We reveal for the first time through a theoretical first-principles study that the adsorption of a nonmagnetic $\pi$-conjugated organic molecule on a ferromagnetic surface locally increases the strength of the magnetic exchange interaction between the magnetic atoms binding directly to the molecule. This magnetic hardening effect leads to the creation of a local molecular mediated magnetic unit with a stable magnetization direction and an enhanced barrier for the magnetization switching as compared to the clean surface. Remarkably, such a hybrid organic-ferromagnetic system exhibits also a spin-filter functionality with sharp spin-split molecularlike electronic features at the molecular site.

Recently, extensive vortex lattice metastability was reported in ${\mathrm{MgB}}_2$ in connection with a second-order rotational phase transition. However, the mechanism responsible for these well-ordered metastable vortex lattice phases is not well understood. Using small-angle neutron scattering, we studied the vortex lattice in ${\mathrm{MgB}}_2$ as it was driven from a metastable to the ground state through a series of small changes in the applied magnetic field. Our results show that metastable vortex lattice domains persist in the presence of substantial vortex motion and directly demonstrate that the metastability is not due to vortex pinning. Instead, we propose that it is due to the jamming of counterrotated vortex lattice domains which prevents a rotation to the ground state orientation.

Quantum criticality in the normal and superconducting states of the heavy-fermion metal ${\mathrm{CeCoIn}}_5$ is studied by measurements of the magnetic Grüneisen ratio ${\Gamma }_H$ and specific heat in different field orientations and temperatures down to 50 mK. A universal temperature over magnetic field scaling of ${\Gamma }_H$ in the normal state indicates a hidden quantum critical point at zero field. Within the superconducting state, the quasiparticle entropy at constant temperature increases upon reducing the field towards zero, providing additional evidence for zero-field quantum criticality.

We present a high-pressure NMR study of the overdoped iron pnictide superconductor ${\mathrm{NaFe}}_{0.94}{\mathrm{Co}}_{0.06}\mathrm{As}$. The low-energy antiferromagnetic spin fluctuations in the normal state, manifest as the Curie-Weiss upturn in the spin-lattice relaxation rate $1/{}^{75}T_{1}T$, first increase strongly with pressure but fall again at $P>P_{\mathrm{opt}}=2.2\mathrm{GPa}$. Neither long-ranged magnetic order nor a structural phase transition is encountered up to 2.5 GPa. The superconducting transition temperature $T_c$ shows a pressure dependence identical to the spin fluctuations. Our observations demonstrate that magnetic correlations and superconductivity are optimized simultaneously as a function of the electronic structure, thereby supporting very strongly a magnetic origin of superconductivity.

Reversibly switching interlayer exchange coupling (IEC) of magnetic semiconductor multilayers between ferromagnetic (FM) and antiferromagnetic (AFM) modes is a difficult but key issue for fabricating semiconductor giant magnetoresistance devices. Here, we show that such tunable IEC is achievable around room temperature in Co-doped ${\mathrm{TiO}}_2/{\mathrm{VO}}_2$ diluted magnetic semiconductor multilayers. On the basis of first-principles calculations of electronic structure and fermiology, it is clarified that, associated with the metal-insulator transition (MIT) of nanosized ${\mathrm{VO}}_2$ spacers, exotic short-range magnetic orders are developed in the multilayers so that the IEC can be tuned reversibly from FM mode to AFM mode by varying temperature crossing the MIT ($\sim 340\mathrm{K}$).

We present spatially resolved measurements of the magnon temperature in a magnetic insulator subject to a thermal gradient. Our data reveal an unexpectedly close correspondence between the spatial dependencies of the exchange magnon and phonon temperatures. These results indicate that if—as is currently thought—the transverse spin Seebeck effect is caused by a temperature difference between the magnon and phonon baths, it must be the case that the magnon temperature is spectrally nonuniform and that the effect is driven by the sparsely populated dipolar region of the magnon spectrum.

The infinite-time average of the expectation values of local fields of any interacting quantum theory after a global quench process are key quantities for matching theoretical and experimental results. For quantum integrable field theories, we show that they can be obtained by an ensemble average that employs a particular limit of the form factors of local fields and quantities extracted by the generalized Bethe ansatz.

We compare the ground-state energies of bosons and fermions with the same form of the Hamiltonian. If both are noninteracting, the ground-state energy of bosons is always lower, owing to Bose-Einstein condensation. However, the comparison is nontrivial when bosons do interact. We first prove that, when the hopping is unfrustrated (all the hopping amplitudes are non-negative), hard-core bosons still must have a lower ground-state energy than fermions. If the hopping is frustrated, bosons can have a higher ground-state energy than fermions. We prove rigorously that this inversion indeed occurs in several examples.

We discuss sequential unambiguous state-discrimination measurements performed on the same qubit. Alice prepares a qubit in one of two possible states. The qubit is first sent to Bob, who measures it, and then on to Charlie, who also measures it. The object in both cases is to determine which state Alice sent. In an unambiguous state discrimination measurement, we never make a mistake, i.e., misidentify the state, but the measurement may fail, in which case we gain no information about which state was sent. We find that there is a nonzero probability for both Bob and Charlie to identify the state, and we maximize this probability. The probability that Charlie’s measurement succeeds depends on how much information about the state Alice sent is left in the qubit after Bob’s measurement, and this information can be quantified by the overlap between the two possible states in which Bob’s measurement leaves the qubit. This Letter is a first step toward developing a theory of nondestructive sequential quantum measurements, which could be useful in quantum communication schemes.

The evolution of a system induced by counterdiabatic driving mimics the adiabatic dynamics without the requirement of slow driving. Engineering it involves diagonalizing the instantaneous Hamiltonian of the system and results in the need of auxiliary nonlocal interactions for matter waves. Here, experimentally realizable driving protocols are found for a large class of single-particle, many-body, and nonlinear systems without demanding the spectral properties as an input. The method is applied to the fast decompression of Bose-Einstein condensates in different trapping potentials.

We search for a spin-dependent $P$- and $T$-violating nucleon-nucleon interaction mediated by light pseudoscalar bosons such as axions or axionlike particles. We employ an ultrasensitive low-field magnetometer based on the detection of free precession of colocated ${}^3\mathrm{He}$ and ${}^{129}\mathrm{Xe}$ nuclear spins using SQUIDs as low-noise magnetic flux detectors. The precession frequency shift in the presence of an unpolarized mass was measured to determine the coupling of pseudoscalar particles to the spin of the bound neutron. For boson masses between 2 and $500\mu \mathrm{eV}$ (force ranges between $3\times {10}^{-4}\mathrm{m}$ and ${10}^{-1}\mathrm{m}$) we improved the laboratory upper bounds by up to 4 orders of magnitude.

Over the last decade, a growing number of quantum-gravity researchers has been looking for opportunities for the first ever experimental evidence of a Planck-length quantum property of spacetime. These studies are usually based on the analysis of some candidate indirect implications of spacetime quantization, such as a possible curvature of momentum space. Some recent proposals have raised hope that we might also gain direct experimental access to quantum properties of spacetime, by finding evidence of limitations to the measurability of the center-of-mass coordinates of some macroscopic bodies. However, I here observe that the arguments that originally led to speculating about spacetime quantization do not apply to the localization of the center of mass of a macroscopic body. And, I also analyze some popular formalizations of the notion of quantum spacetime, finding that when the quantization of spacetime is Planckian for the constituent particles, then for the center of mass of a composite macroscopic body the quantization of spacetime is much weaker than Planckian. These results suggest that the center-of-mass observables of macroscopic bodies should not provide good opportunities for uncovering quantum properties of spacetime. And, they also raise some conceptual challenges for theories of mechanics in quantum spacetime, in which, for example, free protons and free atoms should feel the effects of spacetime quantization differently.

The charmless decays $B^{\pm }\to K^{\pm }{\pi }^{+}{\pi }^{-}$ and $B^{\pm }\to K^{\pm }{K}^{+}{K}^{-}$ are reconstructed using data, corresponding to an integrated luminosity of $1.0{\mathrm{fb}}^{-1}$, collected by LHCb in 2011. The inclusive charge asymmetries of these modes are measured as $A_{CP}(B^{\pm }\to K^{\pm }{\pi }^{+}{\pi }^{-})=0.032\pm 0.008(\mathrm{stat})\pm 0.004(\mathrm{syst})\pm 0.007(J/\psi K^{\pm })$ and $A_{CP}(B^{\pm }\to K^{\pm }{K}^{+}{K}^{-})=-0.043\pm 0.009(\mathrm{stat})\pm 0.003(\mathrm{syst})\pm 0.007(J/\psi K^{\pm })$, where the third uncertainty is due to the $CP$ asymmetry of the $B^{\pm }\to J/\psi K^{\pm }$ reference mode. The significance of $A_{CP}(B^{\pm }\to K^{\pm }{K}^{+}{K}^{-})$ exceeds three standard deviations and is the first evidence of an inclusive $CP$ asymmetry in charmless three-body $B$ decays. In addition to the inclusive $CP$ asymmetries, larger asymmetries are observed in localized regions of phase space.

We present results of a search for $CP$ violation in $B^0\mathrm{\text{−}}{\overline{B}}^0$ mixing with the BABAR detector. We select a sample of $B^0\to D^{*-}X{\ell }^{+}\nu$ decays with a partial reconstruction method and use kaon tagging to assess the flavor of the other $B$ meson in the event. We determine the $CP$ violating asymmetry ${\mathcal{A}}_{CP}\equiv [N(B^0{B}^0)-N({\overline{B}}^0{\overline{B}}^0)]/[N(B^0{B}^0)+N({\overline{B}}^0{\overline{B}}^0)]=(0.06\pm {0.17}_{-0.32}^{+0.38})\%$, corresponding to ${\Delta }_{CP}=1-|q/p|=(0.29\pm {0.84}_{-1.61}^{+1.88})\times {10}^{-3}$.

Various theories beyond the standard model predict new particles with masses in the sub-eV range with very weak couplings to ordinary matter. A new $P$-odd and $T$-odd interaction between polarized and unpolarized nucleons proportional to $\overrightarrow{K}·\overrightarrow{r}$ is one such possibility, where $\overrightarrow{r}$ is the distance between the nucleons and $\overrightarrow{K}$ is the spin of the polarized nucleon. Such an interaction involving a scalar coupling $g_s$ at one vertex and a pseudoscalar coupling $g_p$ at the polarized nucleon vertex can be induced by the exchange of spin-0 bosons. We used the NMR cell test station at Northrop Grumman Corporation to search for NMR frequency shifts in polarized ${}^{129}\mathrm{Xe}$ and ${}^{131}\mathrm{Xe}$ when a nonmagnetic zirconia rod is moved near the NMR cell. Long ($T_2\sim 20\mathrm{s}$) spin-relaxation times allow precision measurements of the NMR frequency ratios, which are insensitive to magnetic field fluctuations. Combined with existing theoretical calculations of the neutron spin contribution to the nuclear angular momentum in xenon nuclei, the measurements improve the laboratory upper bound on the product $g_s{g}_p^n$ by 2 orders of magnitude for distances near 1 mm. The sensitivity of this technique can be increased by at least two more orders of magnitude.

By using observables that only depend on charged particles (tracks), one can efficiently suppress pileup contamination at the LHC. Such measurements are not infrared safe in perturbation theory, so any calculation of track-based observables must account for hadronization effects. We develop a formalism to perform these calculations in QCD, by matching partonic cross sections onto new nonperturbative objects called track functions which absorb infrared divergences. The track function $T_i(x)$ describes the energy fraction $x$ of a hard parton $i$ which is converted into charged hadrons. We give a field-theoretic definition of the track function and derive its renormalization group evolution, which is in excellent agreement with the pythia parton shower. We then perform a next-to-leading order calculation of the total energy fraction of charged particles in $e^{+}{e}^{-}\to$ hadrons. To demonstrate the implications of our framework for the LHC, we match the pythia parton shower onto a set of track functions to describe the track mass distribution in Higgs plus one jet events. We also show how to reduce smearing due to hadronization fluctuations by measuring dimensionless track-based ratios.

We investigate the interaction between a single atom and optical pulses in a coherent state with a controlled temporal envelope. In a comparison between a rising exponential and a square envelope, we show that the rising exponential envelope leads to a higher excitation probability for fixed low average photon numbers, in accordance with a time-reversed Weisskopf-Wigner model. We characterize the atomic transition dynamics for a wide range of the average photon numbers and are able to saturate the optical transition of a single atom with $\approx 50$ photons in a pulse by a strong focusing technique.

We apply entropy removal by measurement and feedback to a cold atomic spin ensemble. Using quantum nondemolition probing by Faraday rotation measurement, and feedback by weak optical pumping, we drive the initially random collective spin variable $\widehat{\mathbf{F}}$ toward the origin $\widehat{\mathbf{F}}=\mathbf{0}$. We use input-output relations and ensemble quantum noise models to describe this quantum control process and identify an optimal two-round control procedure. We observe 12 dB of spin noise reduction, or a factor-of-63 reduction in phase-space volume. The method offers a nonthermal route to generation of exotic entangled states in ultracold gases, including macroscopic singlet states and strongly correlated states of quantum lattice gases.

Recent measurements of Efimov resonances for a number of ultracold atom species have revealed an unexpected universality, in which three-body scattering properties are determined by the van der Waals length of the two-body interaction potential. To investigate whether this universality extends to heteronuclear mixtures, we measure loss rate coefficients in an ultracold trapped gas of ${}^{40}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ atoms. We find an Efimov-like resonance in the rate of inelastic collisions between ${}^{40}\mathrm{K}{}^{87}\mathrm{Rb}$ Feshbach molecules and ${}^{87}\mathrm{Rb}$ atoms. However, we do not observe any Efimov-related resonances in the rates of inelastic collisions between three atoms. These observations are compared to previous measurements by the LENS group of Efimov resonances in a ${}^{41}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$ mixture as well as to recent predictions.

We report on the evolution of the thickness-dependent electronic band structure of the two-dimensional layered-dichalcogenide molybdenum disulfide (${\mathrm{MoS}}_2$). Micrometer-scale angle-resolved photoemission spectroscopy of mechanically exfoliated and chemical-vapor-deposition-grown crystals provides direct evidence for the shifting of the valence band maximum from $\overline{\Gamma }$ to $\overline{K}$, for the case of ${\mathrm{MoS}}_2$ having more than one layer, to the case of single-layer ${\mathrm{MoS}}_2$, as predicted by density functional theory. This evolution of the electronic structure from bulk to few-layer to monolayer ${\mathrm{MoS}}_2$ had earlier been predicted to arise from quantum confinement. Furthermore, one of the consequences of this progression in the electronic structure is the dramatic increase in the hole effective mass, in going from bulk to monolayer ${\mathrm{MoS}}_2$ at its Brillouin zone center, which is known as the cause for the decreased carrier mobility of the monolayer form compared to that of bulk ${\mathrm{MoS}}_2$.