Dependence of topological phase on nuclear spin SS and spin modulation vector in van der Waals Magnets

Recently non-chiral spin structures on the surface of the van der Waals (vdW) magnets have been observed down to monolayers. We provide a Hamiltonian to analyze the electronic properties of these materials. The Hamiltonian takes into account the arbitrary background spin structures and large atomic spin $S$ of the materials. The large spin-\emph{S} treatment is necessary as magnetic atoms of the vdW magnets can have spin $S > 1/2$. In this work the Hamiltonian is solved for the spin spirals with azimuthal and polar degrees of freedom -- this spin structure was recently observed in Fe$_{3}$GeTe$_{2}$. We methodically analyze the Hamiltonian for both integer and half-integer spins in the honeycomb lattice. It shows emerging topological hall effect emerges irrespective of the spin. The Chern number, hence the topological phase, depends on the spin \emph{S}, and interestingly only on the azimuthal angle of the spin vector. These results will be useful for the design of the topological electronics devices based on vdW magnets.Comment: 9 Pages, 5 figure

Spin-flip induced superfluidity in a ring of spinful hard-core bosons

The t - J Hamiltonian of the spinful hard-core bosonic ring in the Nagaoka limit is solved. The energy spectrum becomes quantized due to presence of spin, where each energy level corresponds to a cyclic permutation state of the spin chains. The ground state is true ferromagnetic when the ring contains N = 2, 3 spinful hard-core bosons; for all other N it is a mixture of the ferromagnetic and non-ferromagnetic states. This behaviour is different from the fermionic ring, where ground state is true ferromagnetic only for N = 3. It is shown that the intrinsic spin generated gauge fields are analogous to the synthetic gauge fields generated by rotation of either the condensate or the confining potential. It is argued that the low lying excited levels of the spin flipped states intrinsically support the superfluidity. Possible ways to experimentally verify these results are also discussed.Comment: main text in 6 pages, 2 figures; supplementary materials in 7 page

Topological Hall effect induced by classical large-spin background: su(2)su(2) path-integral approach

The $su(2)$ coherent-state path-integral technique is employed to study lattice electrons strongly coupled to a quantum spin background. In the large-spin limit it is replaced with its classical counterpart that breaks the time-reversal symmetry. The fermions propagating through a classical large-spin texture may then exhibit the topological Hall effect which arises even for a zero scalar spin chirality of the underlying spin background.Comment: 6 pages, 9 figure

Anisotropic zero-resistance onset in organic superconductors

We study the coexistence of superconductivity (SC) and density-wave state and reconcile various puzzling experimental data in organic superconductors (TMTSF)$_{2}$PF$_{6}$ and (TMTSF)$_{2}$ClO$_{4}$. The anisotropic resistance drop above $T_c$ is qualitatively described by nascent isolated SC islands within a bulk analytical model. However, the observed anisotropic SC onset is explained only when the finite size and flat needle shape of samples is considered. Our results pave a way to estimate the volume fraction and the typical size of SC islands in far from the sample surface, and apply to many inhomogeneous superconductors, including high-$T_c$ cuprate or Fe-based ones.Comment: 8 pages with Supplementary materia

A method to estimate the volume fraction and shape of superconducting domains in organic superconductors

In highly anisotropic organic superconductor (TMTSF)$_2$ClO$_4$, superconducting (SC) phase coexists with metallic and spin density wave phases in the form of domains. Using the Maxwell-Garnett approximation (MGA), we provide a method to calculate the volume ratio and the shape of these embedded SC domains from resistivity data. Due to percolation of SC domains, the zero resistance can be achieved even when the SC volume ratio $\phi =\phi_c \ll 1$. This percolation threshold $\phi_c$ depends on the shape and size of SC domains and of the sample, and may be anisotropic. Using our theory we find $\phi$ for various cooling rates of (TMTSF)$_{2}$ClO$_{4}$ samples. We also analyze the effect of disorder on the shape of SC domains. We found that the SC domains have oblate shape, being the shortest along the interlayer z-axis. This contradicts the widely assumed filamentary superconductivity along z-axis, used to explain the anisotropic superconductivity onset. We show that this anisotropic resistivity drop at the SC transition can be described by the analytical MGA theory with anisotropic background resistance, while the anisotropic $T_c$ can be explained\cite{kochev-AnisotropicZeroresistanceOnset-2020} by considering a finite size and flat shape of the samples. Due to a flat/needle sample shape, the probability of percolation via SC domains is the highest along the shortest sample dimension (z-axis), and the lowest along the sample length (x-axis). Our theory can be applied to other heterogeneous superconductors, where the size $d$ of SC domains is much larger than the SC coherence length $\xi$, e.g. cuprates, iron based or organic superconductors. It is also applicable when the spin/charge-density wave domains are embedded inside a metallic background, or vice versa.Comment: 13 Pages, 6 Figure

The evolution of electron dispersion in the series of rare-earth tritelluride compounds obtained from their charge-density-wave properties and susceptibility calculations

We calculated electron susceptibility of rare-earth tritelluride compounds RTe$_3$ as a function of temperature, wave vector and electron-dispersion parameters. Comparison of results obtained with the available experimental data on the transition temperature and on the wave vector of a charge-density wave in these compounds allowed us to predict values and the evolution of electron-dispersion parameters with the variation of atomic number of rare-earth element R.Comment: 6 pages, 6 figure

Evolution of Shape and Volume Fraction of Superconducting Domains with Temperature and Anion Disorder in (TMTSF)₂ClO₄

In highly anisotropic organic superconductor (TMTSF)2ClO4, superconducting (SC) phase coexists with metallic and spin-density wave phases in the form of domains. Using the Maxwell-Garnett approximation (MGA), we calculate the volume ratio and estimate the shape of these embedded SC domains from resistivity data at various temperature and anion disorder, controlled by the cooling rate or annealing time of (TMTSF)2ClO4 samples. We found that the variation of cooling rate and of annealing time affect differently the shape of SC domains. In all cases the SC domains have oblate shape, being the shortest along the interlayer z-axis. This contradicts the widely assumed filamentary superconductivity along the z-axis, used to explain the anisotropic superconductivity onset. We show that anisotropic resistivity drop at the SC onset can be described by the analytical MGA theory with anisotropic background resistance, while the anisotropic Tc can be explained by considering a finite size and flat shape of the samples. Due to a flat/needle sample shape, the probability of percolation via SC domains is the highest along the shortest sample dimension (z-axis), and the lowest along the sample length (x-axis). Our theory can be applied to other heterogeneous superconductors, where the size d of SC domains is much larger than the SC coherence length ξ, e.g., cuprates, iron-based or organic superconductors. It is also applicable when the spin/charge-density wave domains are embedded inside a metallic background, or vice versa