Understanding thermal transport in nanoscale systems presents important challenges to both theory and experiment. In particular, the concept of local temperature at the nanoscale appears difficult to justify. Here, we propose a theoretical approach where we replace the temperature gradient with controllable external blackbody radiations. The theory recovers known physical results, for example, the linear relation between the thermal current and the temperature difference of two blackbodies. Furthermore, our theory is not limited to the linear regime and goes beyond accounting for nonlinear effects and transient phenomena. Since the present theory is general and can be adapted to describe both electron and phonon dynamics, it provides a first step toward a unified formalism for investigating thermal and electronic transport.

Thermal activation tends to destroy the magnetic stability of small magnetic nanoparticles, with crucial implications for ultrahigh density recording among other applications. Here we demonstrate that low-blocking-temperature ferromagnetic (FM) Co nanoparticles ($T_B<70K$) become magnetically stable above 400 K when embedded in a high-Néel-temperature antiferromagnetic (AFM) NiO matrix. The origin of this remarkable $T_B$ enhancement is due to a magnetic proximity effect between a thin CoO shell (with low Néel temperature, $T_N$, and high anisotropy, $K_{\mathrm{AFM}}$) surrounding the Co nanoparticles and the NiO matrix (with high $T_N$ but low $K_{\mathrm{AFM}}$). This proximity effect yields an effective antiferromagnet with an apparent $T_N$ beyond that of bulk CoO, and an enhanced anisotropy compared to NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations via core-shell exchange-bias coupling, leading to the observed $T_B$ increase. Mean-field calculations provide a semiquantitative understanding of this magnetic-proximity stabilization mechanism.

The quantum oscillations of the magnetoresistance under ambient and high pressure have been studied for ${\mathrm{WTe}}_2$ single crystals, in which extremely large magnetoresistance was discovered recently. By analyzing the Shubnikov–de Haas oscillations, four Fermi surfaces are identified, and two of them are found to persist to high pressure. The sizes of these two pockets are comparable, but show increasing difference with pressure. At 0.3 K and in 14.5 T, the magnetoresistance decreases drastically from $1.25\times {10}^5\%$ under ambient pressure to $7.47\times {10}^3\%$ under 23.6 kbar, which is likely caused by the relative change of Fermi surfaces. These results support the scenario that the perfect balance between the electron and hole populations is the origin of the extremely large magnetoresistance in ${\mathrm{WTe}}_2$.

A first-principles approach to the construction of concentration-temperature magnetic phase diagrams of metallic alloys is presented. The method employs self-consistent total energy calculations based on the coherent potential approximation for partially ordered and noncollinear magnetic states and is able to account for competing interactions and multiple magnetic phases. Application to the ${\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\mathrm{Pt}$ “magnetic chameleon” system yields the sequence of magnetic phases at $T=0$ and the $c-T$ magnetic phase diagram in good agreement with experiment, and a new low-temperature phase is predicted at the Mn-rich end. The importance of non-Heisenberg interactions for the description of the magnetic phase diagram is demonstrated.

We demonstrate the existence of hidden topological angles (HTAs) in a large class of quantum field theories and quantum mechanical systems. HTAs are distinct from theta parameters in the Lagrangian. They arise as invariant angles associated with saddle points of the complexified path integral and their descent manifolds (Lefschetz thimbles). Physical effects of HTAs become most transparent upon analytic continuation in $n_f$ to a noninteger number of flavors, reducing in the integer $n_f$ limit to a ${\mathbb{Z}}_2$ valued phase difference between dominant saddles. In $\mathcal{N}=1$ super Yang-Mills theory we demonstrate the microscopic mechanism for the vanishing of the gluon condensate. The same effect leads to an anomalously small condensate in a QCD-like $SU(N)$ gauge theory with fermions in the two-index representation. The basic phenomenon is that, contrary to folklore, the gluon condensate can receive both positive and negative contributions in a semiclassical expansion. In quantum mechanics, a HTA leads to a difference in semiclassical expansion of integer and half-integer spin particles.

We present the first measurement for helium atoms of the tune-out wavelength at which the atomic polarizability vanishes. We utilize a novel, highly sensitive technique for precisely measuring the effect of variations in the trapping potential of confined metastable ($2{}^3{S}_1$) helium atoms illuminated by a perturbing laser light field. The measured tune-out wavelength of $413.0938(9_{\text{stat}})(20_{\text{syst}})\mathrm{nm}$ compares well with the value predicted by a theoretical calculation [413.02(9) nm] which is sensitive to finite nuclear mass, relativistic, and quantum electrodynamic effects. This provides motivation for more detailed theoretical investigations to test quantum electrodynamics.

We study collisional heating in a cold ${}^7\mathrm{Li}\text{−}{}^{87}\mathrm{Rb}$ mixture near a broad Feshbach resonance at 661 G. At the high field slope of the resonance, we find an enhanced three-body recombination rate that we interpret as a heteronuclear Efimov resonance. With improved Feshbach spectroscopy of two further resonances, a model for the molecular potentials has been developed that now consistently explains all known Feshbach resonances of the various Li-Rb isotope mixtures. The model is used to determine the scattering length of the observed Efimov state. Its value of $-1870a_0$ Bohr radii supports the currently discussed assumption of universality of the three-body parameter also in heteronuclear mixtures.

We show that the one-particle density matrix $\rho$ can be used to characterize the interaction-driven many-body localization transition in closed fermionic systems. The natural orbitals (the eigenstates of $\rho$) are localized in the many-body localized phase and spread out when one enters the delocalized phase, while the occupation spectrum (the set of eigenvalues of $\rho$) reveals the distinctive Fock-space structure of the many-body eigenstates, exhibiting a steplike discontinuity in the localized phase. The associated one-particle occupation entropy is small in the localized phase and large in the delocalized phase, with diverging fluctuations at the transition. We analyze the inverse participation ratio of the natural orbitals and find that it is independent of system size in the localized phase.

We explore the microwave radiation emitted from a biased double quantum dot due to the inelastic tunneling of single charges. Radiation is detected over a broad range of detuning configurations between the dot energy levels, with pronounced maxima occurring in resonance with a capacitively coupled transmission line resonator. The power emitted for forward and reverse resonant detuning is found to be in good agreement with a rate equation model, which considers the hybridization of the individual dot charge states.

We present the first experimental observation of a topological transition in a non-Hermitian system. In contrast to standard methods for examining topological properties, which involve probing edge (or surface) states, we monitor the topological transition by employing bulk dynamics only. The system is composed of a lattice of evanescently coupled optical waveguides, and non-Hermitian behavior is engineered by inducing bending loss by spatially “wiggling” every second waveguide.

Several recent experiments have established by measuring the Mandel $Q$ parameter that the number of Rydberg excitations in ultracold gases exhibits sub-Poissonian statistics. This effect is attributed to the Rydberg blockade that occurs due to the strong interatomic interactions between highly excited atoms. Because of this blockade effect, the system can end up in a state in which all particles are either excited or blocked: a jamming limit. We analyze appropriately constructed random-graph models that capture the blockade effect, and derive formulae for the mean and variance of the number of Rydberg excitations in jamming limits. This yields an explicit relationship between the Mandel $Q$ parameter and the blockade effect, and comparison to measurement data shows strong agreement between theory and experiment.

${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$ is a unique case of a hyperkagome 3D corner sharing triangular lattice that can be decorated with quantum spins. It has spurred a lot of theoretical interest as a spin liquid candidate. We present a comprehensive set of NMR data taken on both the ${}^{23}\mathrm{Na}$ and ${}^{17}\mathrm{O}$ sites. We show that disordered magnetic freezing of all Ir sites sets in below $T_f\sim 7\mathrm{K}$, well below $J=300\mathrm{K}$, with a drastic slowing down of fluctuations to a static state revealed by our $T_1$ measurements. Above typically $2T_f$, physical properties are relevant to the spin liquid state induced by this exotic geometry. While the shift data show that the susceptibility levels off below 80 K, $1/T_1$ has little variation from 300 K to $2T_f$. We discuss the implication of our results in the context of published experimental and theoretical work.

Iron pnictides and selenides display a variety of unusual magnetic phases originating from the interplay between electronic, orbital, and lattice degrees of freedom. Using powder inelastic neutron scattering on the two-leg ladder ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$, we fully characterize the static and dynamic spin correlations associated with the ${\mathrm{Fe}}_4$ block state, an exotic magnetic ground state observed in this low-dimensional magnet and in ${\mathrm{Rb}}_{0.89}{\mathrm{Fe}}_{1.58}{\mathrm{Se}}_2$. All the magnetic excitations of the ${\mathrm{Fe}}_4$ block state predicted by an effective Heisenberg model with localized spins are observed below 300 meV and quantitatively reproduced. However, the data only account for $16(3){\mu }_B^2$ per ${\mathrm{Fe}}^{2+}$, approximatively $2/3$ of the total spectral weight expected for localized $S=2$ moments. Our results highlight how orbital degrees of freedom in iron-based magnets can conspire to stabilize an exotic magnetic state.

New mechanisms for achieving direct electric field control of ferromagnetism are highly desirable in the development of functional magnetic interfaces. To that end, we have probed the electric field dependence of the emergent ferromagnetic layer at ${\mathrm{CaRuO}}_3/{\mathrm{CaMnO}}_3$ interfaces in bilayers fabricated on ${\mathrm{SrTiO}}_3$. Using polarized neutron reflectometry, we are able to detect the ferromagnetic signal arising from a single atomic monolayer of ${\mathrm{CaMnO}}_3$, manifested as a spin asymmetry in the reflectivity. We find that the application of an electric field of $600\mathrm{kV}/\mathrm{m}$ across the bilayer induces a significant increase in this spin asymmetry. Modeling of the reflectivity suggests that this increase corresponds to a transition from canted antiferromagnetism to full ferromagnetic alignment of the ${\mathrm{Mn}}^{4+}$ ions at the interface. This increase from $1{\mu }_B$ to $2.5–3.0{\mu }_B$ per Mn is indicative of a strong magnetoelectric coupling effect, and such direct electric field control of the magnetization at an interface has significant potential for spintronic applications.

The paucity of observed supermassive black hole binaries (SMBHBs) may imply that the gravitational wave background (GWB) from this population is anisotropic, rendering existing analyses suboptimal. We present the first constraints on the angular distribution of a nanohertz stochastic GWB from circular, inspiral-driven SMBHBs using the 2015 European Pulsar Timing Array data. Our analysis of the GWB in the $\sim 2–90\mathrm{nHz}$ band shows consistency with isotropy, with the strain amplitude in $l>0$ spherical harmonic multipoles $\lesssim 40\%$ of the monopole value. We expect that these more general techniques will become standard tools to probe the angular distribution of source populations.

We present a measurement of the fundamental parameter of the standard model, the weak mixing angle ${\sin }^2{\theta }_{\text{eff}}^{\ell }$ which determines the relative strength of weak and electromagnetic interactions, in $p\overline{p}\to Z/{\gamma }^{*}\to e^{+}{e}^{-}$ events at a center of mass energy of 1.96 TeV, using data corresponding to $9.7{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the D0 detector at the Fermilab Tevatron. The effective weak mixing angle is extracted from the forward-backward charge asymmetry as a function of the invariant mass around the $Z$ boson pole. The measured value of ${\sin }^2{\theta }_{\text{eff}}^{\ell }=0.23147\pm 0.00047$ is the most precise measurement from light quark interactions to date, with a precision close to the best LEP and SLD results.