We report the charge doping of ${\mathrm{KCr}}_3{\mathrm{As}}_3$ via H intercalation. We show that the previously reported ethanol bath deintercalation of ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ to ${\mathrm{KCr}}_3{\mathrm{As}}_3$ has a secondary effect whereby H from the bath enters the quasi-one-dimensional ${\mathrm{Cr}}_6{\mathrm{As}}_6$ chains. Furthermore, we find that—contrary to previous interpretations—the difference between nonsuperconducting as-grown ${\mathrm{KCr}}_3{\mathrm{As}}_3$ samples and superconducting hydrothermally annealed samples is not a change in crystallinity but due to charge doping, with the latter treatment increasing the H concentration in the CrAs tubes, effectively electron doping the 133 compound. These results suggest another stoichiometry ${\mathrm{KH}}_x{\mathrm{Cr}}_3{\mathrm{As}}_3$, that superconductivity arises from a suppressed magnetic order via a tunable parameter and paves the way for a charge-doped phase diagram in these materials.

We compute the scaling properties of the localization length ${\xi }_2$ of two interacting particles in a one-dimensional chain with diagonal disorder, and the connectivity properties of the Fock states. We analyze record large system sizes (up to $N=20000$) and disorder strengths (down to $W=0.5$). We vary the energy $E$ and the on-site interaction strength $u$. At a given disorder strength, the largest enhancement of ${\xi }_2$ occurs for $u$ of the order of the single-particle bandwidth and for two-particle states with energies at the center of the spectrum, $E=0$. We observe a crossover in the scaling of ${\xi }_2$ with the single-particle localization length ${\xi }_1$ into the asymptotic regime for ${\xi }_1>100$ ($W<1.0$). This happens due to the recovery of translational invariance and momentum conservation rules in the matrix elements of interconnected Fock eigenstates for $u=0$. The entrance into the asymptotic scaling is manifested through a nonlinear scaling function ${\xi }_2/{\xi }_1=F\left(u{\xi }_1\right)$.

We study the orbitally frustrated singlet-triplet models that emerge in the context of spin-orbit coupled Mott insulators with $t_{2g}^4$ electronic configuration. In these compounds, low-energy magnetic degrees of freedom can be cast in terms of three-flavor “triplon” operators describing the transitions between spin-orbit entangled $J=0$ ionic ground state and excited $J=1$ levels. In contrast to a conventional, flavor-isotropic O(3) singlet-triplet models, spin-orbit entangled triplon interactions are flavor-and-bond selective and thus highly frustrated. In a honeycomb lattice, we find close analogies with the Kitaev spin model—an infinite number of conserved quantities, no magnetic condensation, and spin correlations being strictly short-ranged. However, due to the bosonic nature of triplons, there are no emergent gapless excitations within the spin gap, and the ground state is a strongly correlated paramagnet of dense triplon pairs with no long-range entanglement. Using exact diagonalization, we study the bosonic Kitaev model and its various extensions, which break exact symmetries of the model and allow magnetic condensation of triplons. Possible implications for magnetism of ruthenium oxides are discussed.

We studied the magnetic properties, in particular dynamics, of the correlated spins associated with natural defects in the organic spin chain compounds ${\left(o\text{-DMTTF}\right)}_{2}X(X=\mathrm{Br},\mathrm{Cl})$ by means of electron spin resonance (ESR) spectroscopy. Both materials exhibit spin-Peierls transitions at temperatures around 50 K [P. Foury-Leylekian et al., Phys. Rev. B 84, 195134 (2011)], which allow a separation of the properties of defects inside the chains from the magnetic response of the spin chains. Indeed, continuous-wave ESR measurements performed over a wide temperature range evidence the evolution of the spin dynamics from being governed by the spins in the chains at elevated temperatures to a low-temperature regime which is dominated by defects within the spin-dimerized chains. Such defects polarize the antiferromagnetically coupled spins in their vicinity, thereby leading to a finite local alternating magnetization around the defect site which can be described in terms of a soliton, i.e., a spin-$\frac{1}{2}$ quasiparticle built of many correlated spins, pinned to the defect. In addition, contributions of triplon excitations of the spin-dimerized state to the ESR response below the transition temperature were observed, which provides a spectroscopic estimate for the spin gap of the studied systems. Moreover, details of spin dynamics deep in the spin-Peierls phase were investigated by pulse ESR experiments which revealed Rabi oscillations as signatures of coherent spin dynamics. The latter is a prerequisite for a selective manipulation of the defect-induced soliton spin states which is, for instance, relevant in the context of quantum computation. From a comparison of the characteristic damping times of the Rabi oscillations with measurements of the spin relaxation times by means of primary-echo decay and Carr-Purcell-Meiboom-Gill methods, it becomes evident that inhomogeneities in local magnetic fields strongly contribute to the soliton decoherence.

The mysterious pseudogap (PG) phase of cuprate superconductors has been the subject of intense investigation over the last 30 years, but without a clear agreement about its origin. Owing to a recent observation in Raman spectroscopy, of a precursor in the charge channel, on top of the well known fact of a precursor in the superconducting channel, we present here a novel idea: the PG is formed through a Higgs mechanism, where two kinds of preformed pairs, in the particle-particle and particle-hole channels, become entangled through a freezing of their global phase. Remarkably, this entanglement is equivalent to fractionalizing a Cooper pair density wave (PDW) into its elementary parts; the particle-hole pair, giving rise to both density modulations and current modulations, and the particle-particle counterpart, leading to the formation of Cooper pairs. From this perspective, the “fractionalized PDW” becomes the central object around the formation of the pseudogap. The “locking” of phases between the charge and superconducting modes gives a unique explanation for the unusual global phase coherence of short-range charge modulations, observed below $T_c$ on phase sensitive scanning tunneling microscopy (STM). A simple microscopic model enables us to estimate the mean-field values of the precursor gaps in each channel and the PG energy scale, and to compare them to the values observed in Raman scattering spectroscopy. We also discuss the possibility of a multiplicity of orders in the PG phase and give an overview of the phase diagram.

Iron-based binary chalcogenide superconductors FeSe and FeS have attracted much recent attention due to their complex orbital-selective correlations and Cooper pairing, offering the minimal model system holding the key properties to understanding the physics of high-$T_c$ superconductors. Here, using density functional plus dynamical mean-field theory method with full self-consistency over the charge density, we study the effect of electronic correlations on the electronic structure, magnetic properties, orbital-dependent band renormalizations, and Fermi surface of the tetragonal phase of bulk FeS. We perform a direct structural optimization of the $P_4/nmm$ crystal structure of paramagnetic FeS, minimizing the total energy of FeS with respect to the lattice constant $a$ and the internal coordinate $z_{\mathrm{S}}$ of atom S. Our results show an anomalous sensitivity of the electronic structure and magnetic properties of FeS (intrinsic to all Fe-based chalcogenide and pnictide superconductors) to fine details of its crystal structure, e.g., to a small variation of the chalcogen coordinate $z_{\mathrm{S}}$. Upon expansion of the lattice volume (which can be realized, e.g., in ${\mathrm{FeS}}_{1-x}{\mathrm{Se}}_x$ and ${\mathrm{FeS}}_{1-x}{\mathrm{Te}}_x$), we observe a remarkable change of the electronic structure of FeS which is associated with a complete reconstruction of the Fermi-surface topology (Lifshitz transition). This behavior is ascribed to a correlation-induced shift of the Van Hove singularity associated with the Fe $xy$ and $xz/yz$ orbitals at the $M$ point across the Fermi level. The Lifshitz phase transition is accompanied by a significant growth of local magnetic moments and emergence of strong orbital-selective correlations. It is seen as a pronounced anomaly (“kink”) in the total energies upon expansion of the lattice, associated with a remarkable enhancement of compressibility. This behavior is accompanied by an orbital-dependent formation of local moments, a crossover from itinerant to localized orbital-selective moment behavior of the Fe $3d$ electrons. While exhibiting weak effective mass enhancement of the Fe $3d$ states $m^{*}/m\sim 1.3–1.4$, correlation effects reveal a strong impact on a position of the Van Hove singularity originating from the Fe $xz/yz$ and $xy$ orbitals at the $M$ point, implying a complex interplay between electronic correlations and band structure effects in FeS. Our results suggest a complex interplay between electronic correlations, magnetism, and lattice degrees of freedom in FeS.

The integer quantum Hall to insulator transition (IQHIT) is a paradigmatic quantum critical point. Key aspects of this transition, however, remain mysterious, due to the simultaneous effects of quenched disorder and strong interactions. We study this transition using a composite fermion (CF) representation, which incorporates some of the effects of interactions. As we describe, the transition also marks an IQHIT of CFs: this suggests that the transition may exhibit “self-duality.” We show the explicit equivalence of the electron and CF Lagrangians at the critical point via the corresponding nonlinear sigma models, revealing the self-dual nature of the transition. We show analytically that the resistivity tensor at the critical point is ${\rho }_{xx}^c={\rho }_{xy}^c=\frac{h}{e^2}$, which are consistent with the expectations of self-duality, and in rough agreement with experiments.

Nanoscale solid-solid contacts are key elements which determine the electrical and thermal behavior of modern electronic devices and micro- and nanoelectromechanical systems. Here we show that simultaneous measurements of the shear force and the heat flow in nanoscale junctions reveal a linear correlation between thermal conductance and maximal shear force, confirming the ballistic nature of heat transport in the junction. Furthermore, we find that here the shear strength and thermal conductance in nanoscale contacts for materials where heat transport is phonon dominated can be linked via the fundamental material properties of heat capacity and group velocity of the heat carriers.

At the heart of the study of topological insulators lies a fundamental dichotomy: Topological invariants are defined in infinite systems but surface states as their main footprint only exist in finite systems. In the slab geometry, namely, infinite in two planar directions and finite in the perpendicular direction, the 2D topological invariant was shown to display three different types of behavior. The perpendicular Dirac velocity turns out to be a critical control parameter discerning between different qualitative situations. When it is zero, the three types of behavior extrapolate to the three 3D topologically distinct phases: trivial, weak, and strong topological insulators. We show analytically that the boundaries between types of behavior are topological phase transitions of particular significance since they allow us to predict the 3D topological invariants from finite-thickness transitions. When the perpendicular Dirac velocity is not zero, we identify a new phase with surface states but no band inversion at any finite thickness, disentangling these two concepts which are closely linked in 3D. We also show that at zero perpendicular Dirac velocity, the system is gapless in the 3D bulk and therefore not a topological insulating state, even though the slab geometry extrapolates to the 3D topological phases. Finally, in a parameter regime with strong dispersion perpendicular to the surface of the slab, we encounter the unusual case that the slab physics displays nontrivial phases with surface states but nevertheless extrapolates to a 3D trivial state.

The recent development of efficient chirality sorting techniques has opened the way to the use of single-walled carbon nanotubes (SWCNTs) in a plethora of nanoelectronic, photovoltaic, and optoelectronic applications. However, to understand the excitation processes undergone by SWCNTs, it is necessary to have highly efficient and accurate computational methods to describe their optical and electronic properties, methods which have until now been unavailable. Here we employ linear combinations of atomic orbitals (LCAOs) to represent the Kohn-Sham (KS) wave functions and perform highly efficient time-dependent density functional theory (TDDFT) calculations in the frequency domain using our LCAO-TDDFT-$k\text{−}\omega$ code to model the optical absorbance and energy loss spectra and spatial distribution of the exciton charge densities in SWCNTs. By applying the GLLB-SC derivative discontinuity correction to the KS eigenenergies, we reproduce the measured $E_{11}$ and $E_{22}$ transitions within $\sigma \lesssim 70$ meV and the optical absorbance and electron energy loss spectra semiquantitatively for a set of 15 semiconducting and 4 metallic chirality sorted SWCNTs. Furthermore, our calculated electron hole density difference $\mathrm{\Delta }\rho (\mathbf{r},\omega )$ resolves the spatial distribution of the measured excitations in SWCNTs. These results open the path towards the computational design of optimized SWCNT nanoelectronic, photovoltaic, and optoelectronic devices in silico.

Current approaches of discovering new topological quantum materials, such as (magnetic) Weyl materials, by screening material databases or by rational design suffer from experimental challenges in growing the material in the desired geometry, hampering their potential applications. We take an approach of defect-engineering a topology starting from systems with experimentally well-controlled growth. Using SnTe, an experimentally viable topological crystalline insulator, as our model system, we elucidate the different effects of breaking time-reversal, inversion, and mirror symmetries using magnetic dopant defects, to controllably induce a topological phase transition. To this effect, we compute the thermodynamics, magnetism, and band-structure topology of magnetically doped SnTe using a full ab initio approach. In this process, we have discovered that Cr-doped SnTe is a Weyl semimetal. We have computed its classic Fermi-arc experimental signature on the (001) surface and also showed that it has a large anamolous Hall conductivity (AHC) of $\sim 250 {\mathrm{\Omega }}^{-1}{\text{cm}}^{-1}$. We predict a large Curie temperature ($T_c$) of 62 K even at $3.3\%$ doping, with both $T_c$ and AHC being tunable with Cr concentration, suggesting potential for room-temperature applications. Our study in general paves the way for defect-designing room-temperature topological quantum phases.

We study the interplay of competing interactions in spin-$\frac{1}{2}$ triangular Heisenberg model through tuning the first- ($J_1$), second- ($J_2$), and third-neighbor ($J_3$) couplings. Based on a large-scale density-matrix renormalization group calculation, we identify a quantum phase diagram of the system and discover a gapless chiral spin-liquid (CSL) phase in the intermediate $J_2$ and $J_3$ regime. This CSL state spontaneously breaks time-reversal symmetry with finite scalar chiral order, and it has gapless excitations implied by a vanishing spin triplet gap and a finite central charge on the cylinder. Moreover, the central charge grows rapidly with the cylinder circumference, indicating emergent spinon Fermi surfaces. To understand the numerical results we propose a parton mean-field spin-liquid state, the $U\left(1\right)$ staggered flux state, which breaks time-reversal symmetry with chiral edge modes by adding a Chern insulator mass to Dirac spinons in the $U\left(1\right)$ Dirac spin liquid. This state also breaks lattice rotational symmetries and possesses two spinon Fermi surfaces driven by nonzero $J_2$ and $J_3$, which naturally explains the numerical results. This realizes an example of a gapless CSL state with coexisting spinon Fermi surfaces and chiral edge states, demonstrating the rich family of interesting quantum phases emergent from competing interactions in triangular-lattice magnets.

We report an investigation on the mixed-valence state for $\mathrm{Yb}X{\mathrm{Cu}}_4$ compounds with $X$ = Mg, Cd, In, and Sn. The x-ray diffraction measurements on $X$ = Sn have revealed a cubic ${\mathrm{AuBe}}_5$-type structure with the lattice constant $a=7.123\AA$. The Kondo temperature of $X$ = Sn is determined to be $T_{\mathrm{K}}=503$ K from the value of magnetic susceptibility. We have observed a mixing of the ${\mathrm{Yb}}^{2+}$ and ${\mathrm{Yb}}^{3+}$ electronic states from the x-ray absorption spectra at 300 K and a change in their relative intensities with varying $X$ atoms, indicating a different degree of valence mixing in $\mathrm{Yb}X{\mathrm{Cu}}_4$. Our data clearly exhibit correlations among the energy of the ${\mathrm{Yb}}^{3+}$ peak, the degree of valence mixing, and the Kondo temperature. These results suggest that the $5p$ electrons of $X$ atoms play an important role in the mixed-valence state of $\mathrm{Yb}X{\mathrm{Cu}}_4$ by controlling the number of the Yb $4f$ electrons.

We use a recently proposed class of tensor-network states to study phase transitions in string-net models. These states encode the genuine features of the string-net condensate such as, e.g., a nontrivial perimeter law for Wilson loops expectation values, and a natural order parameter detecting the breakdown of the topological phase. In the presence of a string tension, a quantum phase transition occurs between the topological phase and a trivial phase. We benchmark our approach for ${\mathbb{Z}}_2$ string nets and capture the second-order phase transition which is well known from the exact mapping onto the transverse-field Ising model. More interestingly, for Fibonacci string nets, we obtain first-order transitions in contrast with previous studies but in qualitative agreement with mean-field results.

Collective charge-density wave (CDW) transport was measured under a high magnetic field in ${\mathrm{NbSe}}_3$ microbridges which have been cut transversely and at an angle to the chains' direction. We give evidence that the CDW sliding is driven by the Hall voltage generated by the interchain current of normal carriers. We have discovered a reentrance effect of the Hall-driven sliding above a crossover temperature at which the Hall constant has been known to change signs. For the narrow channel, cut at ${45}^{\circ }$ relative to the chain axis, we observed an evolution from the Hall-driven sliding at low temperatures, to the conventional sliding at higher temperatures, which corroborates with falling of the Hall constant. In this course, the nonlinear contribution to the conductivity coming from the collective sliding changes signs. The quantization of Shapiro steps, generated presumably by a coherent sequence of phase slips, indicates that their governing changes from the applied voltage to the current.

Non-Hermiticity in the Weyl Hamiltonian leads to the realization of Weyl exceptional rings and flat bands inside the Weyl exceptional rings. Recently, the platform of non-Hermitian physics is extended to many-body or disordered systems where quasiparticles possess finite lifetime. Here, we clarify that the deviation from the unitarity limit in a disordered Weyl semimetal leads to the generation of Weyl exceptional rings regardless of the detail of the scattering potential. In the case of topological Dirac semimetals, hybrid point rings and flat bands without vorticity of complex-energy eigenvalues are realized. This scenario is applicable to any Weyl or Dirac semimetals. These effects are detectable by using photoemission or quasiparticle interference experiments.

In a recent experiment [A. Donarini et al., Nat. Commun. 10, 381 (2019)], electronic transport through a carbon nanotube quantum dot was observed to be suppressed by the formation of a quantum-coherent “dark state.” In this paper we consider theoretically the counting statistics and waiting-time distribution of this dark-state-limited transport. We show that the statistics are characterized by giant super-Poissonian Fano factors and long-tailed waiting-time distributions, both of which are signatures of the bistability and extreme electron bunching caused by the dark state.