The spin coherence of an ensemble of electrons bound to fluorine donors in epitaxially grown ZnSe layers is studied by time-resolved pump-probe Kerr rotation. Long-lived spin dephasing with decay times up to $T_2^{*}=33$ ns is found for a sample with a low fluorine concentration of $1\times {10}^{15}$ cm${}^{-3}$ at cryogenic temperatures. The time is close to the limit set by nuclear-spin fluctuations, for which we measure a strength of 1.65 mT. We find $T_2^{*}$ to be constant up to 40 K, with a strong drop for higher temperatures. The dephasing time also shortens with increasing fluorine concentration, indicating an interaction between the spins at different fluorine centers.

Small alkali clusters do not submerge in liquid helium nanodroplets but instead survive predominantly in high spin states that reside on the surface of the nanodroplet. However, a recent theoretical prediction by Stark and Kresin [Phys. Rev. B 81, 085401 (2010)], based on a classical description of the energetics of bubble formation for a fully submerged alkali cluster, suggests that the alkali clusters can submerge on energetic grounds when they exceed a critical size. Following recent work on sodium clusters, where ion yield data from electron impact mass spectrometry was used to obtain the first experimental evidence for alkali cluster submersion, we report here on similar experiments for potassium clusters. Evidence is presented for full cluster submersion at $n>80$ for K${}_n$ clusters, which is in good agreement with the recent theoretical prediction. In an additional observation, we report “magic number” sizes for both K${}_n$${}^{+}$ and K${}_n$${}^{2+}$ ions derived from helium droplets, which are found to be consistent with the jellium model.

We develop a technique for constructing Bloch-like functions for 2D ${\mathbb{Z}}_2$ insulators (i.e., quantum spin-Hall insulators) that are smooth functions of $\mathbf{k}$ on the entire Brillouin-zone torus. As the initial step, the occupied subspace of the insulator is decomposed into a direct sum of two “Chern bands,” that is, topologically nontrivial subspaces with opposite Chern numbers. This decomposition remains robust independent of underlying symmetries or specific model features. Starting with the Chern bands obtained in this way, we construct a topologically nontrivial unitary transformation that rotates the occupied subspace into a direct sum of topologically trivial subspaces, thus facilitating a Wannier construction. The procedure is validated and illustrated by applying it to the Kane-Mele model.

We perform Raman scattering experiments on natural graphite in magnetic fields up to 45 T, observing a series of peaks due to interband electronic excitations over a much broader magnetic field range than previously reported. We also explore electron-phonon coupling in graphite via magnetophonon resonances. The Raman $G$ peak shifts and splits as a function of magnetic field, due to the magnetically tuned coupling of the $E_{2g}$ optical phonons with the $K$- and $H$-point inter-Landau-level excitations. The analysis of the observed anticrossing behavior allows us to determine the electron-phonon coupling for both $K$- and $H$-point carriers. In the highest field range ($>$35 T) the $G$ peak narrows due to suppression of electron-phonon interaction.

The coherent spin dynamics of resident carriers, electrons, and holes in semiconductor nanostructures is studied theoretically under the conditions of periodical optical excitation using short laser pulses and in an external magnetic field. The generation and dephasing of spin polarization in an ensemble of carrier spins, for which the relaxation time of individual spins exceeds the repetition period of the laser pulses, are analyzed. Accumulation of the spin polarization is manifested either as resonant spin amplification or as mode locking of carrier spin coherences. It is shown that both regimes have the same origin, while their appearance is determined by the optical pump power and the spread of spin precession frequencies in the ensemble.

The $R$BaCo${}_4$O${}_7$ (R $=$ Ca, Y, Tb, Ho, Tm, Yb, Lu) compounds provide a novel topology for studying the competition between triangular geometry and magnetic order. Here, we report the structural and magnetic behavior of the Lu member of this series via neutron and synchrotron x-ray diffraction, magnetization, and resistivity measurements. We determined sequential phase transitions and a strong competition between a stable and a metastable low-temperature state that critically depends on controlled cooling rates and the associated heat removal kinetics. No evidence for long-range ordered magnetism was detected by neutron diffraction at any temperature. However, very slow spin dynamics are evidenced by time-dependent neutron diffraction measurements and can be explained by several competing magnetic phases with incommensurate short-range correlations coexisting in this material.

We provide a thorough study of a carbon divacancy, a point defect expected to have a large impact on the properties of graphene. Low-temperature scanning tunneling microscopy imaging of irradiated graphene on different substrates enabled us to identify a common twofold symmetry point defect. Our first-principles calculations reveal that the structure of this type of defect accommodates two adjacent missing atoms in a rearranged atomic network formed by two pentagons and one octagon, with no dangling bonds. Scanning tunneling spectroscopy measurements on divacancies generated in nearly ideal graphene show an electronic spectrum dominated by an empty-states resonance, which is ascribed to a nearly flat, spin-degenerated band of $\pi$-electron nature. While the calculated electronic structure rules out the formation of a magnetic moment around the divacancy, the generation of an electronic resonance near the Fermi level reveals divacancies as key point defects for tuning electron transport properties in graphene systems.

We present results of detailed experimental and theoretical studies of all-optical magnetization reversal by single circularly-polarized laser pulses in ferrimagnetic rare earth—transition metal (RE–TM) alloys Gd${}_x$Fe${}_{90-x}$Co${}_{10}$ ($20\%<x<28\%$). Using single-shot time-resolved magneto-optical microscopy and multiscale simulations, we identified and described the unconventional path followed by the magnetization during the reversal process. This reversal does not involve precessional motion of magnetization but is governed by the longitudinal relaxation and thus has a linear character. We demonstrate that this all-optically driven linear reversal can be modeled as a result of a two-fold impact of the laser pulse on the medium. First, due to absorption of the light and ultrafast laser-induced heating, the medium is brought to a highly nonequilibrium state. Simultaneously, due to the ultrafast inverse Faraday effect the circularly polarized laser pulse acts as an effective magnetic field of the amplitude up to $\sim$20 T. We show that the polarization-dependent reversal triggered by the circularly polarized light is feasible only in a narrow range (below 10%) of laser fluences. The duration of the laser pulse required for the reversal can be varied from $\sim$40 fs up to at least $\sim$1700 fs. We also investigate experimentally the role of the ferrimagnetic properties of GdFeCo in the all-optical reversal. In particular, the optimal conditions for the all-optical reversal are achieved just below the ferrimagnetic compensation temperature, where the magnetic information can be all-optically written by a laser pulse of minimal fluence and read out within just 30 ps. We argue that this is the fastest write-read event demonstrated for magnetic recording so far.

We report on the superconducting properties of $A_x$Fe${}_{2-y}$Se${}_2$ ($A$$=$Rb, K) single crystals studied with the muon spin relaxation or rotation ($\mu$SR) technique. At low temperatures, close to 90$\%$ of the sample volumes exhibit large-moment magnetic order which impedes the investigation of their superconducting properties by $\mu$SR. On the other hand, about 10$\%$ of the sample volumes remain paramagnetic and clearly show a superconducting response. The temperature dependence of the superconducting carrier density was analyzed within the framework of a single $s$-wave gap scenario. The zero-temperature values of the in-plane magnetic penetration depths ${\lambda }_{\mathrm{ab}}\left(0\right)=258\left(2\right)$ and 225(2) nm and the superconducting gaps $\Delta \left(0\right)=7.7\left(2\right)$ and 6.3(2) meV have been determined for $A$ $=$ Rb and K, respectively. The microscopic coexistence and/or phase separation of superconductivity and magnetism is discussed.

We perform first-principles calculations based on density functional theory of the spin-resolved conductance of poly(vinylidene fluoride)- (PVDF) based multiferroic tunnel junctions (MFTJs). We consider Co/PVDF/O/Co (0001) MFTJs with one oxidized interface, representing the different experimental growth conditions for the two interfaces. We demonstrate that this natural asymmetry leads to multiple resistance states associated with different magnetization configurations of the electrodes and ferroelectric polarization orientations of the barrier. Our results indicate very high tunability of the tunneling magnetoresistance and electroresistance effects, which could be useful for logic and memory applications.

We present a detailed analysis of spin-orbit coupling (SOC) in $\pi$-conjugated organic materials and its effects on spin characteristics including the spin-relaxation time, spin-diffusion length, and $g$ factor. While $\pi$ electrons are responsible for low-energy electrical and optical processes in $\pi$-conjugated organic solids, $\sigma$ electrons must be explicitly included to properly describe the SOC. The SOC mixes up- and down-spin states and, in the context of spintronics, can be quantified by an admixture parameter in the electron and hole polaron states in $\pi$-conjugated organics. Molecular geometry fluctuations such as ring torsion, which are common in soft organic materials and may depend on sample preparation, are found to have a strong effect on the spin mixing. The SOC-induced spin mixing leads to spin flips as polarons hop from one molecule to another, giving rise to spin relaxation and diffusion, which are examined by the time-dependent perturbation theory and density-matrix theory. The spin-relaxation rate is found to be proportional to the carrier hopping rate, or equivalently, carrier mobility. The spin-diffusion length depends on the spin mixing and hopping distance but is insensitive to the carrier mobility. An applied electric field causes spin drift and gives rise to upstream and downstream spin-diffusion lengths in the hopping-conduction regime. The SOC influences the $g$ factor of the polaron state and makes it deviate from the free-electron value. The deviation is due to the mixing of different orbitals in the polaron state, which does not include the spin mixing within the same orbital, and therefore underestimates the SOC strength. In particular, the $g$ factor is not sensitive to the molecular geometry fluctuations, where the spin mixing within the same orbital is dominant. The SOCs in tris-(8-hydroxyquinoline) aluminum (Alq${}_3$) and in copper phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu $3d$ orbitals in the latter. The theory quantitatively explains the recent measured spin-diffusion lengths in Alq${}_3$ from muon spin rotation and in CuPc from spin-polarized two-photon photoemission.

We discuss different approximations for effective low-energy interactions in multiband models for weakly correlated electrons. In the study of Fermi-surface instabilities of the conduction band(s), the standard approximation consists only keeping those terms in the bare interactions that couple only to the conduction band(s), while corrections due to virtual excitations into bands away from the Fermi surface are typically neglected. Here, using a functional renormalization group approach, we present an improved truncation for the treatment of the effective interactions in the conduction band that keeps track of the generated three-particle interactions (six-point term) and hence allows one to include important aspects of these virtual interband excitations. Within a simplified two-patch treatment of the conduction band, we demonstrate that these corrections can have a rather strong effect in parts of the phase diagram by changing the critical scales for various orderings and the phase boundaries.

In order to identify the gap structure of CeIrIn${}_5$, we measured field-angle-resolved specific heat $C\left(\phi \right)$ by conically rotating the magnetic field $H$ around the $c$ axis at low temperatures down to 80 mK. We revealed that $C\left(\phi \right)$ exhibits a fourfold angular oscillation, whose amplitude decreases monotonically by tilting $H$ out of the $ab$ plane. Detailed microscopic calculations based on the quasiclassical Eilenberger equation confirm that the observed features are uniquely explained by assuming the $d_{x^2-y^2}$-wave gap. These results strongly indicate that CeIrIn${}_5$ is a $d_{x^2-y^2}$-wave superconductor and suggest the universal pairing mechanism in Ce$M$In${}_5$ ($M=\text{Co}$, Rh, and Ir).

We show how the onset of a non-Slater antiferromagnetic ordering in a correlated material can be detected by optical spectroscopy. Using dynamical mean-field theory we identify two distinctive features: The antiferromagnetic ordering is associated with an enhanced spectral weight above the optical gap, and well-separated spin-polaron peaks emerge in the optical spectrum. Both features are indeed observed in LaSrMnO${}_4$ [A. Gössling et al., Phys. Rev. B 77, 035109 (2008)].

Although carbon nanotubes consist of honeycomb carbon, they have never been fabricated from graphene directly. Here, it is shown by quantum molecular-dynamics simulations and classical continuum-elasticity modeling, that graphene nanoribbons can, indeed, be transformed into carbon nanotubes by means of twisting. The chiralities of the tubes thus fabricated can be not only predicted but also externally controlled. This twisting route is an opportunity for nanofabrication, and is easily generalizable to ribbons made of other planar nanomaterials.

The superconductivity discovered in iron pnictides is intimately related to a nematic ground state, where the $C_4$ rotational symmetry is broken via the structural and magnetic transitions. We here study the nematicity in NaFeAs with polarization-dependent angle-resolved photoemission spectroscopy. A uniaxial pressure was applied on the sample to overcome the twinning effect in the low temperature $C_2$-symmetric state and obtain a much simpler electronic structure than that of a twinned sample. We found the electronic structure undergoes an orbital-dependent reconstruction in the nematic state, primarily involving the $d_{xy}$- and $d_{yz}$-dominated bands. These bands strongly hybridize with each other, inducing a band splitting, while the $d_{xz}$-dominated bands only exhibit an energy shift without any reconstruction. These findings suggest that the spin fluctuations at high temperatures and their coupling with the orbital degree of freedom could be the dominant force to drive the nematicity, while the ferro-orbital ordering between $d_{xz}$ and $d_{yz}$ orbitals can only play a minor role here.

Resonance diffraction in the periodic array of graphene microribbons is theoretically studied following a recent experiment [L. Ju et al., Nature Nanotech. 6, 630 (2011)]. Systematic studies over a wide range of parameters are presented. It is shown that a much richer resonant picture would be observable for higher relaxation times of charge carriers: More resonances appear and transmission can be totally suppressed. The comparison with the absorption cross-section of a single ribbon shows that the resonant features of the periodic array are associated with leaky plasmonic modes. The longest-wavelength resonance provides the highest visibility of the transmission dip and has the strongest spectral shift and broadening with respect to the single-ribbon resonance, due to collective effects.

We present computational evidence of polaronic hole trapping and migration in lithium peroxide (Li${}_2$O${}_2$), a material of interest in lithium-air batteries. We find that the hole forms in the ${\pi }^{*}$ antibonding molecular orbitals of the peroxide (O${}_2^{2-}$) anion, and that this trapped hole induces significant local lattice distortion, forming a polaron. Our study finds migration barriers for the free polaron to be between 68 and 152 meV, depending on the hopping direction. This low barrier suggests that this material might not be as insulating as previously assumed, provided that the formation of carriers can be achieved. One transport limitation may arise from lithium vacancies, which we find to strongly bind to the polaron. This result, in combination with previous experimental results, suggests that electronic conductivity in this material is likely to be determined by vacancy diffusion.

The interplay between spin transport and thermoelectricity offers several novel ways of generating, manipulating, and detecting nonequilibrium spin in a wide range of materials. Here, we formulate a phenomenological model in the spirit of the standard model of electrical spin injection to describe the electronic mechanism coupling charge, spin, and heat transport and employ the model to analyze several different geometries containing ferromagnetic (F) and nonmagnetic (N) regions: F, F/N, and F/N/F junctions, which are subject to thermal gradients. We present analytical formulas for the spin-accumulation and spin-current profiles in those junctions that are valid for both tunnel and transparent (as well as intermediate) contacts. For F/N junctions, we calculate the thermal spin-injection efficiency and the spin-accumulation-induced nonequilibrium thermopower. We find conditions for countering thermal spin effects in the N region with electrical spin injection. This compensating effect should be particularly useful for distinguishing electronic from other mechanisms of spin injection by thermal gradients. For F/N/F junctions, we analyze the differences in the nonequilibrium thermopower (and chemical potentials) for parallel and antiparallel orientations of the F magnetizations, as evidence and a quantitative measure of the spin accumulation in N. Furthermore, we study the Peltier and spin Peltier effects in F/N and F/N/F junctions and present analytical formulas for the heat evolution at the interfaces of isothermal junctions.

Spin-phonon coupling effects, as reflected in phonon frequency shifts between ferromagnetic (FM) and $G$-type antiferromagnetic (AFM) configurations in cubic CaMnO${}_3$, SrMnO${}_3$, BaMnO${}_3$, LaCrO${}_3$, LaFeO${}_3$, and La${}_2$(CrFe)O${}_6$, are investigated using density-functional methods. The calculations are carried out both with a hybrid-functional Heyd-Scuseria-Ernzerhof (HSE) approach and with a DFT + $U$ approach using a $U$ that has been fitted to HSE calculations. The phonon frequency shifts obtained in going from the FM to the AFM spin configuration agree well with those computed directly from the more accurate HSE approach, but are obtained with much less computational effort. We find that in the $A$MnO${}_3$ materials class with $A=$ Ca, Sr, and Ba, this frequency shift decreases as the $A$ cation radius increases for the $\Gamma$ phonons, while it increases for $R$-point phonons. In La$M$O${}_3$ with $M=$ Cr, Fe, and Cr/Fe, the phonon frequencies at $\Gamma$ decrease as the spin order changes from AFM to FM for LaCrO${}_3$ and LaFeO${}_3$, but they increase for the double perovskite La${}_2$(CrFe)O${}_6$. We discuss these results and the prospects for bulk and superlattice forms of these materials to be useful as multiferroics.