Spectra of the differential tunneling conductivity for ultrathin lead films grown on Si(111)7x7 single crystals with a thickness from 9 to 50 monolayers have been studied by low-temperature scanning tunneling microscopy and spectroscopy. The presence of local maxima of the tunneling conductivity is typical for such systems. The energies of maxima of the differential conductivity are determined by the spectrum of quantum-confined states of electrons in a metallic layer and, consequently, the local thickness of the layer. It has been shown that features of the microstructure of substrates, such as steps of monatomic height, structural defects, and inclusions of other materials covered with a lead layer, can be visualized by bias-modulation scanning tunneling spectroscopy.
We present the observation of strain induced sign reversal of anisotropic magnetoresistance (AMR) in LCMO (LCMO) ultrathin films (thickness 4 nm) deposited on STO (001) substrate (STO). We have also observed unusually large AMR in LCMO/STO thin films with thickness of 6 nm below but close to its Curie temperature (TC) which decrease as the film thickness increases. The sign reversal of AMR (with a maximum value of - 6) with magnetic field or temperature for the 4 nm thin film may be attributed to the increase in tensile strain in the plane of the thin film which in turn facilitates the rotatio…
We report the epitaxial growth of PdTe$_2$ ultrathin films on topological insulator Bi$_2$Se$_3$. A prominent Moiré pattern was observed in STM measurements. The Moiré periodicity increases as film thickness decreases, indicating a lattice expansion of epitaxial PdTe$_2$ thin films with lower thicknesses. In addition, our simulations based on Moiré Metrology reveal uniaxial lattice strains at the edge of PdTe$_2$ domains, and anisotropic strain distributions throughout the Moiré supercell with a net change in lattice strain up to ~2.9%. Our DFT calculations show that this strain effect leads t…
We present a detailed study of the ground-state magnetic structure of ultrathin Fe films on the surface of fcc Ir(001). We use the spin-cluster expansion technique in combination with the relativistic disordered local moment scheme to obtain parameters of spin models and then determine the favored magnetic structure of the system by means of a mean field approach and atomistic spin dynamics simulations. For the case of a single monolayer of Fe we find that layer relaxations very strongly influence the ground-state spin configurations, whereas Dzyaloshinskii-Moriya (DM) interactions and biquadr…
In this paper, a method is presented to create and characterize mechanically robust, free standing, ultrathin, oxide films with controlled, nanometer-scale thickness using Atomic Layer Deposition (ALD) on graphene. Aluminum oxide films were deposited onto suspended graphene membranes using ALD. Subsequent etching of the graphene left pure aluminum oxide films only a few atoms in thickness. A pressurized blister test was used to determine that these ultrathin films have a Young's modulus of 154 \pm 13 GPa. This Young's modulus is comparable to much thicker alumina ALD films. This behavior indic…
Well-ordered ultrathin films of NiO have been prepared on an Ag(001) substrate using molecular beam epitaxy. With the help of angle-resolved two-photon photoemission (2PPE) a series of image potential states (IPS) for film thicknesses of 2--4 monolayers (ML) has been identified. By time-resolved 2PPE, the lifetimes of the first three IPS and their dependence on the oxide film thickness have been determined. While the lifetimes of the (n=1) IPS are all in the range of 27--42 fs, the values for the (n=2) IPS decrease from 85 fs for 2 ML to 33 fs for 4 ML. These differences are discussed in terms…
We show that it is possible to deposit, by mechanical exfoliation on SiO2/Si wafers, atomically thin mica flakes down to a single monolayer thickness. The optical contrast of these mica flakes on top of a SiO2/Si substrate, which depends on their thickness, the illumination wavelength and the SiO2 substrate thickness, can be quantitatively accounted for by a Fresnel law based model. The preparation of atomically thin insulating crystalline sheets will enable the fabrication of ultrathin defect-free insulating substrates, dielectric barriers or planar electron tunneling junctions. Additionally,…
We discuss the optical response peculiarities for ultrathin plasmonic films of finite lateral size. Due to their plasma frequency spatial dispersion caused by the spatial confinement of the electron motion, the film dielectric permittivity tensor is spatially dispersive as well and so nonlocal. Such a confinement induced nonlocality can result in peculiar magneto-optical effects. For example, the frequency dependence of the magnetic permeability of the film exhibits a sharp resonance structure shifting to the red as the film aspect ratio increases. The properly tuned ultrathin plasmonic films …
We have investigated planar metal - insulator - metal tunnel junctions with aluminum oxide as dielectricum. These oxide barriers were grown on an aluminum electrode in pure oxygen at room temperature till saturation. We observed discrete barrier widths separated by Δs \approx 0.38 nm, corresponding to the addition of one layer of oxygen atoms. The minimum thickness of s_0 \approx 0.54 nm is due to a double layer of oxygen. We found a strong and systematic dependence of the barrier height Φ_0 on the thickness s like Φ_0 \approx 2.5 eV / s^2(nm), which nearly coincides with the kinetic electron …
We present room temperature ferromagnetic resonance (FMR) studies of polycrystalline ||Pt/10 nm Cu/t Co/10 nm Cu/Pt|| films as a function of Co layer thickness (1 < t < 10 nm) grown by evaporation and magnetron sputtering. FMR was studied with a high frequency broadband coplanar waveguide (up to 25 GHz) using a flip-chip method. The resonance field and the linewidth were measured as a function of the ferromagnetic layer thickness. The evaporated films exhibit a lower magnetization density (Ms = 1131 emu/cm^3) compared to the sputtered films (Ms= 1333 emu/cm^3), with practically equal perpendic…
In quantum matters hosting electron-electron correlation and spin-orbit coupling, spatial inhomogeneities, arising from competing ground states, can be essential for determining and understanding topological properties. A prominent example is Hall anomalies observed in SrRuO3 films, which were interpreted in terms of either magnetic skyrmion-induced topological Hall effect (THE) or inhomogeneous anomalous Hall effect (AHE). To clarify this ambiguity, we systematically investigated the AHE of SrRuO3 ultrathin films with controllable inhomogeneities in film thickness (tSRO). By harnessing the st…
We have studied the magneto-optical spectra of ultrathin magnetic films deposited on Si substrates coated with an oxide layer (SiOx). We find that the Kerr rotation angle and the ellipticity of ~1 nm thick CoFeB thin films, almost transparent to visible light, show a strong dependence on the thickness of the SiOx layer. The Kerr signal from the 1 nm CoFeB thin film can be larger than that of ~100 nm thick CoFeB films for a given SiOx thickness and light wavelength. The enhancement of the Kerr signal occurs when optical interference takes place within the SiOx layer. Interestingly, under such r…
The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)/Alq3/Si hybrid structures was systematically studied by means of atomic force microscopy, SQUID magnetometry and nuclear magnetic resonance (NMR). All used techniques pointed out that the barrier thickness of 2 nm is required to obtain a magnetically good cobalt layer on top of Alq3. 59Co NMR measurements revealed that the AlOx barrier gives rise to the formation of an interface layer with "defective" cobalt favouring growth of "bulk" cobalt with good magnetic properties.
Ultrathin FeSe films grown on SrTiO$_{3}$ substrates are a recent milestone in atomic material engineering due to their important role in understanding unconventional superconductivity in Fe-based materials. Using femtosecond time- and angle-resolved photoelectron spectroscopy, we study phonon frequencies in ultrathin FeSe/SrTiO$_{3}$ films grown by molecular beam epitaxy. After optical excitation, we observe periodic modulations of the photoelectron spectrum as a function of pump-probe delay for 1 unit cell, 3 unit cell, and 60 unit cell thick FeSe films. The frequencies of the coherent inten…
We use Ca doping during growth of one and two monolayer thick MgO films on Ag(100) to identify the adsorption sites of individual adatoms with scanning tunneling microscopy. For this we combine atomic resolution images of the bare MgO layer with images of the adsorbates and the substitutional Ca atoms taken at larger tip-sample distance. For Ho atoms, the adsorption sites depend on MgO thickness. On the monolayer, they are distributed on the O and bridge sites according to the abundance of those sites, 1/3 and 2/3 respectively. On the MgO bilayer, Ho atoms populate almost exclusively the O sit…
We study theoretically confinement related effects in the optical response of thin plasmonic films of controlled variable thickness. While being constant for relatively thick films, the plasma frequency is shown to acquire spatial dispersion typical of two-dimensional materials such as graphene, gradually shifting to the red with the film thickness reduction. The dissipative loss, while decreasing at any fixed frequency, gradually goes up at the plasma frequency as it shifts to the red with the film thickness reduced. These features offer a controllable way to tune spatial dispersion and relat…
There is an ever increasing interest in the development of plasmonic 2D nanomaterials, with widespread applications in optoelectronics, high resolution microscopy, imaging and sensing, among others. With the current ability of ultrathin noble metal film deposition down to a few monolayers in thickness, there is a need for an analytical expression of the thickness dependent complex dielectric function for predicting optical properties for arbitrary thicknesses. The free and bound electron contributions to the dielectric function are dealt with independently, since their influences affect separa…
We study transport and optical properties of the surface states which lie in the bulk energy gap of a thin-film topological insulator. When the film thickness is comparable with the surface state decay length into the bulk, the tunneling between the top and bottom surfaces opens an energy gap and form two degenerate massive Dirac hyperbolas. Spin dependent physics emerges in the surface bands which are vastly different from the bulk behavior. These include the surface spin Hall effects, spin dependent orbital magnetic moment, and spin dependent optical transition selection rule which allows op…
The inverse spinel ferrimagnetic NiCo$_2$O$_4$ possesses high magnetic Curie temperature $T_C$, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo$_2$O$_4$ thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo$_2$O$_4$ films on MgAl$_2$O$_4$ substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films …
Thermal transport by phonons in films with thicknesses of less than 10 nm is investigated in a soft system (Lennard-Jones argon) and a stiff system (Tersoff silicon) using two-dimensional lattice dynamics calculations and the Boltzmann transport equation. This approach uses a unit cell that spans the film thickness, which removes approximations related to the finite cross-plane dimension required in typical three-dimensional-based approaches. Molecular dynamics simulations are performed to obtain finite-temperature structures for the lattice dynamics calculations and to predict thermal conduct…
The physics of electrons, photons, and their plasmonic interactions changes greatly when one or more dimensions are reduced down to the nanometer scale. For example, graphene shows unique electrical, optical, and plasmonic properties, which are tunable through gating or chemical doping. Similarly, ultrathin metal films (UTMFs) down to atomic thickness can possess new quantum optical effects, peculiar dielectric properties, and predicted strong plasmons. However, truly two-dimensional plasmonics in metals has so far elusive because of the difficulty in producing large areas of sufficiently thin…
All-optical control dynamics of magnetization in sub-10 nm metallic thin films are investigated, as these films with quantum confinement undergo unique interactions with femtosecond laser pulses. Our theoretical derivations based on the free electron model show that the density of states at Fermi level (DOS_F) and electron-phonon coupling coefficients (G_ep) in ultrathin metals have very high sensitivity to film thickness within a few Angstroms. As DOS_F and G_ep depend on thickness, we show that completely different magnetization dynamics characteristics emerge compared with bulk metals. Our …
Layered quasi-two-dimensional transition metal dichalcogenides (TMDCs), which can be readily made in ultrathin films, offer excellent opportunities for studying how dimensionality affects electronic structure and physical properties. Among all TMDCs, NbSe2 is of special interest; bulk NbSe2 hosts a charge-density-wave phase at low temperatures and has the highest known superconducting transition temperature, and these properties can be substantially modified in the ultrathin film limit. Motivated by these effects, we report herein a study of few-layer NbSe2 films, with a well-defined single-do…
High-T_c superconductors confined to two dimension exhibit novel physical phenomena, such as superconductor-insulator transition. In the Bi_2Sr_2CaCu_2O_8+x (Bi2212) model system, despite extensive studies, the intrinsic superconducting properties at the thinness limit have been difficult to determine. Here we report a method to fabricate high quality single-crystal Bi2212 films down to half-unit-cell thickness in the form of graphene/Bi2212 van der Waals heterostructure, in which sharp superconducting transitions are observed. The heterostructure also exhibits a nonlinear current-voltage char…
Various-thickness Ag islands were prepared on Si(111)-7$\times$7 using the one-step deposition at a high substrate temperature. An electronic state centered at -0.40$\sim$-0.15eV versus E$_{Fermi}$, detectable on the surface of the Ag islands thinner than 9 layers, was created by the electronic hybridization between Ag and Si at the Ag-Si interface. Scanning tunneling microscopy/spectroscopy and density functional theory revealed that the thickness of Ag islands determined the strength of the hybridization, leading to a modulation to the energy and intensity of the state on the surface.
For a ultrathin NiO(001) film of 4 monolayers (ML) thickness grown on Ag(001), the vibrational properties have been determined by high-resolution electron energy loss spectroscopy (HREELS). For the well-ordered pseudomorphically grown film, nine phonon modes have been identified and their dispersions have been revealed along the Gamma-X high-symmetry direction. The comparison with phonon data for a 25 ML thick NiO(001) film shows that the NiO (001) phonon properties are already fully developed at 4 ML. Significant differences are found for the surface-localized phonon S6 which has an increased…
We address, by means of ab-initio calculations, the origin of the correlation that has been observed experimentally between the chemisorption energy of CO on nanoscale Cu(001) supported films and quantum-size effects. The calculated chemisorption energy shows systematic oscillations, as a function of film thickness, with a periodicity corresponding to that of quantum-well states at the surface-Brillouin-zone center crossing the Fermi energy. We explain this trend based on the oscillations, with film thickness, of the decay length on the vacuum side of the quantum-well states at the Fermi energ…
Novel quantum phenomena, including high-temperature superconductivity, topological properties, and charge/spin density waves, appear in low-dimensional conductive materials. It is possible to artificially create low-dimensional systems by fabricating ultrathin films, quantum wires, or quantum dots with flat interfaces. Some experiments have been performed on ultrathin compounds of strongly correlated electron systems. However, since it is technically difficult to control multiple elements precisely, most of the properties of artificially fabricated low-dimensional compounds fall into uncharted…
We report broadband ferromagnetic resonance (FMR) measurements based on a coplanar waveguide (CPW) of ultrathin magnetic film structures that comprise in-plane/out-of-plane decoupled layers deposited on nonmagnetic buffer layers of various thickness or other buffer structures with a diverse sheet resistance. We show that the excitation of the fundamental mode can be substantially (up to 10 times) enhanced in the structures deposited on buffer layers with a low sheet resistance in comparison to the structures deposited on thin or weakly conducting buffer layers. The results are analyzed in term…
We studied the spectral dynamics of single fluorescent dye molecules embedded in ultrathin films (5 - 100 nm) of the amorphous polymer polyisobutylene at cryogenic temperatures and its variation with film thickness. Noticeable portion of molecules in the ensemble show a behavior which is inconsistent with the standard tunneling model: Their spectral lines are subject to irreversible spectral jumps, continuous shifting, and abrupt chaotic changes of the linewidth or jumping rate. In films thinner than 100 nm, the occurrence of "non-standard" spectral behavior increases with decreasing sample th…
There is an attractive Casimir-Lifshitz force between two silica surfaces in a liquid (bromobenze or toluene). We demonstrate that adding an ultrathin (5-50Å) metallic nanocoating to one of the surfaces results in repulsive Casimir-Lifshitz forces above a critical separation. The onset of such quantum levitation comes at decreasing separations as the film thickness decreases. Remarkably the effect of retardation can turn attraction into repulsion. From that we explain how an ultrathin metallic coating may prevent nanoelectromechanical systems from crashing together.
The superconductor PdTe$_2$ is known to host bulk Dirac bands and topological surface states. The coexistence of superconductivity and topological surface states makes PdTe$_2$ a promising platform for exploring topological superconductivity and Majorana bound states. In this work, we report the layer-by-layer molecular beam epitaxy growth and spectroscopic characterization of high quality PdTe$_2$ films with thickness down to 3 monolayers (ML). In the 3 ML PdTe$_2$ film, we observed spin-polarized surface resonance states, which are isolated from the bulk bands due to the quantum size effects…
In this paper, we show that ultrathin GaN membranes having a thickness of 15 nm and planar dimensions of 12x184 microns act as memristive devices. This fact is due to the migration of the negatively charged deep traps, which form in the volume of the membrane during the fabrication process, towards the unoccupied surface states of the suspended membranes. The time constant of the migration process is of the order of tens of second and varies with the current or voltage sweep.
A film-coupled metamaterial structure is numerically investigated for enhancing the light absorption in an ultrathin photovoltaic layer of crystalline gallium arsenide (GaAs). The top subwavelength concave grating and the bottom metallic film could not only effectively trap light with the help of wave interference and magnetic resonance effects excited above the bandgap, but also practically serve as electrical contacts for photon-generated charge collection. The energy absorbed by the active layer is greatly enhanced in the film-coupled metamaterial structure, resulting in significant enhance…
The critical current density $J_{c0}$ required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (STT-MRAM) cell is proportional to the product of the damping parameter, saturation magnetization and thickness of the free layer, $αM_S t_F$. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of sub-monolayer thickness, we have reduced $t_F$ while minimizing $α$ and maximizing $M_S$, ultimately leading to lower $J_{c0}…
We report on a combined structural and electronic analysis of niobium ultrathin films (from 2 to 10 nm) deposited in ultra-high vacuum on atomically flat R-plane sapphire wafers. A textured polycrystalline morphology is observed for the thinnest films showing that hetero-epitaxy is not achieved under a thickness of 3.3nm, which almost coincides with the first measurement of a superconducting state. The superconducting critical temperature rise takes place on a very narrow thickness range, of the order of a single monolayer (ML). The thinnest superconducting sample (3 nm/9ML) has an offset crit…
Bulk 1T$^\prime$-MoTe$_2$ shows a structural phase transition from 1T$^\prime$ to Weyl semimetallic (WSM) $ T_{d} $ phase at $\sim$ 240 K. This phase transition and transport properties in the two phases have not been investigated on ultra-thin crystals. Here we report electrical transport, $1/f$ noise and Raman studies in ultra-thin 1T$^\prime$-MoTe$_2$ ($\sim$ 5 to 16 nm thick) field-effect transistors (FETs) devices as a function of temperature. The electrical resistivities for thickness 16 nm and 11 nm show maxima at temperatures 208 K and 178 K, respectively, making a transition from semi…
The ability to break Kirchhoff s law is of fundamental importance in thermal radiation. Various nonreciprocal emitters have been proposed to break the balance between absorption and emission. However, the thicknesses of the nonreciprocal materials are usually larger than 1/10 times of the wavelength. Besides, the previous proposed nonreciprocal emitters are complex, thus they can hardly be fabricated in experiment to verify the Kirchhoff s law for nonreciprocal materials. In this paper, we investigate the nonreciprocal thermal radiation of the magnetic Weyl semimetal (MWSM) film atop of the me…
Observation of the surface hybridization gap in the electrical transport properties of the ultrathin topological insulator (Bi$_{1-x}$Sb$_{x}$)$_2$Te$_3$
The reflectance difference (RD) as a function of film thickness was measured during Pb deposition on Si(111)-(6x6)Au surface at 105 K. The oblique incident s- and p-polarized light with energy range 0.25-0.60 eV was used. The component of the dielectric function tensor parallel to the surface was determined from the data of experiments and it oscillation due to quantum size effects (QSE) with period of 2 ML within the Pb monolayer-by-monolayer growth was observed.
Interfacial electron-phonon coupling in ultrathin films has attracted much interest; it can give rise to novel effects and phenomena, including enhanced superconductivity. Here we report an observation of strong kinks in the energy dispersions of quantum well states in ultrathin Yb films grown on graphite. These kinks, arising from interfacial electron-phonon coupling, are most prominent for films with a preferred ("magic") thickness of 4 monolayers, which are strained and hole doped by the substrate. The energy position of the kinks agrees well with the optical phonon energy of graphite, and …
We present a method for fabricating Josephson junctions and superconducting quantum interference devices (SQUIDs) which is based on the local anodization of niobium strip lines 3 to 6.5 nm-thick under the voltage-biased tip of an Atomic Force Microscope. Microbridge junctions and SQUID loops are obtained either by partial or total oxidation of the niobium layer. Two types of weak link geometries are fabricated : lateral constriction (Dayem bridges) and variable thickness bridges. SQUIDs based on both geometries show a modulation of the maximum Josephson current with a magnetic flux periodic wi…
Engineering nanophotonic mode dispersions in ultrathin, planar structures enables significant control over infrared perfect absorption (PA) and thermal emission characteristics. Here, using simulations, the wavelength and angular ranges over which ultrathin, low loss, epsilon-near-zero (ENZ) films on a reflecting surface most efficiently absorb and re-radiate are identified, and the design parameters that tailor the ENZ mode dispersion within these limits are investigated. While the absorption is spectrally limited to wavelengths where the refractive index ($n$) lies below unity, the angular l…
By combining first principles theoretical calculations and experimental optical and structural characterization such as spectroscopic ellipsometry, X-ray spectroscopy, and electron microscopy, we study the dielectric permittivity and plasmonic properties of ultrathin TiN films at an atomistic level. Our results indicate a remarkably persistent metallic character of ultrathin TiN films and a progressive red shift of the plasmon energy as the thickness of the film is reduced. The microscopic origin of this trend is interpreted in terms of the characteristic two-band electronic structure of the s…
We report a magnetotransport study of spin relaxation in 1.4-21.2 nm epitaxial SrIrO$_3$ thin films coherently strained on SrTiO$_3$ substrates. Fully charge compensated semimetallic transport has been observed in SrIrO$_3$ films thicker than 1.6 nm, where the charge mobility at 10 K increases from 45 cm$^2$/Vs to 150 cm$^2$/Vs with decreasing film thickness. In the two-dimensional regime, the charge dephasing and spin-orbit scattering lengths extracted from the weak localization/anti-localization effects show power-law dependence on temperature, pointing to the important role of electron-elec…
The discovery of high-temperature superconductivity in FeSe/STO has trigged great research interest to reveal a range of exotic physical phenomena in this novel material. Here we present a temperature dependent magnetotransport measurement for ultrathin FeSe/STO films with different thickness and protection layers. Remarkably, a surprising linear magnetoresistance (LMR) is observed around the superconducting transition temperatures but absent otherwise. The experimental LMR can be reproduced by magnetotransport calculations based on a model of magnetic field dependent disorder induced by spin …
We consider the diffusion of markers in a layered medium, with the lateral diffusion coefficient being the function of hight. We show that the probability density of the lateral displacements follows one-dimensional Batchelor's equation with time-dependent diffusion coefficient governed by the particles' redistribution in height. For the film of a finite thickness the resulting mean squared displacement exhibits superdiffusion at short times and crosses over to normal diffusion at long times. The approach is used for description of experimental results on inhomogeneous molecular diffusion in t…
