We investigate the magnetic ordering and the magnetization dynamics (from kHz to THz time scales) of the double perovskite ${\mathrm{Lu}}_2{\mathrm{MnCoO}}_6$ using elastic neutron diffraction, muon spin relaxation, and micro-Hall magnetization measurements. This compound is known to be a type II multiferroic with the interesting feature that a ferromagneticlike magnetization hysteresis loop couples to an equally hysteretic electric polarization in the bulk of the material despite a zero-field magnetic ordering of the type $\uparrow \uparrow \downarrow \downarrow$ along Co-Mn spin chains. Here we explore the unusual dynamics of this compound and find extremely strong fluctuations, consistent with the axial next-nearest-neighbor Ising (ANNNI) model for frustrated spin chains. We identify three temperature scales in ${\mathrm{Lu}}_2{\mathrm{MnCoO}}_6$ corresponding to the onset of highly fluctuating long-range order below $T_N=50\pm 3$ K identified from neutron scattering, the onset of magnetic and electric hysteresis, with change in kHz magnetic and electric dynamics below a 30 K temperature scale, and partial freezing of $\sim \mathrm{MHz}$ spin fluctuations in the muon spin relaxation data below $12\pm 3$ K. Our results provide a framework for understanding the multiferroic behavior of this compound and its hysteresis and dynamics.

We have systematically studied the low-temperature specific heat of the ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$ single crystals covering the whole superconducting dome. Using the nonsuperconducting heavily overdoped $x=0.3$ sample as a reference for the phonon contribution to the specific heat, we find that the normal-state electronic specific heats in the superconducting samples may have a nonlinear temperature dependence, which challenges previous results in the electron-doped Ba-122 iron-based superconductors. A model based on the presence of ferromagnetic spin fluctuations may explain the data between $x=0.1$ and $x=0.15$, suggesting the important role of Fermi-surface topology in understanding the normal-state electronic states.

We construct a systematic high-frequency expansion for periodically driven quantum systems based on the Brillouin-Wigner (BW) perturbation theory, which generates an effective Hamiltonian on the projected zero-photon subspace in the Floquet theory, reproducing the quasienergies and eigenstates of the original Floquet Hamiltonian up to desired order in $1/\omega$, with $\omega$ being the frequency of the drive. The advantage of the BW method is that it is not only efficient in deriving higher-order terms, but even enables us to write down the whole infinite series expansion, as compared to the van Vleck degenerate perturbation theory. The expansion is also free from a spurious dependence on the driving phase, which has been an obstacle in the Floquet-Magnus expansion. We apply the BW expansion to various models of noninteracting electrons driven by circularly polarized light. As the amplitude of the light is increased, the system undergoes a series of Floquet topological-to-topological phase transitions, whose phase boundary in the high-frequency regime is well explained by the BW expansion. As the frequency is lowered, the high-frequency expansion breaks down at some point due to band touching with nonzero-photon sectors, where we find numerically even more intricate and richer Floquet topological phases spring out. We have then analyzed, with the Floquet dynamical mean-field theory, the effects of electron-electron interaction and energy dissipation. We have specifically revealed that phase transitions from Floquet-topological to Mott insulators emerge, where the phase boundaries can again be captured with the high-frequency expansion.

How to tune two-dimensional electron gas at interface of heterostructures is becoming an important question for both fundamental physics and electronic applications. Here, using density functional theory calculations, we find that the polarization state of ${\mathrm{BaTiO}}_3$ in metal capped ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructures changes dramatically, depending on the termination of ${\mathrm{BaTiO}}_3$ and the different metal layers ($M=\text{Al}$, Fe, Pt Au). Most interestingly, for Pt on the BaO-terminated ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$, interface conductivity can be tuned. With a paraelectric state in ${\mathrm{BaTiO}}_3, \text{Pt}\_\mathrm{BaTiO}{}_3/{\mathrm{SrTiO}}_3$ remains insulating at interface, while when ${\mathrm{BaTiO}}_3$ is ferroelectric, a hole- or electron-conducting ${\mathrm{BaTiO}}_3/{\mathrm{SrTiO}}_3$ interface can be realized, depending on its polarization direction in ${\mathrm{BaTiO}}_3$. This conducting interface and the top Pt layer screen the depolarization field, and thus stabilize the ferroelectricity in ${\mathrm{BaTiO}}_3$. Our result provides important clues for the reversibly tunable conductivity at oxide interfaces.

We discuss the influence of a rapid thermal annealing step on the magneto-optical emission properties of In(Ga)As/GaAs quantum dots. We map out a strong influence of the growth and annealing parameters on the excitons' effective Landé $g$ factors and in particular on their diamagnetic coefficients, which we directly correlate with the modification of the emitters' shape and material composition. In addition, we study the excitons' spontaneous emission lifetime as a function of the annealing temperature and the dot height and observe a strong increase of the emission rate with the quantum dot volume. The corresponding increase in oscillator strength yields fully consistent results with the analysis of the diamagnetic behavior. Specifically, we demonstrate that a rapid thermal annealing step of $850{}^{\circ }\mathrm{C}$ can be employed to increase the oscillator strength of as-grown InAs/GaAs QDs by more than a factor of 2.

We have studied the cyclotron decay time of a Landau-quantized two-dimensional electron gas as a function of temperature (0.4–100 K) at a fixed magnetic field ($\pm 1.25\mathrm{T}$) using terahertz time-domain spectroscopy in a gallium arsenide quantum well with a mobility of ${\mu }_{dc}=3.6\times {10}^6{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and a carrier concentration of $n_s=2\times {10}^{11}{\mathrm{cm}}^{-2}$. We find a cyclotron decay time that is limited by superradiant decay of the cyclotron ensemble and a temperature dependence that may result from both dissipative processes as well as a decrease in $n_s$ below $1.5\mathrm{K}$. Shubnikov–de Haas characterization determines a quantum lifetime, ${\tau }_q=1.1\mathrm{ps}$, which is significantly faster than the corresponding dephasing time, ${\tau }_s=66.4\mathrm{ps}$, in our cyclotron data. This is consistent with small-angle scattering as the dominant contribution in this sample, where scattering angles below $\theta \le {13}^{\circ }$ do not efficiently contribute to dephasing. Above $50\mathrm{K}$, the cyclotron oscillations show a strong reduction in both the oscillation amplitude and lifetime that result from polar optical phonon scattering.

The buried interface between a molecular thin film and the metal substrate is generally not accessible to the photoemission experiment. With the example of a sexiphenyl ($6\mathrm{P}$) bilayer on Cu we show that photoemission tomography can be used to study the electronic level alignment and geometric structure, where it was possible to assign the observed orbital emissions to the individual layers. We further study the Cs doping of this bilayer. Initial Cs exposure leads to a doping of only the first interface layer, leaving the second layer unaffected except for a large energy shift. This result shows that it is in principle possible to chemically modify just the interface, which is important to issues like tuning of the energy level alignment and charge transfer to the interface layer. Upon saturating the film with Cs, photoemission tomography shows a complete doping ($6p^{4-}$) of the bilayer, with the molecular geometry changing such that the spectra become dominated by $\sigma$-orbital emissions.

Using first-principles density-functional theory, we have investigated the electronic and magnetic properties of recently synthesized and characterized $5d$ double-perovskites $\mathrm{S}{\mathrm{r}}_{2}B\mathrm{Os}{\mathrm{O}}_6(B=\mathrm{Y},\mathrm{In},\mathrm{Sc})$. The electronic structure calculations show that in all compounds the $\mathrm{O}{\mathrm{s}}^{5+}$ ($5d^3$) site is the only magnetically active one, whereas ${\mathrm{Y}}^{3+}, \mathrm{I}{\mathrm{n}}^{3+}$, and $\mathrm{S}{\mathrm{c}}^{3+}$ remain in nonmagnetic states with Sc/Y and In featuring $d^0$ and $d^{10}$ electronic configurations, respectively. Our studies reveal the important role of closed-shell ($d^{10}$) versus open-shell ($d^0$) electronic configurations of the nonmagnetic sites in determining the overall magnetic exchange interactions. Although the magnetic $\mathrm{O}{\mathrm{s}}^{5+}$ ($5d^3$) site is the same in all compounds, the magnetic superexchange interactions mediated by nonmagnetic Y/In/Sc species are strongest for $\mathrm{S}{\mathrm{r}}_2\mathrm{ScOs}{\mathrm{O}}_6$, weakest for $\mathrm{S}{\mathrm{r}}_2\mathrm{InOs}{\mathrm{O}}_6$, and intermediate in the case of the Y ($d^0$) due to different energy overlaps between Os-$5d$ and Y/In/Sc-$d$ states. This explains the experimentally observed substantial differences in the magnetic transition temperatures of these materials, despite an identical magnetic site and underlying magnetic ground state. Furthermore, short-range Os-Os exchange interactions are more prominent than long-range Os-Os interactions in these compounds, which contrasts with the behavior of other $3d-5d$ double perovskites.

Time-resolved magnetic sensing is of great importance from fundamental studies to applications in physical and biological sciences. Recently, the nitrogen-vacancy defect center in diamond has been developed as a promising sensor of magnetic fields under ambient conditions. However, methods to reconstruct time-resolved magnetic fields with high sensitivity are not yet fully developed. Here, we propose and demonstrate a sensing method based on spin echo and Haar wavelet transformation. Our method is exponentially faster in reconstructing time-resolved magnetic fields with comparable sensitivity than existing methods. It is also easier to implement in experiments. Furthermore, the wavelet's unique features enable our method to extract information from the whole signal with only part of the measuring sequences. We then explore this feature for a fast detection of simulated nerve impulses. These results will be useful to time-resolved magnetic sensing with quantum probes at nanoscale.

The coexistence of ferroelectricity and magnetism in $AB{\mathrm{O}}_3$ perovskite oxides is rare, a phenomenon that has become known as the ferroelectric “$d^0$ rule.” Recently, the perovskite ${\mathrm{BiCoO}}_3$ has been shown experimentally to be isostructural with ${\mathrm{PbTiO}}_3$, while simultaneously the $d^6{\mathrm{Co}}^{3+}$ ion has a high-spin ground state with $C$-type antiferromagnetic ordering. It has been suggested that the hybridization of Bi $6s$ states with the O $2p$ valence band stabilizes the polar phase, however, we have recently demonstrated that ${\mathrm{Co}}^{3+}$ ions in the perovskite structure can facilitate a ferroelectric distortion via the Co $3d$–O $2p$ covalent interaction [L. Weston, et al., Phys. Rev. Lett. 114, 247601 (2015)]. In this paper, using accurate hybrid density functional calculations, we investigate the atomic, electronic, and magnetic structure of ${\mathrm{BiCoO}}_3$ to elucidate the origin of the multiferroic state. To begin with, we perform a more general first-principles investigation of the role of $d$ electrons in affecting the tendency for perovskite materials to exhibit a ferroelectric distortion; this is achieved via a qualitative trend study in artificial cubic and tetragonal $\mathrm{La}B{\mathrm{O}}_3$ perovskites. We choose La as the $A$ cation so as to remove the effects of Bi $6s$ hybridization. The lattice instability is identified by the softening of phonon modes in the cubic phase, as well as by the energy lowering associated with a ferroelectric distortion. For the $\mathrm{La}B{\mathrm{O}}_3$ series, where $B$ is a $d^0–d^8$ cation from the $3d$ block, the trend study reveals that increasing the $d$ orbital occupation initially removes the tendency for a polar distortion, as expected. However, for high-spin $d^5–d^7$ and $d^8$ cations a strong ferroelectric instability is recovered. This effect is explained in terms of increased pseudo-Jahn-Teller (PJT) $p-d$ vibronic coupling. The PJT effect is described by the competition between a stabilizing force ($K_0$) that favors the cubic phase, and a vibronic term that drives the ferroelectric state ($K_v$). The recovery of the lattice instability for high-spin $d^5–d^7$ and $d^8$ cations is due to (i) a reduction in $K_0$ due to a significant volume increase arising from population of the $\sigma$-bonded axial $d{e}_g$ orbitals, and (ii) an increase in the $K_v$ contribution arising from increased $p-d$ hybridization; our calculations suggest that the former mechanism is dominant. Surprisingly, we are able to show that, in some cases unpaired electron spins actually drive ferroelectricity, rather than inhibit it, which represents a shift in the understanding of how ferroelectricity and magnetism interact in perovskite oxides. It follows, that for the case of ${\mathrm{BiCoO}}_3$, the ${\mathrm{Co}}^{3+}$ ion plays a major role in the ferroelectric lattice instability. Importantly, the ferroelectric polarization is greatly enhanced when the ${\mathrm{Co}}^{3+}$ ion is in the high-spin state, when compared to the nonmagnetic, low-spin state, and a large coupling of the electric and magnetic polarization is present. Generally, for $d^5–d^7 B$ cations in $AB{\mathrm{O}}_3$ perovskites, an inherent and remarkably strong magnetoelectric coupling exists via the multiferroic crossover effect, whereby switching the spin state strongly affects the ferroelectric polarization and, potentially, manipulation of the polarization with an externally applied electric field could induce a spin-state transition. This novel effect is demonstrated for ${\mathrm{BiCoO}}_3$, for which the ground spin state is switched by reducing the internal ferroelectric polarization. These results provide a deeper insight into perovskite ferroelectrics and multiferroics.

We study the emergence of charge-ordered phases within a $\pi$-loop-current $\left(\pi \mathrm{LC}\right)$ model for the pseudogap based on a three-band model for underdoped cuprate superconductors. Loop currents and charge ordering are driven by distinct components of the short-range Coulomb interactions: loop currents result from the repulsion between nearest-neighbor copper and oxygen orbitals, while charge order results from repulsion between neighboring oxygen orbitals. We find that the leading $\pi \mathrm{LC}$ phase has an antiferromagnetic pattern similar to previously discovered staggered flux phases, and that it emerges abruptly at hole dopings $p$ below the Van Hove filling. Subsequent charge-ordering tendencies in the $\pi \mathrm{LC}$ phase reveal that diagonal $d$-charge density waves (dCDWs) are suppressed by the loop currents while axial order competes more weakly. In some cases we find a wide temperature range below the loop-current transition, over which the susceptibility towards an axial dCDW is large. In these cases, short-range axial charge order may be induced by doping-related disorder. A unique feature of the coexisting dCDW and $\pi \mathrm{LC}$ phases is the emergence of an incommensurate modulation of the loop currents. If the dCDW is biaxial (checkerboard) then the resulting incommensurate current pattern breaks all mirror and time-reversal symmetries, thereby allowing for a polar Kerr effect.

We show that ${}^{63}\mathrm{Cu}$-NMR spectra place strong constraints on both the nature and the concentration of oxygen defects in ortho-II ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$. Systematic deviation from ideal ortho-II order is revealed by the presence of inequivalent Cu sites in either full or empty chains. The results can be explained by two kinds of defects: oxygen clustering into additional chains, or fragments thereof, most likely present at all concentrations $\left(6.4<y<6.6\right)$, and oxygen vacancies randomly distributed in the full chains for $y<6.50$ only. Furthermore, the remarkable reproducibility of the spectra in different samples with optimal ortho-II order $\left(y\simeq 6.55\right)$ shows that chain-oxygen disorder, known to limit electronic coherence, is ineluctable because it is inherent to these compounds.

We use neutron resonance spin echo and Larmor diffraction to study the effect of uniaxial pressure on the tetragonal-to-orthorhombic structural $\left(T_s\right)$ and antiferromagnetic (AF) phase transitions in iron pnictides ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2 \left(x=0,0.03,0.12\right),{\mathrm{SrFe}}_{1.97}{\mathrm{Ni}}_{0.03}{\mathrm{As}}_2$, and ${\mathrm{BaFe}}_2\left({\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}\right){}_2$. In antiferromagnetically ordered ${\mathrm{BaFe}}_{2-x}{\mathrm{Ni}}_x{\mathrm{As}}_2$ and ${\mathrm{SrFe}}_{1.97}{\mathrm{Ni}}_{0.03}{\mathrm{As}}_2$ with $T_N$ and $T_s \left(T_N\le T_s\right)$, a uniaxial pressure necessary to detwin the sample also increases $T_N$, smears out the structural transition, and induces an orthorhombic lattice distortion at all temperatures. By comparing temperature and doping dependence of the pressure induced lattice parameter changes with the elastoresistance and nematic susceptibility obtained from transport and ultrasonic measurements, we conclude that the in-plane resistivity anisotropy found in the paramagnetic state of electron underdoped iron pnictides depends sensitively on the nature of the magnetic phase transition and a strong coupling between the uniaxial pressure induced lattice distortion and electronic nematic susceptibility.

We study the slow quenching dynamics (characterized by an inverse rate ${\tau }^{-1}$) of a one-dimensional transverse Ising chain with nearest neighbor ferromagentic interactions across the quantum critical point (QCP) and analyze the Loschmidt overlap measured using the subsequent temporal evolution of the final wave function (reached at the end of the quenching) with the final time-independent Hamiltonian. Studying the Fisher zeros of the corresponding generalized “partition function,” we probe nonanalyticities manifested in the rate function of the return probability known as dynamical phase transitions (DPTs). In contrast to the sudden quenching case, we show that DPTs survive in the subsequent temporal evolution following the quenching across two critical points of the model for a sufficiently slow rate; furthermore, an interesting “lobe” structure of Fisher zeros emerge. We have also made a connection to topological aspects studying the dynamical topological order parameter $\left[{\nu }_D\left(t\right)\right]$ as a function of time $\left(t\right)$ measured from the instant when the quenching is complete. Remarkably, the time evolution of ${\nu }_D\left(t\right)$ exhibits drastically different behavior following quenches across a single QCP and two QCPs. In the former case, ${\nu }_D\left(t\right)$ increases stepwise by unity at every DPT (i.e., $\mathrm{\Delta }{\nu }_D=1$). In the latter case, on the other hand, ${\nu }_D\left(t\right)$ essentially oscillates between 0 and 1 (i.e., successive DPTs occur with $\mathrm{\Delta }{\nu }_D=1$ and $\mathrm{\Delta }{\nu }_D=-1$, respectively), except for instants where it shows a sudden jump by a factor of unity when two successive DPTs carry a topological charge of the same sign.

This paper describes a phase-resolved ferromagnetic resonance (FMR) measurement using a heterodyne method. Spin precession is driven by microwave fields and detected by 1550 nm laser light that is modulated at a frequency slightly shifted with respected to the FMR driving frequency. The evolving phase difference between the spin precession and the modulated light produces a slowly oscillating Kerr rotation signal with a phase equal to the precession phase plus a phase due to the path length difference between the excitation microwave signal and the optical signal. We estimate the accuracy of the precession phase measurement to be 0.1 rad. This heterodyne FMR detection method eliminates the need for field modulation and allows a stronger detection signal at higher intermediate frequency where the $1/f$ noise floor is reduced.

A rise in the superconducting critical temperature $T_c$ of more than two times (exceeding 20 K) is discovered in point contacts created between iron chalcogenide FeSe single crystals and Cu. The possible reasons for such a $T_c$ increase in point contacts are discussed. The most likely cause for this may be the interfacial carrier doping and/or interfacial enhanced electron-phonon interaction.

Symmetry-protected topological (SPT) phases exhibit nontrivial order if symmetry is respected but are adiabatically connected to the trivial product phase if symmetry is not respected. However, unlike the symmetry-breaking phase, there is no local order parameter for SPT phases. Here we employ a tensor-network method to compute the topological invariants characterized by the simulated modular $S$ and $T$ matrices to study transitions in a few families of two-dimensional (2D) wave functions which are ${\mathbb{Z}}_N$ $\left(N=2\text{and}3\right)$ symmetric. We find that in addition to the topologically ordered phases, the modular matrices can be used to identify nontrivial SPT phases and detect transitions between different SPT phases as well as between symmetric and symmetry-breaking phases. Therefore modular matrices can be used to characterize various types of gapped phases in a unifying way.

Using ambient pressure x-ray and high pressure neutron diffraction, we studied the pressure effect on structural and magnetic properties of multiferroic ${\mathrm{Mn}}_{1\text{−}x}{\mathrm{Co}}_x{\mathrm{WO}}_4$ single crystals ($x=0$, 0.05, 0.135, and $0.17$) and compared it with the effects of doping. Both Co doping and pressure stretch the Mn-Mn chain along the $c$ direction. At high doping level ($x=0.135$ and $0.17$), pressure and Co doping drive the system in a similar way and induce a spin-flop transition for the $x=0.135$ compound. In contrast, magnetic ground states at lower doping level $(x=0$ and $0.05$) are robust against pressure but experience a pronounced change upon Co substitution. As Co introduces both chemical pressure and magnetic anisotropy into the frustrated magnetic system, our results suggest the magnetic anisotropy is the main driving force for the Co induced phase transitions at low doping level, and chemical pressure plays a more significant role at higher Co concentrations.