Quantum spin liquids can be well described in the language of gauge theory. While most theoretical effort has been focused on gauge theories with a familiar vector gauge field, there exists a stable class of quantum spin liquids described by higher-rank tensor gauge fields. Here, the authors focus on a class of stable three-dimensional spin liquids described by symmetric tensor $U$(1) gauge fields. They find that these spin liquids feature an exotic class of excitations that are restricted to motion along lower-dimensional subspaces, a phenomenon seen earlier in fracton models. They show how this subdimensional behavior follows naturally from a set of higher-moment charge conservation laws that place severe restrictions on particle motion. This work opens up an exciting new direction in the field of spin liquids.

Berry connection is conventionally defined as a static gauge field in the Brillouin zone. Here, the authors show that for three-dimensional (3D) time-reversal invariant superconductors, a generalized Berry gauge field behaves as a fluctuating field of a momentum-space Chern-Simons gauge theory, which is the effective theory of the system. Furthermore, the gapless nodal lines in the momentum space which may arise in such superconductors play exactly the role of Wilson loop observables, and their linking and knot invariants modify the gravitational theta angle which can be evaluated from the Chern-Simons action. This angle induces a topological gravitomagnetoelectric effect where a temperature gradient induces a rotational energy flow. The authors also show how this idea may be potentially generalized to realize topological strings in six-dimensional phase space, where the physical space defects play the role of topological D-branes.

The dynamics of the phase separation of very dilute ${}^3$He-${}^4$He solid solutions is studied using high-sensitivity low-temperature NMR techniques. This paper compares the results observed at very low concentrations (down to 16 ppm ${}^3$He) with previous results up to 2%. The growth of ${}^3$He nanodroplets after phase separation is analyzed and shown to follow a one-third power-law dependence at long times as expected for Ostwald ripening. The time scale for the dynamics is governed by the relatively fast quantum diffusion in solid helium, allowing the long-term behavior of the ripening process to be explored by experiments lasting a few days rather than several months, as in the case of metallic alloys.

Here, the authors make connection to the foundation of quantum mechanics. As stated by Schrödinger, entanglement is “the characteristic trait of quantum mechanics” and seemingly at odds with the description of nature. In this joint experimental and theoretical work, the authors discuss the possibility of spin entanglement of two freely propagating electrons. These have been created by a collision of a spin-polarized primary electron with the Shockley surface state of the Cu(111) surface. The contribution of this state can clearly be identified in the experiment and allows use of its particular features. The electron scattering study done here resembles the scenario envisioned by the seminal works of Einstein-Podolsky-Rosen and Schrödinger to work out the peculiar nature of entanglement.

Evidence for hydrodynamic flow of electrons was recently reported in a number of transport experiments in high-mobility semiconductor and graphene nanodevices. The electron liquid in these systems moves in the presence of a long-range disorder potential, which makes the entropy per electron position dependent. The resulting nonisentropic flow is markedly different from Stokesian flow: the resistivity is governed not only by the viscosity of the electron fluid but also by its thermal conductivity. Here, the authors show that the weak-field magnetoresistance (MR) in this hydrodynamic regime is caused by the modification of the flow pattern by the Lorentz force. The resulting MR is positive and is controlled by the shear viscosity alone. This may help separate the viscous and the thermal contributions to the resistivity.

The problem of the half-filled Landau level has recently received renewed interest. The ground state of the half-filled lowest Landau level is believed to be a Fermi liquid of an emergent “composite fermion” quasiparticle. In the past, Halperin-Lee-Read (HLR) theory was employed to describe the low-energy physics of this Fermi-liquid state. Recently, an alternative theory based on the idea of the Dirac composite fermion has been proposed, in which particle-hole symmetry is manifestly realized. In this paper, a previously unknown exact relationship between response functions at finite frequency and wave number is derived. It is shown that this relationship can be accommodated by the Dirac composite fermion theory, but is violated by the HLR theory.

Antiferromagnetic spin systems have been extensively studied but still yield surprising new physics, especially on geometrically frustrated lattices and in the quantum regime. Less well studied are ferrimagnets, whose net magnetization is positive but less than that of the fully polarized state. Here, the authors study a frustrated ferrimagnetic spin Hamiltonian on a kagome lattice with both ferromagnetic and antiferromagnetic couplings, inspired by density functional theory analysis of the material Cu${}_3$V${}_2$O${}_7$(OH${}_2$)$\cdot$2H${}_2$O (volborthite). This model displays a very broad plateau in the magnetization curve at 1/3-full magnetization. We incorporate quantum effects via a semiclassical 1/$S$ expansion and find that they further stabilize the plateau. When the frustrating coupling becomes strong enough to almost destabilize the plateau, we find an instability to bound-magnon “exciton” condensation, which results in an exotic “chiral liquid” phase with relativistic excitations that breaks inversion (but no other) symmetry.

Modeling the dislocation glide through atomic-scale simulations in pure solids and alloys, the authors show that in the course of the plastic deformation the variation of the crystal’s zero point energy (ZPE) and the dislocation potential energy barriers are of opposite sign. The multiplicity of situations in which this same trend has been observed allows us to conclude that quantum fluctuations, giving rise to crystal ZPE, make dislocation glide easier in most materials.

An exciting recent development is the ability to drive a quantum material into a highly excited, but still phase coherent state, using intense laser sources. Although such a state can be extremely short-lived, it can be long enough for detection using ultrafast spectroscopy. The authors here show how intense laser pulses can generate a new type of nonequilibrium superconducting phase. The authors demonstrate theoretically that the pump induces a “twisted” BCS state that subsequently evolves coherently after the cessation of the pulse. They show how the nonlinear coupling of the pump pulse light can induce coherent dynamics of $\mathrm{\Delta }(t)$, consistent with the experiment [R. Matsunaga et al. Phys. Rev. Lett. 111, 057002 (2013)]. Moreover, the authors show that more intense pump pulses can create a far-from-equilibrium phase of gapless superconductivity, originally predicted in the context of ultracold atomic gases. The terahertz pump is found to be much more efficient than the interaction quench at producing this gapless phase. These results open the door to engineering and observing new dynamical phases and phase transitions in quantum materials.

Here, the authors discover a missing link between antiferromagnetism and the Hall effect by introducing a theoretical framework based on a novel concept, cluster multipole (CMP), to characterize macroscopic magnetization of antiferromagnets. Whereas the anomalous Hall effect (AHE) is usually observed in ferromagnets and explained as an outcome of the macroscopic dipole magnetization, CMP theory reveals that a certain type of antiferromagnetic (AFM) structure induces the AHE despite no net magnetization. The new order parameters enable us to characterize the AHE in the AFM states and explain the AHE in the AFM states of Mn${}_3$Ir and Mn${}_{3}Z$ ($Z$ = Sn, Ge), for which the large AHE has recently been studied. Furthermore, the theory can deal with the AHE in antiferromagnets on an equal footing with that in simple ferromagnets. The authors compare the AHE in antiferromagnetic Mn${}_{3}Z$ Mn${}_{3}Z$ and ferromagnetic bcc Fe based on first-principles calculations and find out their similarity with respect to the CMP moments. The theory brings on a significant step forward in our current understanding of anomalous current in condensed matter, and the obtained knowledge could be crucial in the future for the design of antiferromagnetic devices, e.g., with possible spintronics-related applications.

Traveling-wave parametric amplifiers may be fabricated from superconducting films that exhibit highly nonlinear kinetic inductance. The coplanar waveguide of such a microwave device, extending to a meter or more in length but compacted to reside on a chip of the order of a square centimeter, is engineered with periodic variations in its width. These width variations, or loadings, alter the dispersion characteristics of a nonlinear current propagating along the waveguide, changing its group velocity and modulation behavior. A strong pump and a small signal injected into one end of the waveguide mix parametrically in the presence of the nonlinear kinetic inductance. Engineered dispersion induces the favorable conditions of overall phase matching, leading to generation of idler products as well as signal amplification of wide bandwidth, high dynamic range, and low noise, making the device of particular use in quantum computing and photon detection. The authors present a theoretical framework in which signal gain may be calculated solely from loading design. This involves construction of a metamaterial band theory of the engineered dispersion, which is used as a basis to describe the mixing of nonlinear traveling waves.

Topological phenomena in two-dimensional materials are characterized by an electrical conductivity that is restricted to the edges of a sample. A heterostructure of the semiconductors indium arsenide and gallium antimonide, InAs/GaSb, can be a quantum spin Hall insulator (QSHI), which exhibits spin-resolved channels at the edges but has an insulating bulk. The QSHI is linked to a nontrivial inverted band structure that emerges only for certain thicknesses of the InAs and GaSb layers. Here, the authors have exposed InAs/GaSb samples to variable compressive and tensile strain through piezo-electric elements. It is observed that the system’s electrical properties are very susceptible to even small values of such mechanical deformation. Band structure calculations reveal the impact of strain on the nontrivial electronic band structure and highlight the potential of strain-engineering for the observation of the quantum spin Hall insulating phase in this material system.

Here, the authors show that in molecules there is an emergent spin molecular-orbital coupling (SMOC) that is not merely inherited from the atomic scale. This causes the spin of electrons to become entangled with currents running around the molecule. Molecules that are symmetric only under rotation by 2$\pi$/$N$, provide a striking example. For odd $N$ there is a ladder of orbital states and SMOC allows the system to climb up or down the ladder, much like spin-orbit coupling in atoms. However, for even $N$ there is a ring of states – SMOC can move the system around in either directions, without ever reaching a minimum or maximum. This is a consequence of the umklapp spin-orbit scattering present in even-$N$ molecules that is precluded in odd-$N$ molecules by time-reversal symmetry. The authors show that SMOC is large in organometallic complexes and explore how synthetic chemistry can be used to control SMOC. Such control has numerous potential applications.

Bosonic symmetry protected topological (SPT) phases are bosonic analogs of free-fermion topological insulators and superconductors, but require interactions to be realized. Previously, a wide range of bosonic SPT phases protected by on-site symmetries has been systematically investigated, which are found to be related to group cohomology theory. However, a systematic understanding of SPT phases protected by spatial symmetries is still lacking. Here, the authors present systematic constructions of tensor-network wave functions for bosonic SPT phases protected by a general symmetry group SG involving both on-site and spatial symmetries. They find, in spatial dimension $d=1,2,3$, that a wide range of bosonic SPT phases are classified by the group cohomology $H^{d+1}$SG,U(1), in which the time-reversal symmetry and mirror symmetries should be treated as anti-unitary operations. They also provide generic tensor-network wave functions for these SPT phases that are useful for numerical simulations. As a by-product, the authors demonstrate a generic connection between rather conventional symmetry-enriched topological phases and SPT phases via an anyon condensation mechanism.

Weyl semimetals (WSM) have quickly gained popularity since their discovery and they are now the subject of intense theoretical and experimental research. In general, the WSM is categorized as being either type I or type II. The type-I WSM is characterized by broken inversion symmetry, while the type II by broken Lorentz symmetry. Due to this characteristic difference in the Fermi surfaces of the two types, it is necessary to extend our understanding of WSMs to include the different physical properties that having a tilted spectrum offers. In this vein, disorder-induced phase transitions have been well established for type-I WSMs, while the converse is not the case for type-II WSMs. In this work, the authors demonstrate that the type-I WSM with a nonzero tilting term undergoes a quantum phase transition to the type-II WSM in the presence of random on-site electric and magnetic impurity disorder. They show that the Fermi velocity decreases with increasing disorder strength and, therefore, a type-I to type-II transition occurs before the insulating transition. The conclusion is that the type-I WSM with nonzero tilt will always undergo a disorder-induced transition to the type-II WSM phase.

In topological insulators (TIs), spin-orbit coupling leads to the emergence of metallic topological surface states crossing the fundamental band gap. They exhibit a Dirac-cone-like dispersion with a helical spin structure. Bi${}_2$Se${}_3$(111) is the most prominent prototypical TI featuring a simple band structure with a single Dirac cone close to the Fermi level around the center of the surface Brillouin zone. Due to the interesting spin structure, TIs have emerged as promising materials in the field of spintronics and optospintronics. Ultrafast light pulses might pave a way to control spin currents. In this context, a profound knowledge about the dispersion and the spin polarization of both the occupied and the unoccupied electronic states is required. The authors present a joint experimental and theoretical study on the unoccupied electronic states of the topological insulator Bi${}_2$Se${}_3$. They discuss spin- and angle-resolved inverse photoemission results in comparison with calculations for both the intrinsic band structure and, within the one-step model of (inverse) photoemission, the expected spectral intensities. This allows them to unravel the intrinsic spin texture of the unoccupied bands at the surface of Bi${}_2$Se${}_3$.

An invisibility cloak makes itself and the object inside undetectable from the outside. For a cloak designed using transformation optics, light is bent around the object by changing the sizes and shapes of the local dispersion surfaces of the metamaterials surrounding the object. Here, the authors have found another way to build a cloak by using a concept called pseudomagnetic field, which accelerates the photon in a direction perpendicular to its velocity. It corresponds to shifting local dispersion surfaces. The new cloak bends the light in an asymmetric fashion for light passing the left and right side of the object for a fixed polarization of pseudospin. A truncation of the inner materials of the cloak creates an additional asymmetric transmission behavior. Furthermore, the new method also enables one to design more optical devices, such as a retroreflector.

The interplay of electronic correlations and spin-orbit coupling results in a rich phenomenology in semimetals with linear or quadratic band touching points. Remarkably, the associated low-energy physics and ground-state properties can often be described by effective field theories with enlarged symmetries such as rotation or Lorentz invariance. Realistic band structures of actual materials, however, are typically anisotropic and thus only allow for cubic rotation invariance. Here, the authors study the influence of anisotropy on the ground state of three-dimensional semimetals described by the Luttinger Hamiltonian, which captures the quadratic band touching present in HgTe, the pyrochlore iridates $R_2$Ir${}_2$O${}_7$, or strongly correlated half-Heusler compounds such as YPtBi. They highlight that a sufficient amount of anisotropy can induce qualitatively new ground states in these systems, namely a scale-invariant non-Fermi liquid state, or different octupolar tensor orders that break time-reversal invariance. The latter configurations share features of magnets and nematic order, so one could call them “nemagnetic”. The authors clarify their very natural interpretation as all-in-all-out or spin ice configurations for electrons in the pyrochlore iridates.

There have long been considerable efforts to study the many-body quantum phase diagram and quantum phase transitions of interacting bosonic particles in solid-state systems. The particular aim is the realization and identification of Bose-Einstein condensation, a coherent ground state with all particles condensed into a macroscopic many-body ground-state wave function. The authors probe photogenerated excitonic ensembles in coexistence with a two-dimensional hole system confined in GaAs double quantum well structures and electrostatically trapped by local gate electrodes using photoluminescence and resonant inelastic light scattering experiments. In this work, the authors observe a collective excitation mode at the transferred in-plane momentum and an energy of only 0.44 meV. This mode is interpreted as a plasma excitation of the two-dimensional excess hole subsystem coupled to the excitonic system. In this respect, the two subsystems behave like coupled oscillators. The plasmon energy is determined by the many-body interaction between the photogenerated exciton and hole subsystems. The low-energy excitation spectrum particularly of the excitonic system can play a key role in identifying Bose-Einstein condensation of excitonic ensembles, where, for example, roton excitations are expected.

The author investigates in a nonperturbative way the effects of Rashba interaction and electromagnetic field on the edge states of a two-dimensional topological insulator. He shows that the electron dynamics is equivalent to a problem of massless Dirac fermions propagating with an inhomogeneous velocity, unveiling interesting consequences. The Rashba coupling always enhances the electron velocity and, despite the inelastic and time-reversal breaking processes induced by the electromagnetic field, no backscattering occurs without interaction. Furthermore, when the photoexcitation occurs far from the Rashba region, the latter effectively acts as a “superluminal gate”, boosting the photoexcited wavepacket outside the light-cone determined by the bare Fermi velocity $v_F$. In contrast, for an electric pulse overlapping the Rashba region, the emerging wave packets are squeezed in a manner that depends on the overlap area. The author also discusses the effects of electron-electron interaction, for both intraspin and interspin density-density coupling. The results suggest that Rashba interaction, often considered as an unwanted disorder effect, may be exploited to tailor the shape and the propagation time of photoexcited spin-polarized wave packets.