The dynamical fluctuations in approaches such as dynamical mean-field theory (DMFT) allow for the self-consistent optimization of a local fragment, hybridized with a true correlated environment. We show that these correlated environmental fluctuations can instead be efficiently captured in a wave function perspective in a computationally cheap, frequency-independent, zero-temperature approach. This allows for a systematically improvable, short-time wave function analog to DMFT, which entails a number of computational and numerical benefits. We demonstrate this approach to solve the correlated dynamics of the paradigmatic Bethe lattice Hubbard model, as well as detailing cluster extensions in the one-dimensional Hubbard chain where we clearly show the benefits of this rapidly convergent description of correlated environmental fluctuations.

We present a theoretical study of photoabsorption in $n$-doped two-dimensional (2D) and quasi-2D semiconductors that takes into account the interaction of the photocreated exciton with Fermi-sea (FS) electrons through (i) Pauli blocking, (ii) Coulomb screening, and (iii) excitation of FS electron-hole pairs—that we here restrict to one. The system we tackle is thus made of one exciton plus zero or one FS electron-hole pair. At low doping, the system ground state is predominantly made of a “trion-hole”—a trion (two opposite-spin electrons plus a valence hole) weakly bound to a FS hole—with a small exciton component. As the trion is poorly coupled to photon, the intensity of the lowest absorption peak is weak; it increases with doping, thanks to the growing exciton component, due to a larger coupling between two-particle and four-particle states. Under a further doping increase, the trion-hole complex is less bound because of Pauli blocking by FS electrons, and its energy increases. The lower peak then becomes predominantly due to an exciton dressed by FS electron-hole pairs, that is, an exciton-polaron. As a result, the absorption spectra of $n$-doped semiconductor quantum wells show two prominent peaks, the nature of the lowest peak turning from trion-hole to exciton-polaron under a doping increase. Our work also nails down the physical mechanism behind the increase with doping of the energy separation between the trion-hole peak and the exciton-polaron peak, even before the anticrossing, as experimentally observed.

The competition between interactions and dissipative processes in a quantum many-body system can drive phase transitions of different order. Exploiting a combination of cluster methods and quantum trajectories, we show how the systematic inclusion of (classical and quantum) nonlocal correlations at increasing distances is crucial to determine the structure of the phase diagram, as well as the nature of the transitions in strongly interacting spin systems. In practice, we focus on the paradigmatic dissipative quantum Ising model: In contrast to the nondissipative case, its phase diagram is still a matter of debate in the literature. When dissipation acts along the interaction direction, we predict important quantitative modifications of the position of the first-order transition boundary. In the case of incoherent relaxation in the field direction, our approach confirms the presence of a second-order transition, while does not support the possible existence of multicritical points. Potentially, these results can be tested in up-to-date quantum simulators of Rydberg atoms.

The spin resonance of two-dimensional (2D) electrons confined in a high-quality 4.5-nm AlAs quantum well was studied in the regime of the integer quantum Hall effect. The electron $g$-factor extracted from the magnetic field position of the spin resonance at a fixed microwave frequency demonstrated a strong nonlinear dependence on the magnetic field with discontinuities around even filling factors. The value of the $g$-factor tended to increase with a decrease of the magnetic field around each odd filling. Furthermore, the $g$-factor at the exactly odd filling factor $\nu$ turned out to be dependent on $\nu$, suggesting the entanglement between the spin degree of freedom and the orbital motion of the electrons in the regime of the integer quantum Hall effect. This suggestion is further supported, as all the experimental data are well described, when a Dresselhaus-type spin-orbit interaction is introduced into the Hamiltonian of a single electron in the quantizing magnetic field. Surprisingly, such excellent agreement was observed even in the case of tilted magnetic fields. The fitting procedure allowed us to determine the strength of the Dresselhaus spin-orbit term in a 2D electron system and to extract the fundamentally important Dresselhaus constant for bulk AlAs. Unexpectedly, not only was single spin resonance observed around even fillings, it tended to split into two well-resolved lines. Yet this finding remains a mystery and highlights the need for further experimental and theoretical efforts.

The two-electron exchange coupling in a nanowire double quantum dot (DQD) is shown to possess Moriya's anisotropic superexchange interaction under the influence of both the Rashba and Dresselhaus spin-orbit couplings (SOCs) and a Zeeman field. We reveal the controllability of the anisotropic exchange interaction via tuning the SOC and the direction of the external magnetic field. The exchange interaction can be transformed into an isotropic Heisenberg interaction, but the uniform magnetic field becomes an effective inhomogeneous field whose measurable inhomogeneity reflects the SOC strength. Moreover, the presence of the effective inhomogeneous field gives rise to an energy-level anticrossing in the low-energy spectrum of the DQD. By fitting the analytical expression for the energy gap to the experimental spectroscopic detections [S. Nadj-Perge et al., Phys. Rev. Lett. 108, 166801 (2012)], we obtain the complete features of the SOC in an InSb nanowire DQD.

A lattice of interacting Majorana modes can occur in a superconducting film on a topological insulator in a magnetic field. The phase diagram as a function of interaction strength for the square lattice was analyzed recently using a combination of mean field theory and renormalization group methods, and was found to include second order phase transitions. One of these corresponds to spontaneous breaking of an emergent U(1) symmetry, for attractive interactions. Despite the fact that the U(1) symmetry is not exact, this transition was claimed to be in a supersymmetric universality class when time reversal symmetry is present and in the conventional XY universality class otherwise. Another second order transition was predicted for repulsive interactions with time reversal symmetry to be in the same universality class as the transition occurring in the Gross-Neveu model, despite the fact that the U(1) symmetry is not exact in the Majorana model. We analyze these phase transitions using a modified $\epsilon$ expansion, confirming the previous conclusions. We also use soft supersymmetry breaking to prove an exact relation for the linear order fermion mass beta function.

We explore deformations of finite chains of noninteracting fermions at half-filling which give rise to large correlations between their extremes. After a detailed study of the Su-Schrieffer-Heeger model, the tradeoff curve between end-to-end correlations and the energy gap of the chains is obtained using machine-learning techniques, paying special attention to the scaling behavior with the chain length. We find that edge-dimerized chains, where the second and penultimate hoppings are reinforced, are very often close to the optimal configurations. Our results allow us to conjecture that, given a fixed gap, the maximal attainable correlation falls exponentially with the system size. Study of the entanglement entropy and contour of the optimal configurations suggest that the bulk entanglement pattern is minimally modified from the clean case.

We present an extensive numerical study of a type of frustrated itinerant magnetism on the pyrochlore lattice. In this theory, the pyrochlore magnet can be viewed as a cross-linking network of Kondo or double-exchange chains. Contrary to models based on Mott insulators, this itinerant magnetism approach provides a natural explanation for several spin and orbital superstructures observed on the pyrochlore lattice. Through extensive Monte Carlo simulations, we obtain the phase diagrams at two representative electron filling fractions $n=\frac{1}{2}$ and $\frac{2}{3}$. In particular, we show that an intriguing glassy magnetic state characterized by ordering wave vectors $\mathbf{q}=\left(\frac{1}{3},\frac{1}{3},1\right)$ gives a rather satisfactory description of the low-temperature phase recently observed in spinel ${\mathrm{GeFe}}_2{\mathrm{O}}_4$.

The magnetic field of a cuboidal cluster of eight magnetic spheres is measured. It decays with the inverse seventh power of the distance. This corresponds formally to a multipole named a dotriacontapole. This strong decay is explained on the basis of dipole-dipole interaction and the symmetry of the ensuing ground state of the cuboidal cluster. A method to build such dotriacontapoles is provided.

Measurements of temperature-dependent resistance and magnetization under hydrostatic pressures up to 2.13 GPa are reported for single-crystalline, superconducting ${\mathrm{BaBi}}_3$. A temperature-pressure phase diagram is determined and the results suggest three different superconducting phases $\alpha$, $\beta$, and $\gamma$ in the studied pressure range. We further show that the occurrence of the three superconducting phases is intuitively linked to phase transitions at higher temperature, which are likely first order and structural in nature. With the $\alpha$ phase being the ambient-pressure tetragonal structure ($P4/mmm$), our first-principles calculations suggest the $\beta$ phase has a cubic structure ($Pm\text{−}3m$) and the $\gamma$ phase has a distorted tetragonal structure where the Bi atoms are moved out of the face-centered position. Finally, an analysis of the evolution of the superconducting upper critical field with pressure further confirms these transitions in the superconducting state and suggests a possible change of band structure or a Lifshitz transition near 1.54 GPa in $\gamma$ phase. Given the large atomic numbers of both Ba and Bi, our results establish ${\mathrm{BaBi}}_3$ as a good candidate for the study of the interplay of structure with superconductivity in the presence of strong spin-orbit coupling.

Motivated by recent attention to a potential antiferromagnetic quantum critical point at $x_c\sim 0.19$, we have used inelastic neutron scattering to investigate the low-energy spin excitations in crystals of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ bracketing $x_c$. We observe a peak in the normal-state spin-fluctuation weight at $\sim 20\mathrm{meV}$ for both $x=0.21$ and 0.17, inconsistent with quantum critical behavior. The presence of the peak raises the question of whether low-energy spin fluctuations limit the onset of superconducting order. Empirically evaluating the spin gap ${\mathrm{\Delta }}_{\mathrm{spin}}$ in the superconducting state, we find that ${\mathrm{\Delta }}_{\mathrm{spin}}$ is equal to the coherent superconducting gap ${\mathrm{\Delta }}_c$ determined by electronic spectroscopies. To test whether this is a general result for other cuprate families, we have checked through the literature and find that ${\mathrm{\Delta }}_c\le {\mathrm{\Delta }}_{\mathrm{spin}}$ for cuprates with uniform $d$-wave superconductivity. We discuss the implications of this result.

Using first-principle Hybrid Monte Carlo (HMC) simulations, we carry out an unbiased study of the competition between spin-density wave (SDW) and charge-density wave (CDW) order in the extended Hubbard model on the two-dimensional hexagonal lattice at half filling. We determine the phase diagram in the space of on-site and nearest-neighbor couplings $U$ and $V$ in the region $V<U/3$, which can be simulated without a fermion sign problem, and find that a transition from semimetal to a SDW phase occurs at sufficiently large $U$ for basically all $V$. Tracing the corresponding phase boundary from $V=0$ to the $V=U/3$ line, we find evidence for critical scaling in the Gross-Neveu universality class for the entire boundary. With rather high confidence we rule out the existence of the CDW ordered phase anywhere in the range of parameters considered. We also discuss several improvements of the HMC algorithm which are crucial to reach these conclusions, in particular the improved fermion action with exact sublattice symmetry and the complexification of the Hubbard-Stratonovich field to ensure the ergodicity of the algorithm.

Here, we report a combined study of transport and thermodynamic measurements on the layered pnictide material ${\mathrm{SrAg}}_4{\mathrm{As}}_2$. Upon cooling, a drop in electrical and Hall resistivity, a jump in heat capacity, and an increase in susceptibility and magnetoresistance are observed around 110 K. All suggest nonmagnetic phase transitions emerge at around 110 K, likely associated with structural distortions. In sharp contrast with the first-principles calculations based on the crystal structure at room temperature, quantum oscillations reveal small Fermi pockets with light effective masses, suggesting significant change in the Fermi surface topology caused by the low-temperature structural distortion. No superconductivity emerges in ${\mathrm{SrAg}}_4{\mathrm{As}}_2$ down to 2 K under pressures up to 2.13 GPa; instead, the low-temperature structural distortion moves up linearly to high temperature at a rate of $\approx 13$ K/GPa above 0.89 GPa.

We complement previous functional renormalization group (fRG) studies of the two-dimensional Hubbard model by mean-field calculations. The focus falls on Van Hove filling and the hopping amplitude $t^{\prime }/t=0.341$. The fRG data suggest a quantum critical point (QCP) in this region and in its vicinity a singular fermionic self-energy $\mathrm{Im}\mathrm{\Sigma }\left(\omega \right)/\omega \sim -{\left|\omega \right|}^{-\gamma }$ with $\gamma \approx 0.26$ [K-U. Giering and M. Salmhofer, Self-energy flows in the two-dimensional repulsive Hubbard model, Phys. Rev. B 86, 245122 (2012)]. Here we start a more detailed investigation of this QCP using a bosonic formulation for the effective action, where the bosons couple to the order parameter fields. To this end, we use the channel decomposition of the fermionic effective action developed in C. Husemann and M. Salmhofer, Efficient Parametrization of the Vertex Function, Omega-Scheme, and the (t, t')-Hubbard Model at Van Hove Filling, Phys. Rev. B 79, 195125 (2009), which allows us to perform Hubbard-Stratonovich transformations for all relevant order parameter fields at any given energy scale $\mathrm{\Omega }$. We stop the flow at a scale $\mathrm{\Omega }$ where the correlations of the order parameter field are already pronounced, but the flow is still regular, and derive the effective boson theory. It contains $d$-wave superconducting, magnetic, and density-density interactions. We analyze the resulting phase diagram in the mean-field approximation. We show that the singular fermionic self-energy suppresses gap formation both in the superconducting and magnetic channel already at the mean-field level, thus rounding a first-order transition (without self-energy) to a quantum phase transition (with self-energy). We give a simple effective model that shows the generality of this effect. In the two-dimensional Hubbard model, the effective density-density interaction is peaked at a nonzero frequency, so that solving the mean-field equations already involves a functional equation instead of simply a matrix equation (on a technical level, similar to incommensurate phases). Within a certain approximation, we show that such an interaction leads to a short quasiparticle lifetime.

The optomechanical interaction of a resonant particle with a current sheet, graphene for example, induced by its surface plasmon is investigated. It is predicted that stable equilibrium positions for the particle at finite distances from the sheet can be formed. For high-index dielectric particles the equilibria take place in a finite frequency range close to the Mie resonances. Thus, the excitation of resonances by the plasmon and their coupling to the sheet give rise to the strong repulsion, which competes against the usually prevailing attraction induced by the evanescent tail. The formation of purely optical stable equilibria for particles outside of waveguiding structures can find applications for particle propulsion, sorting, and in various systems that require particle isolation.

Resolving the structure of energy bands in transport experiments is a major challenge in condensed matter physics and material science. Sometimes, however, traditional electrical conductance or resistance measurements only provide very small signals, and thus limit the ability to obtain direct band-structure information. In this case, it has been proven beneficial to employ thermoelectric measurements which are sensitive to the first derivative of the density of states with respect to energy, rather than to its value itself. Due to the large interest in topological effects these days, it is important to identify a similar concept for detecting the Berry curvature in a band structure. Nowadays, the common way to access the Berry curvature directly via measurements is the anomalous Hall effect, but the corresponding signal can be too small to be detected when the topological features of the band structure lie too far off the Fermi level. In this Rapid Communication, we propose to investigate topological band-structure features utilizing the anomalous Nernst effect which is tied to the derivative of the anomalous Hall effect with respect to energy. Thereby, also signatures become resolvable, which are elusive in anomalous Hall measurements. We demonstrate the underlying mechanisms for a minimal effective four-band model and exemplary for the real Heusler compounds ${\mathrm{Co}}_2\mathrm{Fe}X$ ($X=\text{Ge},\text{Sn}$), which host topological nodal lines about 100 meV above the Fermi level. This work demonstrates that anomalous Nernst measurements can be an effective tool for the characterization of topological band structures, both at room temperature and in the quantum transport regime at cryogenic temperatures.

We consider the stability of nodal surfaces in fermionic band systems with respect to the Coulomb repulsion. It is shown that nodal surfaces at the Fermi level are gapped out at low temperatures due to emergent particle-hole orders. Energy dispersion of the nodal surface suppresses the instability through an inhomogeneous phase. We argue that around criticality the order parameter fluctuations can induce superconductivity. We show that by tuning doping and disorder one could access various phases, establishing fermionic nodal surface systems as a versatile platform to study emergent quantum orders.

Axion insulators are magnetic topological insulators in which the nontrivial ${\mathbb{Z}}_2$ index is protected by inversion symmetry instead of time-reversal symmetry. The naturally gapped surfaces of axion insulators give rise to a half-quantized surface anomalous Hall conductivity (AHC), but the sign of the surface AHC cannot be determined from topological arguments. In this paper, we consider topological phenomena at the surface of an axion insulator. To be explicit, we construct a minimal tight-binding model on the pyrochlore lattice and investigate the all-in-all-out (AIAO) and ferromagnetic (FM) spin configurations. We also implement a recently proposed approach for calculating the surface AHC directly, which allows us to explore how the interplay between surface termination and magnetic ordering determines the sign of the half-quantized surface AHC. In the case of AIAO ordering, we show that it is possible to construct a topological state with no protected metallic states on boundaries of any dimension (surfaces, hinges, or corners), although chiral hinge modes do occur for many surface configurations. In the FM case, rotation of the magnetization by an external field offers promising means of control of chiral hinge modes, which can also appear on surface steps or where bulk domain walls emerge at the surface.

We investigate parity-mixing superconductivity in the two-dimensional Hubbard model with Rashba spin-orbit coupling, using cellular dynamical mean-field theory (CDMFT). A superconducting state with mixed singlet $d$-wave and triplet $p$-wave character is found in a wide range of doping. The singlet component decreases with the amplitude of the Rashba spin-orbit coupling, whereas the triplet component increases, but both disappear at about 20% doping. The effect of a Zeeman field is also investigated; it tends to suppress both types of superconductivity but induces nontrivial topological properties: The computed bulk Chern number is nonzero in the mixed superconductivity phase, at least in the underdoped region. A strong suppression of the excitation gap occurs slightly after optimal doping; this might be the sign of a topological transition within the superconducting dome.

When irradiated via high-frequency circularly polarized light, the stroboscopic dynamics in a Heisenberg spin system on a honeycomb lattice develops a next-nearest-neighbor (NNN) Dzyaloshinskii-Moriya (DM) type term, making it a magnonic Floquet topological insulator. We investigate the entanglement generation and its evolution on such systems, particularly an irradiated ferromagnetic XXZ spin-$\frac{1}{2}$ model in a honeycomb lattice as the system parameters are optically tuned. In the high-frequency limit, we compute the lowest quasienergy state entanglement in terms of the concurrence between nearest-neighbor (NN) and NNN pair of spins and witness the entanglement transitions occurring there. For the easy-axis scenario, the unirradiated system forms a product state but entanglement grows between the NNN spin pairs beyond some cutoff DM strength. Contrarily in the easy-planar case, NN and NNN spins remain already entangled in the unirradiated limit. It then goes through an entanglement transition which causes decrease (increase) of the NN (NNN) concurrences down to zero (up to some higher value) at some critical finite DM interaction strength. For a high frequency of irradiation and a suitably chosen anisotropy parameter, we can vary the field strength to witness sudden death and revival of entanglement in the Floquet system. Both exact diagonalization and modified Lanczos techniques are used to obtain the results up to 24 site lattice. We also calculate the thermal entanglement and obtain estimates for the threshold temperatures below which nonzero concurrence can be expected in the system.