Since 2008, iron-based superconductors have been the focus of intense research. Unfortunately, the higher transition temperatures and most promising high-magnetic field properties have been associated with compounds that have to be chemically disordered, making it hard to fully understand their intrinsic properties. A fully ordered compound with near optimal properties would be ideal and CaKFe${}_4$As${}_4$ appears to be that compound. In this work, the authors present detailed anisotropic, structural, thermodynamic, and transport measurements that, in addition to revealing high upper critical fields, also provide the basis for further understanding of this fully ordered, high-$T_c$, iron-based superconductor. It is shown that CaKFe${}_4$As${}_4$ has exceptionally large upper critical fields (in excess of 650 kOe and possibly as high as 900 kOe) as well as high $T_c$ values (35 K), which can be obtained and studied without any disorder-related effects obscuring or complicating the results.

Here, the authors investigate the possibility that particle-hole symmetry in the lowest Landau level at half filling arises as an emergent low-energy symmetry. Specifically, they start with the traditional approach to the half-filled Landau level: an effective field theory known as the composite Fermi liquid pioneered by Halperin, Lee, and Read. In this theory, composite fermions arise when flux is attached to electrons in a half-filled Landau level. However, an equivalent starting point is one where one starts with a filled Landau level, depletes electrons (or equivalently, adds holes) to the point of half filling. One then attaches flux to the holes, and the resulting composite hole description is another viable description of the half-filled Landau level. The authors construct a particle-hole symmetric theory by studying a system consisting of alternating quasi-one-dimensional strips of composite Fermi liquid (CFL) and composite hole liquid (CHL), both of which break particle-hole symmetry. Gauge invariance requires the existence of gapless edge modes at the interface between the CFL and CHL. Out of these modes, they construct a neutral Dirac fermion coupled to an emergent U(1) gauge field. Thus, at distances long compared to the strip width, they demonstrate that the system is described by a Dirac fermion coupled to an emergent gauge field, with an anti-unitary particle-hole symmetry, precisely the form conjectured recently by Son to be the low-energy description of a particle-hole symmetric half-filled Landau level.

The authors perform near-field nanoimaging of various waveguide modes inside thin flakes of WSe${}_2$ by using state-of-the-art near-field scanning optical microscopy. This compound is a prototypical transition metal dichalcogenide with strongly bound excitons. The nanoimaging data provides direct evidence of strong coupling between waveguide photon modes and excitons. Such coupling generates hybrid polaritonic modes that are imaged for the first time as propagative modes in real space. This work uncovers in real space the physics of nanoscale light-matter interactions in van der Waals semiconductors and thus provides guidelines for future applications of this class of materials in nanophotonics and optoelectronics.

The authors here explore the properties of Weyl semimetals and axion insulators coupled to geometry. The novel interplay between axion strings and lattice dislocation defects are studied through both effective field theory and microscopic arguments. Various new phenomena are proposed for a Weyl semimetal with dislocations. This work should be of great interest both in the context of topological semimetal/insulators and for the problem of topological defects in strongly correlated systems.

Strain engineering in graphene is of high current interest because of the possibility to induce new physical phenomena by means of mechanical strain. The authors of this work study the effect of strain on the electron-phonon interaction that plays an important role in the vibrational, thermal, and transport properties of graphene. It has been commonly assumed that Kohn anomalies and electron-phonon coupling in strained graphene occur only for the longitudinal and transverse optical phonons, as in pristine graphene. In contrast, this study shows that uniaxial strain in graphene induces a Kohn anomaly and enhancement of the electron-phonon coupling in the longitudinal acoustic phonon branch. This feature is a major difference between pristine and uniaxially strained graphene, due to the presence of a new intervalley phonon-scattering channel for electronic states close to the Dirac point.

The phenomenon of many-body localization generalizes Anderson localization to interacting systems. Understanding this conceptually novel phenomenon requires a study of many-body eigenstates at finite energy densities. This is a very challenging task since the most efficient numerical methods such as, e.g., the density matrix renormalization group method, can only access the ground state and low lying excitations. In this work, the authors introduce a unitary tensor network based variational method that approximately finds all many-body eigenstates of fully localized Hamiltonians and scales polynomially with system size. The usefulness of their approach is demonstrated by considering the Heisenberg chain in a strongly disordered magnetic field.

Two-dimensional potentials are excellent test beds for the physics of interacting excitonic gases, but engineering highly homogeneous potentials can be challenging. In this work, the authors investigate excitons bound to a single 10-μm scale stacking fault, finding excitonic luminescence with unprecedented homogeneity. The authors further show that stacking-fault-bound excitons are mobile and have a giant permanent dipole moment by means of the magneto-Stark effect. These results indicate that stacking faults may be a promising platform to probe the many-body physics of interacting dipoles in an atomically smooth potential.

The creation of electron-hole pairs by photoexcitation in quasi-two-dimensional semiconductor heterostructures has long provided one motivation for the study of a positive impurity immersed in an electron gas. Improved understanding of such systems is vital to realize the full potential of solar cells and optoelectronic transistors and memory devices. Collective, many-body phenomena dominate the behavior of the hole-in-electron-gas system at high carrier density, while excitonic species such as the exciton (a bound state of an electron and the hole) and the trion (a bound state formed from two electrons and the hole) emerge at low density. Confinement enhances the binding energy in quantum wells compared to bulk samples but, despite much experimental and theoretical work, the properties of excitons and trions are still uncertain. The authors perform diffusion Monte Carlo calculations of a single positive impurity immersed in a two-dimensional, paramagnetic electron gas at zero temperature using the Coulomb ($1/r$) potential. A gradual crossover from the collective, many-body behavior to the trion state is studied over a range of carrier densities and electron-hole mass ratios consistent with recent experiments in GaAs and CdTe quantum wells. Relaxation energies, electron-hole pair correlation functions, and electron-hole center-of-mass momentum densities are reported for these systems and may be used, for example, in density functional theory studies of optoelectronic devices.

In a wide range of experiments where electrical currents are used to inject angular momentum, or spin currents, from a metallic ferromagnet into a nonmagnetic metal, the interface plays a critical role. Whether in the giant magnetoresistance effect or in nonlocal spin valves, a loss of spin polarization of an electrical current crossing the interface is seen to reduce the spin current and lead to smaller overall response in sensors. The authors here compare electrical spin injection to the more recently discovered thermal spin injection in a nonlocal spin valve. They show that, despite a strong reduction of electrical spin injection that they tie to the loss of interfacial spin polarization, thermal spin injection remains a large effect. This highlights that thermal spin injection (also called the spin-dependent Seebeck effect) can proceed by new physical mechanisms not possible in the electrically driven case, potentially involving incoherent spin pumping and collective behavior of magnetization in the oxidized interface layer. Better understanding of these interface effects could lead to new ways to increase sensitivity of next-generation magnetic sensors and efficiency of sources of spin current in metallic systems.

Atomic force microscopy is one of the techniques that allows us to visualize objects that we cannot see directly. The question of what we actually see when we look at an atomic force microscopy image is, however, a subtle one. This paper discusses a scheme that permits the calculation of atomic force microscopy images with relatively little effort. Using electronic density functional theory, the force on the cantilever is calculated as the gradient of the effective potential that governs the electronic structure in density functional theory. In this way, nonclassical contributions to the force can efficiently be taken into account and are translated into an image. Comparison to experimental images shows that the nonclassical contribution to the force can make a visible difference.

The Kitaev model is a toy model for $S$=$\frac{1}{2}$ moments on a honeycomb lattice, which interact via strongly bond-dependent Ising-like interactions. In real material candidates like Na${}_2$IrO${}_3$, small Heisenberg interactions are also present in addition to dominant Kitaev interactions. The doped Kitaev-Heisenberg model has been predicted to show unconventional superconductivity and spin or charge density wave, and bond-order instabilities. In this paper, the authors have succeeded in surface-doping Na${}_2$IrO${}_3$ crystals by argon plasma etching and in changing the conductivity by several orders of magnitude. The doped samples show several unusual transport behaviors, including first order spin or charge density wave-like transitions.

URu${}_2$Si${}_2$ undergoes a second-order phase transition at 17.5 K that has defied attempts to identify its order parameter despite a vast literature extending over the last 30 years. A wide variety of theories have been posed to explain this so-called “hidden order”, some with exotic ground states. An important dividing line between these theories is the character of the 5$f$ orbital: on one side sit theories that require a localized $f^2$ configuration ($f$-manifold crystalline electric field effects, hastatic order, etc.), while on the other sit those that start from an itinerant, partially occupied $f^3$ orbital (band structure + hybridization). Unfortunately, the experimental measures remain muddy on this issue, with indications of both localized $f^2$ and itinerant $f^3$ behavior depending on the experiment. Here, the authors report U $L_{III}$-edge absorption/$L_{\alpha 1}$ emission resonant x-ray emission spectroscopy measurements comparing data from URu${}_2$Si${}_2$ to UCd${}_{11}$, UF${}_4$, and UO${}_2$ data to unravel the various roles of $f$-orbital occupation, delocalization, and ligand-field splitting of the $d$ manifold. The data indicate a dominant delocalized $f^3$ configuration that is likely partially occupied, with no measurable change in occupancy to temperatures as low as 10 K. While these measurements do not rule out a minority localized $f^2$ configuration, any theory relying on such a configuration must account for its relatively small contribution to the Fermi surface.

The ability to explain and predict the behavior in magnetic field is essential for any possible application of multiferroic materials. Motivated by the complex phase diagrams of MnWO${}_4$ and other spiral multiferroics, the authors formulate and investigate the anisotropic next-nearest-neighbor Heisenberg (ANNNH) model, which is a generalization of the celebrated ANNNI model to three component quantum spins. Using real-space mean-field simulations allowed for an unbiased construction of the magnetic phase diagram with multiple commensurate and incommensurate states. In particular, the biaxial anisotropy can explain the puzzling nonferroic reentrant phase of MnWO${}_4$ at high magnetic fields. The employed numerical technique is capable of predicting the temperature and field dependence of the ordering wave vector produced by the competition between helicity and anisotropy. An important issue raised in the present study is the effect of quantum fluctuations on the devil’s staircase known to exist for the ANNNI model, but which is replaced by a “floating” incommensurate phase for the ANNNH model.

An infinite projected entangled-pair state (iPEPS) is a powerful variational tensor network ansatz for two-dimensional ground states in the thermodynamic limit, and can be seen as a natural generalization of a matrix-product state to two dimensions. One of the main challenges in iPEPS simulations is the optimization of the tensors, i.e., finding the optimal variational parameters, in order to have the best representation of the ground state of a given Hamiltonian. The author presents a variational optimization scheme, in which the energy is minimized in an iterative way by sweeping over all the tensors in the ansatz, in a similar spirit as done in the density-matrix renormalization group method. Benchmark results for challenging problems are presented that show that the variational scheme yields considerably more accurate results than the previously best imaginary-time evolution algorithm, with a similar computational cost and with a faster convergence towards the ground state.

Many earlier high-resolution x-ray studies of criticality seemingly invalidated the single-length scaling hypothesis that is predicted by the three-dimensional Ising model, a cornerstone in the theory of critical phenomena. Speculations about the reasons for these discrepancies have concentrated on the role of impurities and on surface-induced strain in the crystalline samples. The general validity of the Ising model for real systems has also been questioned. Here, the authors present x-ray scattering data on the temperature-induced order-disorder transition in $\beta$-brass, nature’s archetypical realization of the Ising model. These investigations unambiguously show that the Ising model does precisely describe the critical behavior observed and, in particular, that the short-range order parameter indeed exhibits single-length scaling. The surface sensitivity has been varied in the experiments without any substantial differences observed.

This paper focuses on developing a new algorithm (En-DMRG) for obtaining highly excited states of disordered and strongly interacting spin systems. This line of research is stimulated by the growing interest in understanding the many-body localized (MBL) state, which emerges in disordered interacting systems at high energy densities through a disorder-driven dynamic phase transition. By studying the one-dimensional Heisenberg spin chain (with up to 72 spins) in a random field, the authors demonstrate the accuracy of the method in obtaining energy eigenstates and the corresponding statistical results of the quantum system in the MBL phase. The method presented in this work has the potential to be applicable in a variety of quantum statistical physics problems, possibly including the MBL phase in higher dimensions.

Ultrathin films of the heavy elements Pd and Pt are widely used in spintronics as converters between charge and spin currents and vice versa. One open question has been whether the strongly paramagnetic nature of these elements influences the interconversion process. The authors show, using a combination of ferromagnetic resonance and x-ray magnetic circular dichroism measurements, that a related spin current absorption process (spin pumping damping) responds to the presence of ferromagnetic order in the polarizable Pd and Pt layers. They observe a change in the thickness dependence of spin current absorption when the Pd and Pt layers are in direct contact with a ferromagnet, compared with when they are in indirect contact. This change correlates with the presence of induced magnetic moments in Pd and Pt as revealed by element-specific x-ray magnetic circular dichroism magnetometry. The results show that the decoherence process for spin current in such layers is not independent of geometry, but rather is influenced by the presence of induced ferromagnetic order.

Arrays of subwavelength slit cavities have been of interest to the acoustic metamaterial community for some years now, giving rise to such phenomena as “enhanced acoustic transmission”, where sound of certain wavelengths can transmit through the slit array structure as if it were not there. This behaviour is widely accepted as a coupling effect between diffraction evanescent waves arising from structure factor, and Fabry-Perot like resonances supported along the length of each slit cavity. Here, the authors explore the effect of altering the spacing, or width, of some of the slits that form the array, thereby forming a “compound” grating, where each periodic unit cell is comprised of multiple slit cavities. The transmission of such structures is experimentally recorded, where it is found that as one increases the number of slit cavities per unit cell, broad and deep minima can appear in the peaks of the primary enhanced acoustic transmission resonance that may have a strong angular dependence. These features owe their existence to the “phase resonance”, where new degrees of freedom available to the near field allow adjacent slit cavities to be out-of-phase with one another and thus destructively interfere with the re-radiated pressure wave. Such behaviour has been studied extensively in the electromagnetic case, where a surface wave band folding picture could be used to predict the dispersive nature of these phase resonances. A similar explanation of the underlying surface wave physics is extended here to the acoustic case.

The authors have studied SmB${}_6$ and Ce${}_3$Bi${}_4$Pt${}_3$. Both of these Kondo insulators show a saturation of the increasing resistivity at low temperature that suggests an additional conduction channel and is consistent with the theoretical prediction that these systems should host robust surface states due to their nontrivial topology. While previous work has shown that the resistance saturation in SmB${}_6$ is due to conducting surface states, this work demonstrates that the origin of the resistance saturation in Ce${}_3$Bi${}_4$Pt${}_3$ is through the bulk of the material and not the surface. The contrasting behavior of the two materials when the surface is disordered with ion-irradiation supports this conclusion, and also demonstrates the sensitivity of the resistance of the surface state in SmB${}_6$ to small levels of disorder. Finally, the low-temperature specific heat of SmB${}_6$ shows a residual linear term, and the origin of this term has been highly debated recently. Through measurements of single crystals and powders, it is shown that this term dominantly arises from the bulk and not the surface state.

The properties of an electron beam can be manipulated by electromagnetic fields in vacuum via the ponderomotive force. Such an interaction is also at the core of the Kapitza-Dirac effect, which describes the diffraction of electrons by an optical standing wave. Here, the authors predict a new type of interaction between electrons and the electromagnetic field, opening up new possibilities for the manipulation of electron beams. If surface plasmon polaritons are tailored to interfere forming a periodic field pattern, it becomes possible to diffract electrons from such near field. With the proper manipulation of the plasmonic fields, orbital angular momentum can be imparted to the electrons, and even the phase of their wave functions can be manipulated. An additional degree of freedom is provided by the possibility to tailor the spatial properties of the light and the materials supporting the surface plasmons. This arbitrary control can be extended to different substrates such as graphene or layered systems and may open up a viable route to create tunable phase plates for electron microscopes.