We use multifractal finite-size scaling to perform a high-precision numerical study of the critical properties of the Anderson localization-delocalization transition in the unitary symmetry class, considering the Anderson model including a random magnetic flux. We demonstrate the scale invariance of the distribution of wave-function intensities at the critical point and study its behavior across the transition. Our analysis, involving more than $4\times {10}^6$ independently generated wave functions of system sizes up to $L^3={150}^3$, yields accurate estimates for the critical exponent of the localization length, $\nu =1.446(1.440,1.452)$, the critical value of the disorder strength, and the multifractal exponents.

Zero-field and transverse-field muon spin rotation/relaxation ($\mu \mathrm{SR}$) experiments were undertaken in order to elucidate the microscopic properties of a strongly coupled superconductor ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$ with $T_{\text{c}}=9.8$ K. The upper critical field extracted from the transverse-field $\mu \mathrm{SR}$ data exhibits significant reduction with respect to the data from thermodynamic measurements indicating the coexistence of two independent length scales in the superconducting state. Accordingly, the temperature-dependent magnetic penetration depth of ${\mathrm{Mo}}_8{\mathrm{Ga}}_{41}$ is described using a model in which two $s$ wave superconducting gaps are assumed. A V for Mo substitution in the parent compound leads to the complete suppression of one superconducting gap, and ${\mathrm{Mo}}_7{\mathrm{VGa}}_{41}$ is well described within the single $s$ wave gap scenario. The reduction in the superfluid density and the evolution of the low-temperature resistivity upon V substitution indicate the emergence of a competing state in ${\mathrm{Mo}}_7{\mathrm{VGa}}_{41}$ that may be responsible for the closure of one of the superconducting gaps.

Surface properties are examined in a chiral $d$-wave superconductor with hexagonal symmetry, whose one-body Hamiltonian possesses intrinsic spin-orbit coupling identical to the one characterizing the topological nature of the Kane-Mele honeycomb insulator. In the normal state, spin-orbit coupling gives rise to spontaneous surface spin currents, whereas in the superconducting state, besides the spin currents, there exist also charge surface currents, due to chiral pairing symmetry. Interestingly, the combination of these two currents results in a surface spin polarization, whose spatial dependence is markedly different on the zigzag and armchair surfaces. We discuss various potential candidate materials, such as SrPtAs, which may exhibit these surface properties.

We report on optical spectroscopy on the lacunar spinels ${\mathrm{GaV}}_4{\mathrm{S}}_8$ and ${\mathrm{GeV}}_4{\mathrm{S}}_8$ in the spectral range from 100 to 23 000 ${\mathrm{cm}}^{-1}$ and for temperatures from 5 to 300 K. These multiferroic spinel systems reveal Jahn-Teller driven ferroelectricity and complex magnetic order at low temperatures. We study the infrared-active phonon modes and the low-lying electronic excitations in the cubic high-temperature phase, as well as in the orbitally and in the magnetically ordered low-temperature phases. We compare the phonon modes in these two compounds, which undergo different symmetry-lowering Jahn-Teller transitions into ferroelectric and orbitally ordered phases, and exhibit different magnetic ground states. We follow the splitting of the phonon modes at the structural phase transition and detect additional splittings at the onset of antiferromagnetic order in ${\mathrm{GeV}}_4{\mathrm{S}}_8$. We observe electronic transitions within the $d$-derived bands of the ${\mathrm{V}}_4$ clusters and document a significant influence of the structural and magnetic phase transitions on the narrow electronic band gaps.

We study the ac current-driven domain wall motion in bilayer ferromagnetic metal (FM)/nonmagnetic metal (NM) nanowires. The solution of the modified Landau-Lifshitz-Gilbert equation including all the spin transfer torques is used to describe motion of the domain wall in the presence of the spin Hall effect. We show that the domain wall center has a second-harmonic frequency response in addition to the known first-harmonic excitation. In contrast to the experimentally observed second-harmonic response in harmonic Hall measurements of spin-orbit torque in magnetic thin films, this second-harmonic response directly originates from spin-orbit torque driven domain wall dynamics. Based on the spin current generated by domain wall dynamics, the longitudinal spin motive force generated voltage across the length of the nanowire is determined. The second-harmonic response introduces additionally a practical field-free and all-electrical method to probe the effective spin Hall angle for FM/NM bilayer structures that could be applied in experiments. Our results also demonstrate the capability of utilizing FM/NM bilayer structures in domain wall based spin-torque signal generators and resonators.

We present a theoretical framework and implementation details for self-energy embedding theory (SEET) with the GW approximation for the treatment of weakly correlated degrees of freedom and configuration interactions solver for handing the strongly correlated degrees. On a series of molecular examples, for which the exact results are known within a given basis, we demonstrate that SEET(CI/GW) is a systematically improvable and well controlled method capable of giving accurate results and well behaved causal self-energies and Green's functions. We compare the theoretical framework of SEET(CI/GW) to that of GW+DMFT and comment on differences between these to approaches that aim to treat both strongly and weakly correlated simultaneously.

The electronic thermoelectric coefficients are analyzed in the vicinity of one and two Anderson localization thresholds in three dimensions. For a single mobility edge, we correct and extend previous studies and find universal approximants which allow us to deduce the critical exponent for the zero-temperature conductivity from thermoelectric measurements. In particular, we find that at nonzero low temperatures the Seebeck coefficient and the thermoelectric efficiency can be very large on the “insulating” side, for chemical potentials below the (zero-temperature) localization threshold. Corrections to the leading power-law singularity in the zero-temperature conductivity are shown to introduce nonuniversal temperature-dependent corrections to the otherwise universal functions which describe the Seebeck coefficient, the figure of merit, and the Wiedemann-Franz ratio. Next, the thermoelectric coefficients are shown to have interesting dependences on the system size. While the Seebeck coefficient decreases with decreasing size, the figure of merit first decreases but then increases, while the Wiedemann-Franz ratio first increases but then decreases as the size decreases. Small (but finite) samples may thus have larger thermoelectric efficiencies. In the last part we study thermoelectricity in systems with a pair of localization edges, the ubiquitous situation in random systems near the centers of electronic energy bands. As the disorder increases, the two thresholds approach each other, and then the Seebeck coefficient and the figure of merit increase significantly, as expected from the general arguments of Mahan and Sofo [J. D. Mahan and J. O. Sofo, Proc. Natl. Acad. Sci. USA 93, 7436 (1996)] for a narrow energy range of the zero-temperature metallic behavior.

Doped free carriers can substantially renormalize electronic self-energy and quasiparticle band gaps of two-dimensional (2D) materials. However, it is still challenging to quantitatively calculate this many-electron effect, particularly at the low doping density that is most relevant to realistic experiments and devices. Here we develop a first-principles-based effective-mass model within the $GW$ approximation and show a dramatic band-gap renormalization of a few hundred meV for typical 2D semiconductors. Moreover, we reveal the roles of different many-electron interactions: The Coulomb-hole contribution is dominant for low doping densities while the screened-exchange contribution is dominant for high doping densities. Three prototypical 2D materials are studied by this method: $h\text{−}\mathrm{BN}$, $\mathrm{Mo}{\mathrm{S}}_2$, and black phosphorus, covering insulators to semiconductors. Especially, anisotropic black phosphorus exhibits a surprisingly large band-gap renormalization because of its smaller density-of-state that enhances the screened-exchange interactions. Our work demonstrates an efficient way to accurately calculate band-gap renormalization and provides quantitative understanding of doping-dependent many-electron physics of general 2D semiconductors.

We present a polarization-resolved photoluminescence study of the exchange interaction effects in a prototype system consisting of an individual ${\mathrm{Fe}}^{2+}$ ion and a single neutral exciton confined in a CdSe/ZnSe quantum dot. A maximal possible number of eight fully linearly polarized lines in the bright exciton emission spectrum is observed, evidencing complete degeneracy lifting in the investigated system. We discuss the conditions required for such a scenario to take place: anisotropy of the electron-hole interaction and the zero-field splitting of the ${\mathrm{Fe}}^{2+}$ ion spin states. Neglecting either of these components is shown to restore partial degeneracy of the transitions, making the excitonic spectrum similar to those previously reported for all other systems of quantum dots with single magnetic dopants.

We investigated the role of lateral force components on the lateral manipulation of intrinsic Si adatoms toward a vacancy site on a Si(111)-$(7\times 7)$ surface using noncontact atomic force microscopy at room temperature. Lateral atom manipulation was accomplished via constant-height scans using a set of tips with varying chemical reactivities. We determined the vertical and lateral force as well as the interaction energy profiles associated with the lateral manipulation of a Si adatom on a Si(111)-$(7\times 7)$ surface. Our results demonstrate that lateral forces do not play a decisive role in the manipulation process while the vertical force component is key for the manipulation process, and the ability to manipulate the Si adatom depends primarily on the chemical nature of the tip apex. Our results further reveal that the tips that exhibit high chemical reactivity with Si adatoms have a sharper interaction energy profile above Si adatoms than tips with less chemical reactivity, indicating the stronger atom-trapping ability of the chemically reactive tips. This characteristic property gives tips the ability to create localized reductions in the energy barrier required for adatom movement, thereby enabling thermally induced adatom hopping toward the tip. These findings can enhance our understanding of the underlying mechanisms involved in the lateral manipulation of intrinsic adatoms of semiconductor surfaces, as well as adsorbate atoms/molecules forming covalent bonds with tip-surface systems, i.e., chemisorption systems.

Low-temperature phonon thermal conductance (PTC) of any 1D system increases proportionally to the temperature. However, here we show that in single- and double-walled carbon nanotubes (CNTs), starting from 3–6 K, the PTC increases faster than the linear function, since the low-frequency modes of dispersion curves, which do not tend to zero together with the wave vector, are excited. To develop the PTC theory, we combine the Landauer's ballistic approach with the simple continuous model proposed for the calculation of the low-frequency phonon spectra of both free nanotubes and those interacting with an environment. The approach obtained is valid not only for commensurate double-walled CNTs, but also for incommensurate ones. The temperature-dependent relation between the PTC of double-walled CNT and those of its constituent SWNTs is obtained and discussed. The low-temperature heat transfer in bulk materials originated from CNTs is also considered and the upper limit of thermal conductivity of such materials is determined. We argue that the ideal material consisting of CNTs can challenge diamond only when the mean length of its defect-free nanotubes reaches at least one hundred of micrometers.

Fracton topological order describes a remarkable phase of matter, which can be characterized by fracton excitations with constrained dynamics and a ground-state degeneracy that increases exponentially with the length of the system on a three-dimensional torus. However, previous models exhibiting this order require many-spin interactions, which may be very difficult to realize in a real material or cold atom system. In this work, we present a more physically realistic model which has the so-called X-cube fracton topological order [Vijay, Haah, and Fu, Phys. Rev. B 94, 235157 (2016)] but only requires nearest-neighbor two-spin interactions. The model lives on a three-dimensional honeycomb-based lattice with one to two spin-1/2 degrees of freedom on each site and a unit cell of six sites. The model is constructed from two orthogonal stacks of $Z_2$ topologically ordered Kitaev honeycomb layers [Kitaev, Ann. Phys.321, 2 (2006)], which are coupled together by a two-spin interaction. It is also shown that a four-spin interaction can be included to instead stabilize 3+1D $Z_2$ topological order. We also find dual descriptions of four quantum phase transitions in our model, all of which appear to be discontinuous first-order transitions.

We investigate the fate of hardcore bosons in a Harper-Hofstadter model which was experimentally realized by Aidelsburger et al. [Nat. Phys. 11, 162 (2015)] at half-filling of the lowest band. We discuss the stability of an emergent fractional Chern insulator (FCI) state in a finite region of the phase diagram that is separated from a superfluid state by a first-order transition when tuning the band topology following the protocol used in the experiment. Since crossing a first-order transition is unfavorable for adiabatically preparing the FCI state, we extend the model to stabilize a featureless insulating state. The transition between this phase and the topological state proves to be continuous, providing a path in parameter space along which an FCI state could be adiabatically prepared. To further corroborate this statement, we perform time-dependent DMRG calculations which demonstrate that the FCI state may indeed be reached by adiabatically tuning a simple product state.

The cubic double perovskite ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ has been investigated by the local probe techniques nuclear magnetic resonance (${}^{89}\mathrm{Y}$ NMR) and muon spin rotation ($\mu \mathrm{SR}$). Both methods confirm the absence of magnetic long-range order in this compound but find evidence for diluted localized paramagnetic moments. NMR spin-lattice relaxation rate $T_1^{-1}$ measurements suggest a slowing down of localized spin moments at low temperatures. An increase of the $\mu \mathrm{SR}$ spin-lattice relaxation rate $\lambda$ confirms the presence of weak magnetism in ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$. However, these findings cannot be explained by the recently suggested excitonic type of magnetism. Instead, they point towards tiny amounts of localized paramagnetic spin centers leading to this magnetic response on the background of a simple nonmagnetic ground state of the $5d^4$ ($J=0)$ electronic configuration of ${\mathrm{Ir}}^{5+}$.

Many Heusler compounds possess magnetic properties well suited for applications as spintronic materials. The pseudobinary ${\mathrm{Mn}}_{0.5}{\mathrm{Fe}}_{0.5}{\mathrm{Ru}}_2\mathrm{Sn}$, formed as a solid solution of two full Heuslers, has recently been shown to exhibit exchange hardening suggestive of two magnetic phases, despite existing as a single chemical phase. We have performed a first-principles study of the chemical and magnetic degrees of freedom in the ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{Ru}}_2\mathrm{Sn}$ pseudobinary to determine the origin of the unique magnetic behavior responsible for exchange hardening within a single phase. We find a transition from antiferromagnetic (AFM) to ferromagnetic (FM) behavior upon replacement of Mn with Fe, consistent with experimental results. The lowest energy orderings in ${\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x{\mathrm{Ru}}_2\mathrm{Sn}$ consist of chemically and magnetically uniform (111) planes, with Fe-rich regions preferring FM ordering and Mn-rich regions preferring AFM ordering, independent of the overall composition. Analysis of the electronic structure suggests that the magnetic behavior of this alloy arises from a competition between AFM-favoring Sn-mediated superexchange and FM-favoring RKKY exchange mediated by spin-polarized conduction electrons. Changes in valency upon replacement of Mn with Fe shifts the balance from superexchange-dominated interactions to RKKY-dominated interactions.

The magnetic oscillations in YBCO high-temperature superconductors have been widely studied over the last decade and consist of three equidistant low frequencies with a central frequency several times more intense than its two shoulders. This remains a puzzle in spite of numerous attempts to explain the corresponding small Fermi-surface pockets. Furthermore, the ARPES data indicate only four Fermi arcs with bilayer splitting, and show no sign of such small areas in the Fermi surface. Here we argue that the magnetic oscillations measured in underdoped bilayer high-temperature superconductors, in particular ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+\delta }$, provide a measure of the interplanar electronic coupling rather than the areas of fine-grain reconstruction of the Fermi surfaces coming from induced charge density waves. This identification is based on the relative intensities of the different peaks, as well as their angular dependence, which points to an effective Fermi surface that is larger than the oscillation frequencies, and is compatible with several indications from ARPES. The dominance of such frequencies with respect to the fundamental frequencies from the Fermi surface is natural for a strongly correlated quasi-two-dimensional electronic system where nonlinear mixings of frequencies are more resistant to sample inhomogeneity.