongly angle-dependent low-energy excitations in the bulk of $alpha$-RuCl$_3$. The excitation gap has a sextuple node structure, and the gap amplitude increases with field, exactly as expected for itinerant Majorana fermions in the Kitaev model.Our thermodynamic results are fully linked with the transport quantization properties, providing the first demonstration of the bulk-edge correspondence in a Kitaev QSL.
We report on a systematic optical investigation of wustite. In addition, the sample under consideration, Fe0.93O, has been characterized in detail by electrical transport, dielectric, magnetic and thermodynamic measurements. From infrared reflectivity experiments, phonon properties, Drude-like conductivity contributions and electronic transitions have been systematically investigated. The phonon modes reveal a clear splitting below the antiferromagnetic ordering temperature, similar to observations in other transition-metal monoxides and in spinel compounds which have been explained in terms of a spin-driven Jahn-Teller effect. The electronic transitions can best be described assuming a crystal-field parameter Dq = 750 cm-1 and a spin-orbit coupling constant lambda = 95 cm-1. A well defined crystal field excitation at low temperatures reveals significant broadening on increasing temperature with an overall transfer of optical weight into dc conductivity contributions. This fact seems to indicate a melting of the on-site excitation into a Drude behavior of delocalized charge carriers. The optical band gap in wustite is close to 1.0 eV at room temperature. With decreasing temperatures and passing the magnetic phase transition we have detected a strong blue shift of the correlation-induced band edge, which amounts more than 15% and has been rarely observed in antiferromagnets.
We have performed high-pressure, electrical resistivity, and specific heat measurements on CeTe3 single crystals. Two magnetic phases with nonparallel magnetic easy axes were detected in electrical resistivity and specific heat at low temperatures. We also observed the emergence of an additional phase at high pressures and low temperatures and a possible structural phase transition detected at room temperature and at 45 kbar, which can possibly be related with the lowering of the charge-density wave transition temperature known for this compound.
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