We report on the magnetic reversal characteristics of exchange coupled ferrimagnetic (FI) $\mathrm{T}{\mathrm{b}}_{19}\mathrm{F}{\mathrm{e}}_{81}/\mathrm{T}{\mathrm{b}}_{36}\mathrm{F}{\mathrm{e}}_{64}$ heterostructures. Both layers are amorphous and exhibit strong perpendicular magnetic anisotropy. The investigated heterostructures consist of a Tb-dominated and a Fe-dominated FI layer. Thus, in the magnetic ground state the net moments of the individual layers are oppositely aligned due to antiferromagnetic coupling of Fe and Tb moments. By cooling the system below 160 K, a large positive and negative exchange bias (EB) effect appears for the Tb- and Fe-dominated layers, respectively. The biasing depends only on the initial magnetization state and is neither affected by a cooling field nor by loop cycling. The phenomenon can be explained by the presence of a hard magnetic Fe-dominated interfacial layer, which forms during the sputter deposition process due to interface mixing and resputtering effects. This interfacial layer acts as a pinning layer below a certain temperature, where its coercivity increases to values larger than the accessible magnetic field range. This assumption is further supported by introducing a 0.9-nm-thick Ru spacer layer, which causes the EB effect to vanish. The EB effect was further investigated for a sample series, where the thickness ratio of the two Tb-Fe layers was varied, while keeping the total thickness of the bilayers constant. Only samples where the individual layers are sufficiently thick reveal double shifted loops, indicating the high sensitivity of the observed bias effect with respect to the magnetic properties of the individual layers and their interfacial area.

We report on an investigation of the redistribution of interlayer coherence in the trilayer cuprate $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{C}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{10}$. The experiment is performed under the same apical-oxygen phonon excitation discussed in the past for the bilayer cuprate $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6.5}$. In $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{C}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{10}$, we observe a similar spectral weight loss at the transverse plasma mode resonance as that seen in $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6.5}$. However, this feature is not accompanied by the light-enhanced interlayer coherence that was found in $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+x}$, for which the transverse plasma mode is observed at equilibrium even in the normal state. These new observations offer an experimental perspective in the context of the physics of light-enhanced interlayer coupling in various cuprates.

We report the critical behavior of a B20 cubic compound MnSi, known by its helical, conical and skyrmion phases in low fields, in its magnetic field-induced ferromagnetic (high-field) phase by means of magnetic-entropy (magnetoresistance) methods. The evaluated critical exponents are $\beta =0.25\pm 0.02(0.19\pm 0.04),\gamma =1.29\pm 0.27(1.32\pm 0.16)$, and $\delta =6.18\pm 0.28(7.79\pm 0.52)$. The self-consistency of the newly adopted methods is established by comparing the thus obtained exponents with those estimated using the modified Arrott's plot method and neutron diffraction. The field controlling parameter is $n\sim 0.5$ and its deviation from the mean-field model exponent confirm the weak first-order and field-induced ferromagnetic behavior. A direct proportional relation between magnetic entropy and magnetoresistance infers itinerant magnetic nature. The collapse of high-field data onto the more generalized magnetic-entropy master curve confirms the field-induced first- to second-order phase transition and thereby tricritical phenomenon in MnSi.

We report the observation of an extremely large magnetoresistance (up to 15 000 000% at 1.8 K in a magnetic field of 9 T) in a simple chemical element, gray arsenic, in which the magnitude of the magnetoresistance increases as approximately the square of the magnetic field strength without any signs of saturation. The Hall-effect study confirms that gray arsenic is a nearly perfect “compensated semimetal,” with a small concentration of very mobile carriers, which lead to an extremely large magnetoresistance. The analysis of Shubnikov–de Haas oscillations reveals a nontrivial π Berry phase, a strong signature of Dirac fermions with three-dimensional dispersion. Furthermore, in the presence of parallel magnetic and electric fields, a weak antilocalization effect and a pronounced negative longitudinal magnetoresistance, which may be linked to novel topological states, are also observed. These findings which uncover the material's basis in gray arsenic not only open avenues in spintronics and magnetic sensor applications but also provide more platforms to study topological materials.

We present electronic and magnetic properties of ${\mathrm{CaMnO}}_3$ (CMO) as obtained from ab initio calculations. We identify the preferable magnetic order by means of density functional theory plus Hubbard $U$ calculations and extract the effective exchange parameters $(J_{ij}$'s) using the magnetic force theorem. We find that the effects of geometrical relaxation at the surface as well as the change of crystal field are very strong and are able to influence the lower-energy magnetic configuration. In particular, our analysis reveals that the exchange interaction between the Mn atoms belonging to the surface and the subsurface layers is very sensitive to the structural changes. An earlier study [A. Filippetti and W. E. Pickett, Phys. Rev. Lett. 83, 4184 (1999)] suggested that this coupling is ferromagnetic and gives rise to the spin-flip (SF) process on the surface of CMO. In our work, we confirm their finding for an unrelaxed geometry, but once the structural relaxations are taken into account, this exchange coupling changes its sign. Thus, we suggest that the surface of CMO should have the same $G$-type antiferromagnetic order as in the bulk. Finally, we show that the suggested SF can be induced in the system by introducing an excess of electrons.

We demonstrate a feedback loop that manifests itself in a strong hysteresis and bistability of the exciton resonance fluorescence signal. Field ionization of photogenerated quantum dot excitons leads to the formation of a charged interface layer that drags the emission line along over a frequency range of more than $30\text{GHz}$. These measurements are well described by a rate equation model. With a time-resolved resonance fluorescence measurement we determined the buildup times for the hole gas in the orders of milliseconds. This internal charge-driven feedback loop could be used to reduce the spectral wandering in the emission spectra of single self-assembled quantum dots.

The electronic structure of Mg and free-base tetraphenylporphyrin films on Ag(100) is investigated by one- and two-photon photoemission in combination with electronic structure calculations using density functional theory and the self-consistent $G{W}_0$ method. We determine the two highest occupied and the nearly degenerate lowest unoccupied molecular orbitals. Higher unoccupied states are seen in an enhanced emission as a final-state effect. For photon energies close to the prominent absorption of the Soret band we observe a strong electron emission attributed to the break up of the bound electron-hole pairs in the $S_2$ excited state. The experimental results on the occupied and unoccupied energy levels for the molecular films on Ag(100) nicely agree with calculated quasiparticle energies and experiments of the molecules in the gas phase.

Quantum systems, excited by an external source of photons, display a photodynamics that is ruled by a subtle balance between radiative or nonradiative energy channels when interacting with metallic nanostructures. We apply and generalize this concept to achieve a quantum probing of multimodal plasmonic resonators by collecting and filtering the broad emission spectra generated by a nanodiamond (ND) hosting a small set of nitrogen-vacancy (NV) color centers attached at the apex of an optical tip. Spatially and spectrally resolved information on the photonic local density of states (ph-LDOS) can be recorded with this technique in the immediate vicinity of plasmonic resonators, paving the way for a complete near-field optical characterization of any kind of nanoresonators in the single photon regime.

Most quasi-one-dimensional (quasi-1D) quarter-filled organic charge-transfer solids (CTS) with insulating ground states have two thermodynamic transitions: a high-temperature metal-insulator transition followed by a low-temperature magnetic transition. This sequence of transitions can be understood within the 1D Peierls-extended Hubbard (PEH) model. However, in some quasi-1D CTS both transitions occur simultaneously in a direct metal to spin-gapped insulator transition. In this second class of materials the organic stack bond distortion pattern does not follow the pattern of a second dimerization of a dimer lattice. These materials also display charge ordering of a large amplitude below the transition. Using quantum Monte Carlo methods we show that the same PEH model can be used to understand both classes of materials, however, within different parameter regions. We discuss the relevance of our work to experiments on several quarter-filled conductors, focusing in particular on the materials (EDO-TTF)${}_{2}X$ and (DMEDO-TTF)${}_{2}X$.

The interaction of a light field with a quantum-mechanical system can be studied in an optically controlled semiconductor quantum dot. When driven by a continuous light field switched on instantaneously, the quantum dot occupation performs Rabi oscillations. Unlike an atomic system, the quantum dot is coupled to phonons, which leads to a damping of the Rabi oscillations. Here we model the time-resolved probe spectra to monitor these dynamics and study the influence of phonons on the spectra. The spectra consist of up to three peaks, similar to the Mollow triplet known from quantum optics. We develop analytical equations within a rate equation model and show that they agree excellently with a numerical solution using a well established correlation expansion approach.

Recent results for the critical exponent of the localization length at the integer quantum Hall transition differ considerably between experimental (${\nu }_{\text{exp}}\approx 2.38$) and numerical (${\nu }_{\text{CC}}\approx 2.6$) values obtained in simulations of the Chalker-Coddington (CC) network model. The difference is at least partially due to effects of the electron-electron interaction present in experiments. Here, we propose a mechanism that changes the value of $\nu$ even within the single-particle picture. We revisit the arguments leading to the CC model and consider more general networks with structural disorder. Numerical simulations of the new model lead to the value $\nu \approx 2.37$. We argue that in a continuum limit the structurally disordered model maps to free Dirac fermions coupled to various random potentials (similar to the CC model) but also to quenched two-dimensional quantum gravity. This explains the possible reason for the considerable difference between critical exponents for the CC model and the structurally disordered model. We extend our results to network models in other symmetry classes.

The elastic constant $C_{11}$ and piezoelectric stress constant $e_{1,11}$ of two-dimensional (2D) dielectric materials comprising h-BN, $2H\text{−}{\mathrm{MoS}}_2$, and other transition-metal dichalcogenides and dioxides are calculated using lattice dynamical theory. The results are compared with corresponding quantities obtained with ab initio calculations. We identify the difference between clamped-ion and relaxed-ion contributions with the dependence on inner strains which are due to the relative displacements of the ions in the unit cell. Lattice dynamics allows us to express the inner-strain contributions in terms of microscopic quantities such as effective ionic charges and optoacoustical couplings, which allows us to clarify differences in the piezoelectric behavior between h-BN and ${\mathrm{MoS}}_2$. Trends in the different microscopic quantities as functions of atomic composition are discussed.

We combine optical and magneto-optical spectroscopies with complementary vibrational and magnetic property measurements to reveal finite length scale effects in nanoscale $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$. Analysis of the $d$-to-$d$ on-site excitations uncovers enhanced color contrast at particle sizes below approximately 75 nm due to size-induced changes in spin-charge coupling that are suppressed again below the superparamagnetic limit. These findings provide a general strategy for amplifying magnetochromism in $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and other iron-containing nanomaterials that may be useful for advanced sensing applications. We also unravel the size dependence of collective excitations in this iconic antiferromagnet.

We report the anisotropic magnetotransport measurement on a noncompound band semiconductor black phosphorus (BP) with magnetic field $\mathit{B}$ up to 16 Tesla applied in both perpendicular and parallel to electric current $\mathit{I}$ under hydrostatic pressures. The BP undergoes a topological Lifshitz transition from band semiconductor to a zero-gap Dirac semimetal state at a critical pressure $P_c$, characterized by a weak localization-weak antilocalization transition at low magnetic fields and the emergence of a nontrivial Berry phase of $\pi$ detected by SdH magneto-oscillations in magnetoresistance curves. In the transition region, we observe a pressure-dependent negative MR only in the $\mathit{B}\parallel \mathit{I}$ configuration. This negative longitudinal MR is attributed to the Adler-Bell-Jackiw anomaly (topological $\mathit{E}·\mathit{B}$ term) in the presence of weak antilocalization corrections.

We have studied by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) single molecular spin hybrids formed upon chemisorbing a polycyclic aromatic, threefold symmetric hydrocarbon molecule on Co(111) nanoislands. The spin-dependent hybridization between the Co $d$ states and the $\pi$ orbitals of the molecule leads to a spin-imbalanced electronic structure of the chemisorbed organic molecule. Spin-sensitive measurements reveal that the spin polarization shows intramolecular variations among the different aromatic rings in spite of the highly symmetric adsorption geometry promoted by symmetry matching of the threefold symmetric molecule and the sixfold symmetric Co(111) lattice. Hence the varying degree of spin polarization on the organic molecule does not stem from a different hybridization of the aromatic rings with the Co atoms, but is proposed to be a consequence of the superposition of the spin polarization of the molecule and the spatially modulated spin polarization of the spin-dependent quantum interference pattern of the Co(111) surface state.

We report on the spin waves and crystal field excitations in single crystal ${\mathrm{LiFePO}}_4$ by inelastic neutron scattering over a wide range of temperatures, below and above the antiferromagnetic transition of this system. In particular, we find extra excitations below $T_N=50$ K that are nearly dispersionless and are most intense around magnetic zone centers. We show that these excitations correspond to transitions between thermally occupied excited states of ${\mathrm{Fe}}^{2+}$ due to splitting of the $S=2$ levels that arise from the crystal field and spin-orbit interactions. These excitations are further amplified by the highly distorted nature of the oxygen octahedron surrounding the iron atoms. Above $T_N$, magnetic fluctuations are observed up to at least 720 K, with an additional inelastic excitation around 4 meV, which we attribute to single-ion effects, as its intensity weakens slightly at 720 K compared to 100 K, which is consistent with the calculated cross sections using a single-ion model. Our theoretical analysis, using the MF-RPA model, provides both detailed spectra of the Fe $d$ shell and estimates of the average ordered magnetic moment and $T_N$. By applying the MF-RPA model to a number of existing spin-wave results from other $\mathrm{Li}M{\mathrm{PO}}_4$ ($M=\mathrm{Mn}$, Co, and Ni), we are able to obtain reasonable predictions for the moment sizes and transition temperatures.

A formalism is presented for treating strongly correlated graphene quantum Hall states in terms of an SO(8) fermion dynamical symmetry that includes pairing as well as particle-hole generators. The graphene SO(8) algebra is isomorphic to an SO(8) algebra that has found broad application in nuclear physics, albeit with physically very different generators, and exhibits a strong formal similarity to SU(4) symmetries that have been proposed to describe high-temperature superconductors. The well-known SU(4) symmetry of quantum Hall ferromagnetism for single-layer graphene is recovered as one subgroup of SO(8), but the dynamical symmetry structure associated with the full set of SO(8) subgroup chains extends quantum Hall ferromagnetism and allows analytical many-body solutions for a rich set of collective states exhibiting spontaneously broken symmetry that may be important for the low-energy physics of graphene in strong magnetic fields. The SO(8) symmetry permits a natural definition of generalized coherent states that correspond to symmetry-constrained Hartree-Fock-Bogoliubov solutions, or equivalently a microscopically derived Ginzburg-Landau formalism, exhibiting the interplay between competing spontaneously broken symmetries in determining the ground state.

We study molecular beam epitaxially grown undoped ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{Sb}$/InAs/AlSb quantum wells with different buffer and barrier designs and varying quantum well width. The highest mobilities were achieved with ${\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{Sb}$ buffers and lower barriers and a quantum well width of 24 nm. These quasi-single-interface InAs/AlSb quantum well devices reached a gate-tuned mobility of $2.4\times {10}^6{\mathrm{cm}}^2$/V s at a density of $1\times {10}^{12}{\mathrm{cm}}^{-2}$ and 1.3 K. In Hall bar devices boundary scattering is found to strongly influence the mobility determination in this mobility regime. Ionized background impurity scattering at low electron densities, device boundary scattering at intermediate electron densities, and intersubband scattering at high electron densities were identified as the most likely dominant scattering processes. Ringlike structures in the Landau fan can be explained using a single-particle model of crossing Landau levels.

We perform a thorough study of the extended Hubbard model featuring local and nearest-neighbor Coulomb repulsion. Using the dynamical mean-field theory we investigated the zero-temperature phase diagram of this model as a function of the chemical doping. The interplay between local and nonlocal interactions drives a variety of phase transitions connecting two distinct charge-ordered insulators, i.e., half filled and quarter filled, a charge-ordered metal and a Mott-insulating phase. We characterize these transitions and the relative stability of the solutions and we show that the two interactions conspire to stabilize the quarter-filled charge-ordered phase.