States In Ultrathin Films Research Articles

Ultrafast Activation and Tuning of Topological Textures in Ferroelectric Nanostructures

AbstractPolar topological textures are recently discovered in ultra‐thin ferroelectrics, including stripe, labyrinth, or bubble states. Enabling to shape on‐demand, these topologies can provide an original platform for potential applications. Here, using first‐principle‐based calculations, it is shown that THz electric field pulses are an efficient means for generating a plethora of topological states in ultrathin films made by a prototypical ferroelectric system, depending on the characteristics of these pulses and films. Consequently, these ultrafast electrical pulses in ultra‐thin ferroelectrics offer key ingredients for mimicking neuronal and synaptic behaviors, including all‐optical devices, and open a new avenue for topological objects to neuromorphic systems.

Distorted 3Q state driven by topological-chiral magnetic interactions

We demonstrate that topological-chiral magnetic interactions can play a key role for magnetic ground states in ultrathin films at surfaces. Based on density functional theory we show that significant chiral-chiral interactions occur in hexagonal Mn monolayers due to large topological orbital moments which interact with the emergent magnetic field. Due to the competition with higher-order exchange interactions superposition states of spin spirals such as the 2Q state or a distorted 3Q state arise. Simulations of spin-polarized scanning tunneling microscopy images suggest that the distorted 3Q state could be the magnetic ground state of a Mn monolayer on Re(0001).

Mapping magnetization states in ultrathin films with Dzyaloshinskii–Moriya interaction

To succeed in the next generation of magnetic storages, nanomagnetic structures must be engineered to allow modulation of magnetization amplitude and spatial period. We demonstrate computationally how magnetic nanostructures states (domains with narrow wall, skyrmions, spin spirals, conical spin spirals, and in-plane magnetization configuration) can be designed in ultrathin films with a Dzyaloshinskii–Moriya interaction (DMI) by adjusting two material parameters: perpendicular magnetic anisotropy characterized by the quality factor Q, and reduced DMI constant . For a broad range of Q and parameters, the magnetization states are mapped in (Q, ) diagrams and characterized by the periodicity (p) of spatial distribution of magnetization and the mean value of the square of an out-of-plane normalized magnetization component . We explain the transitions between different magnetization states and describe phase transition curves in terms of vanishing of the domain wall energy for both and Q > 1 cases. We show that by changing and/or Q and approaching the phase transition curves, the discontinuous transitions accompanied by jumps of both period and amplitude take place.

Correlated electronic states of SrVO3 revealed by angle-resolved photoemission spectroscopy

In this article, we review recent progress in angle-resolved photoemission (ARPES) studies of the Mott-Hubbard-type correlated electron systems SrVO3. It has the d1 electron configuration and is an ideal model compound to study electron correlation effects in normal metal. ARPES studies of bulk single-crystal SrVO3 and CaVO3 have revealed the difference in the mass renormalization of electrons between them. In-situ ARPES studies of thin films fabricated by the pulsed laser deposition method have clarified not only quasi-particle dispersions, which show a kink like high-Tc cuprates, but also finite dispersions in the incoherent part. Self-energy in a wide energy range has been deduced from the ARPES spectral weight using Kramers–Kronig transformation. The obtained self-energy has several structures which yield the incoherent feature and a pseudogap-like dip similar to the high-Tc cuprates. Quantum-well states in ultrathin films of SrVO3 have revealed sub-bands with correlated electrons. These findings of electron correlation effects outlined in the present article would provide a starting point not only for fundamental condensed-matter physics but also for the development of new devices with correlated electrons.

Electrodynamics of the Superconducting State in Ultra-Thin Films at THz Frequencies

We report on terahertz frequency-domain spectroscopy (THz-FDS) experiments in which we measure charge carrier dynamics and excitations of thin-film superconducting systems at low temperatures in the THz spectral range. The characteristics of the set-up and the experimental procedures are described comprehensively. We discuss the single-particle density of states and a theory of electrodynamic absorption and optical conductivity of conventional superconductors. We present the experimental performance of the setup at low temperatures for a broad spectral range from 0.1 - 1.1 THz by the example of ultra-thin films of weakly disordered superconductors niobium nitride (NbN) and tantalum nitride (TaN) with different values of critical temperatures. Furthermore, we analyze and interpret our experimental data within the framework of conventional Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity.

Continuous Theory of Ferroelectric States in Ultrathin Films with Real Electrodes

According to a continuous medium theory, in very thin ferroelectric films with real metallic electrodes (or dead layers near the electrodes) the domain structure reduces to sinusoidal distribution of ferroelectric polarization. Such a sinusoidal structure was considered in 1980s for para-ferroelectric phase transition in a capacitor with dead layers near electrode. We give a review of this theory and its further development for the case of real metallic electrodes. The goal of the general theory is to consistently interpret the experimental data in very thin films with real metallic electrodes. This is illustrated on a recent experimental data for 5-30 nm BaTiO3 films with SrRuO3/SrTiO3 electrodes. The screening length by real metallic electrodes is very small small (<1A), but it has a profound effect on ferroelectric properties and its phase behavior. This general theory also allows to formulate the important open problems and show paths towards their solution. In particular, this is a problem of finding parameters of the system, which can sustain the ferroelectric memory over a desired lifetime.

Electrostatic modification of the conductive properties of amorphous Bi ultrathin films

The application of the field-effect transistor principle to novel materials to achieve electrostatic doping is a relatively new research area with roots that go back to the turn of the 20th century. The technique in principle provides the opportunity to modify the electronic and magnetic properties of materials through controlled and reversible changes in carrier concentration, without altering the degree of disorder or the chemical composition. Electrostatic doping can also serve as a tool for studying quantum critical behavior, by allowing the ground state of a system to be tuned in a controlled fashion. This is precisely what has been done in tuning the transition between insulating and superconducting ground states of ultrathin films of amorphous bismuth.

Simple models for electron and spin transport in barrier–conductor–barrier devices

We use two simple models, a phase accumulation model for quantum well (QW) resonances and a two-current circuit model, to study electron and spin transport in nanoscale systems consisting of barrier–conductor–barrier structures. Such structures include double barrier tunnel junctions, and long chain molecules with weak contacts to two electrodes. The phase accumulation model is applied to QW states in ultrathin films and nanodots sandwiched between two barriers. This model is used to illustrate the difference in the QW confinement effect between thin films and nanodots, and justifies circuit model which describes sequential tunneling. The circuit model for spin transport leads to a generalization of the well-known Julliere’s formula and accounts for the conductivity mismatch. The circuit model is also applied to organic light-emitting diodes and yields a simple formula for the quantum efficiency. The results yield excellent agreement with experiment.

Simulation of spin-polarized scanning tunneling microscopy images of nanoscale non-collinear magnetic structures

We apply an efficient method to calculate spin-polarized scanning tunneling microscopy (SP-STM) images of nanostructures with complex non-collinear magnetic order. The model is based on the spin-polarized version of the Tersoff–Hamann model of STM and the independent orbital approximation for the surface electronic structure. For its application, only the knowledge of the arrangement of the magnetic moments of the surface atoms is required. In spite of its simplifications, calculated SP-STM images of periodic collinear and non-collinear magnetic spin structures are in many cases in excellent agreement with those obtained from computationally much more demanding ab initio calculations. Especially for surfaces of chemically equivalent atoms, the atomic scale SP-STM images are dominated by the magnetic structure and depend much less on the accurate electronic structure. This suggests the application of the method to more complex non-collinear magnetic structures such as domain walls in antiferromagnets, spin-spiral states, spin glasses, or disordered states. Based on the model, we predict SP-STM images of helical spin-spiral states in ultra-thin films.

Modification of Surface States in Ultrathin Films via Hybridization with the Substrate: A Study of Ag on Ge

The Shockley surface state of Ag(111) develops unusual band dispersion relations for Ag films of decreasing thicknesses on Ge(111), as observed by angle-resolved photoemission. Its parabolic dispersion in the thick-film limit shifts toward higher binding energies and splits into multiple bands with dispersions that reflect the valence band structure of Ge including the heavy-hole, light-hole, and split-off bands. The results are explained in terms of a hybridization interaction between the Ag surface state and the Ge substrate states.

Si(111)8x2-InおよびSi(111)7x7上Ag薄膜の量子井戸状態の研究

Si(111)8x2-InおよびSi(111)7x7上Ag薄膜の量子井戸状態の研究

Confinement of Shockley states in ultra thin films of Ag on Cu(111)

Shockley surface states have been studied in Ag epitaxial ultra-thin films on Cu(111) by angle-resolved photoemission (ARPES), scanning tunneling microscopy (STM) and spectroscopy (STS). The morphology of these films is characterized by large Ag islands growing along the step edges of the substrate. In the submonolayer range, the two spectroscopic techniques indicate the existence of two surface states associated with the Cu terraces and the Ag islands suggesting the confinement of the electronic states. Moreover, two different atomic reconstructions depending on the preparation temperature have been evidenced and lead to different surface state energies.

Monte Carlo simulation of the reorientation transition in Heisenberg model with dipole interactions

The change of magnetic states in ultrathin films with temperature have been simulated by Monte Carlo method. A Heisenberg model with long-range dipole interactions was adopted in our calculations. The results were qualitatively in good agreement with the experimental phenomena. That is at low temperatures the magnetization is perpendicular to the plane, and at higher temperatures but below the Curie point, the magnetization is mostly within the plane. In between these two regions, the magnetization seems to be suppressed. The simulations show that the loss of magnetization is a consequence of the special magnetic states in which the local domains orientations are reverse with the neighbor ones.

Temperature dependence of photoemission from quantum-well states in Ag/V(100): Moving surface-vacuum barrier effects

The temperature dependence of angle-resolved photoemission from quantum-well states in ultrathin films of Ag on V(100) has been examined for films from 1--8 ML thickness within the temperature range 45--600 K. Contrary to bulk solids, the photoemission peaks shift to higher binding energy as the temperature is increased. The temperature dependence of the peak widths is linear, consistent with the expected behavior for electron-phonon coupling, but the coupling parameter \ensuremath{\lambda} is found to show a strong oscillatory dependence on film thickness, with some values many times larger than those found for bulk silver. The observations are explained in terms of the influence on both the initial and final states in the photoemission process of the static and dynamic movements of the surface-vacuum interface barrier induced by temperature changes.

Interatomic Resonant Photoemission from Quantum-Well States in Ultrathin Films of Ag on V(100)

Normal emission photoemission spectra from a single ordered monolayer of Ag on V(100) show a sharp and intense peak associated with a quantum-well state. The photon energy dependence of the intensity of this peak shows an identical enhancement profile to the V $3d$ photoemission in the vicinity of the V $3p\ensuremath{\rightarrow}3d$ excitation threshold, characteristic of resonant photoemission. The significance of this result for resonant photoemission characterization of materials is discussed.

Magnetization states in ultrathin films with laterally modulated anisotropies

Micromagnetic theory has been applied to a model system for ultrathin films with laterally modulated anisotropies, consisting of a periodic array of stripes with alternating uniaxial anisotropies. Despite of the discontinuous change of anisotropy, the magnetization direction only changes on a lateral scale given by the exchange length. If the width of one of the two alternating stripes is reduced below a critical value, the magnetization will switch into a uniform state. The variation of the critical width with period, anisotropy constants, exchange constant, and film thickness has been determined using an analytic approach. Non-uniform magnetization states and the magnetization reversal in an external field have been determined using numerical calculations. The results are compared with experimental results for Fe/W(1 1 0)-films.

Theory for the nonlinear optical response of quantum-well states in ultrathin films

We analyze second-harmonic generation (SHG) oscillations due to quantum-well states in ultrathin overlayer films and their dependence on the electronic and magnetic structure of the film and substrate. Depending on the SHG contribution from the surface and the film-substrate interface, and the interplay between the wave-function symmetry (via the dipole matrix elements) and band-structure effects, one obtains different SHG oscillations as a function of film thickness. One may obtain only one period, which is twice that observed in the linear magneto-optical Kerr effect ${\ensuremath{\Lambda}}_{M},$ but also ${\ensuremath{\Lambda}}_{M}$ and an additional, larger period. Thus we explain, within a unified approach, recent experiments on $x$-Au/Co(0001)/Au(111) and $x$-Cu/Fe/Cu(001) films, where different characteristic features of the SHG oscillations and different oscillation periods were observed.

Electronic structure of silver and copper ultrathin films on V(100): Quantum-well states.

Angular-resolved photoemission and inverse-photoemission spectroscopies have been used to investigate the valence-electron states in ultrathin films of silver and copper deposited on a V(100) surface. For both noble metals, discrete s-p derived states are observed within the ${\mathrm{\ensuremath{\Delta}}}_{1}$ gap of the vanadium substrate (approximately \ifmmode\pm\else\textpm\fi{}2 eV around ${\mathit{E}}_{\mathit{F}}$). These states are analyzed using a simple quantum-well picture. For a pseudomorphically grown (centered tetragonal) silver film in the bulklike limit we have determined ${\mathit{k}}_{\mathit{F}}$ (1.19 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$) and the energies of critical points, ${\mathit{X}}_{1}$ (7.60 \ifmmode\pm\else\textpm\fi{} 0.15 eV) and ${\mathit{X}}_{{4}^{\ensuremath{'}}}$ (2.5 \ifmmode\pm\else\textpm\fi{} 0.3 eV) in the E(k) dispersion of the ${\mathrm{\ensuremath{\Delta}}}_{1}$ band in the \ensuremath{\Gamma}-\ensuremath{\Delta}-X direction. The bottom of the ${\mathrm{\ensuremath{\Delta}}}_{1}$ band, i.e., ${\mathrm{\ensuremath{\Gamma}}}_{1}$ point, was estimated to be -6.4\ifmmode\pm\else\textpm\fi{}0.3 eV by fitting the experimentally determined points with a free-electron parabola. In the case of copper overlayers, it was not possible to determine the dispersion of the bulklike ${\mathrm{\ensuremath{\Delta}}}_{1}$ band because Cu films thicker than two monolayers showed poor order. At low coverages (1--2 ML) of both silver and copper, we find that dispersion in ${\mathit{k}}_{\mathrm{\ensuremath{\parallel}}}$ of the discrete s-p quantum-well states is described by a significantly enhanced electron effective mass (${\mathit{m}}^{\mathrm{*}}$g2${\mathit{m}}_{\mathit{e}}$). This is interpreted as due to strong hybridization of these states with the d derived states of the vanadium substrate. \textcopyright{} 1996 The American Physical Society.

Field-induced metastable states in ultrathin films of fcc Fe/Cu(100)

The thickness dependence of both the perpendicular and in-plane magnetization is observed for pseudomorphic ultrathin, fcc Fe epitaxial films grown on room temperature Cu(100). Ferromagnetically ordered 3.5-monolayer-thick films display both in-plane and perpendicular remanence. Perpendicular remanence, lost after a 9.0 kOe static field is applied perpendicular to the film plane, can be restored by either heating or applying large in-plane fields. These field-induced metastable states are interpreted in terms of magnetoelastic effects which modify the exchange and anisotropy constants both perpendicular to and within the film plane.

Magnetic Properties of Superconducting Films

The general Gor'kov equations are solved for a superconducting film in a parallel magnetic field. The method determines the best pairing in the superconducting state without the need for ad hoc assumptions about pairing such as are used in other theories. The critical field ${H}_{c}(T)$ and energy gap $\ensuremath{\Delta}(H, T)$ are determined for temperatures near the transition temperature at zero field ${T}_{c}$. The energy gap in the quasiparticle excitation spectrum is shown to be approximately equal to the spatial average of the order parameter. For films whose thickness $d$ is less than the coherence length ${\ensuremath{\xi}}_{T}$ the Gor'kov equations are nonlocal and differ from the Ginzburg-Landau (GL) equations. In this range we find ${H}_{c}\ensuremath{\propto}{d}^{\ensuremath{-}\frac{3}{2}}$ in agreement with experiment. For films with $d&gt;{\ensuremath{\xi}}_{T}$ the solution of the Gor'kov equations are the same as the GL results, as expected, since this is a local regime. We find that for all $d$ (excepting ultrathin films) and in the temperature range $(1\ensuremath{-}\frac{T}{{T}_{c}})\ensuremath{\ll}1$ the field dependence of the energy gap is the same as that given by the GL equations, i.e, $\frac{\ensuremath{\Delta}(H)}{\ensuremath{\Delta}(0)}={{1\ensuremath{-}{(\frac{H}{{H}_{c}})}^{2}}}^{\frac{1}{2}}$. Thus, nonlocal effects do not change the field dependence of the gap. Most of the experimental data are in accord with this equation. However, some recent results for aluminum films show deviations which we interpret as probably being due to the important role played by energy-level quantization of single-particle states in ultrathin films. The extension of the method to lower temperatures and higher fields is also discussed.