We present a theoretical study on the impact of an active optical layer on the emission properties of an ultrathin luminescent film. While the study can be generalized to any material, we focus here on a simple layered medium composed of a conjugated polymers (CPs) thin film, a zinc oxide layer (ZnO) and a sapphire substrate. The study spreads throughout variable aspects including the effect of the structure parameters on the CP luminescence and radiation pattern and more specifically the influence of the absorption and emission properties of the active layer. Comparing between the passive and active layer cases, the obtained results show that an enhancement of the CP luminescence of more than 20 times can be obtained by using an optically active underlying layer. The results can be explained in terms of photon recycling where the optically active layer acts as a photon reservoir and a secondary light source for the ultra thin film. This general concept is of a special interest for ultra-trace chemosensor.
We report on significant enhancement of the magnetooptical effects in gyrotropic systems of a metallic film perforated by subwavelength hole arrays and a uniform dielectric film magnetized perpendicular to its plane. Calculations, based on a rigorous coupled-wave analysis, demonstrate the Faraday and Kerr effect spectra having several resonance peaks in the near infrared range, some of them coinciding with transmittance peaks. Qualitative analysis revealed that magnetic polaritons being coupled magnetic-film waveguiding modes with surface plasmons play a crucial role in the observed effect.
Recently developed classes of monocrystalline silicon solar microcells (u-cell) can be assembled into modules with characteristics (i.e., mechanically flexible forms, compact concentrator designs, and high-voltage outputs) that would be impossible to achieve using conventional, wafer-based approaches. In this paper, we describe a highly dense, uniform and non-periodic nanocone forest structure of black silicon (bSi) created on optically-thin (30 um) u-cells for broadband and omnidirectional light-trapping with a lithography-free and high-throughput plasma texturizing process. With optimized pl…
Highly confined "spoof" surface plasmon-like (SSP) modes are theoretically predicted to exist in a perforated metal film coated with a thin dielectric layer. Strong modes confinement results from the additional waveguiding by the layer. Spectral characteristics, field distribution, and lifetime of these SSPs are tunable by the holes' size and shape. SSPs exist both above and below the light line, offering two classes of applications: "perfect" far-field absorption and to efficient emission into guided modes. It is experimentally shown that these plasmon-like modes can turn thin, weakly-absorbi…
Pulse laser deposited La2/3Sr1/3MnO3 ultrathin films on SrTiO3 substrates were characterized by polar and longitudinal Kerr magneto-optical spectroscopy. An agreement between experimental and theoretical spectra was achieved for a 10.7 nm thick film, while a distinction in the Kerr effect amplitudes was obtained for a 5 nm thick film. This points to the slight suppression of ferromagnetism by interface effects. Nevertheless, the room temperature ferromagnetism of the 5 nm thick film was clearly demonstrated. All the studied films exhibit magneto-optical properties similar to single crystals an…
Realizing laterally continuous ultra-thin gold films on transparent substrates is a challenge of significant technological importance. In the present work, formation of ultra-thin gold films on fused silica is studied, demonstrating how suppression of island formation and reduction of plasmonic absorption can be achieved by treating substrates with (3-mercaptopropyl) trimethoxysilane prior to deposition. Void-free fi lms with deposition thickness as low as 5.4 nm are realized and remain structurally stable at room temperature. Based on detailed structural analysis of the fi lms by specular and…
Bulk bismuth presents outstanding optical properties, such as a giant infrared refractive index (n near 10) and a negative ultraviolet visible permittivity induced by giant interband electronic transitions. Although such properties are very appealing for applications in nanophotonics, the dielectric function of bismuth nanostructures has been scarcely studied. Here, we determine by spectroscopic ellipsometry the far infrared to ultraviolet dielectric function of pulsed laser deposited bismuth thin films with nominal thickness tBi varied from near 10 nm to several tens of nm. For tBi above 15 n…
Sub-10 nm continuous metal films are promising candidates for flexible and transparent nanophotonics and optoelectronics applications. In this Letter, we demonstrate that monolayer MoS2 is a perspective adhesion layer for the deposition of continuous conductive gold films with a thickness of only 3-4 nm. Optical properties of continuous ultrathin gold films deposited on two-dimensional MoS2 grown by chemical vapor deposition are investigated by spectroscopic ellipsometry over a wide wavelength range (300-3300 nm). Results show that optical losses in ultrathin films increase with decreasing thi…
Here we report the adsorption behaviour of a water soluble laser dye rhodamine onto ultrathin films prepared by LB technique without altering its photophysical behabiour.
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 phenomenon of potential sputtering of ultra-thin Pt films due to impact of 20 keV highly charged $Ar^{q+}$ (q = 3 - 8) ions has been investigated. The results show enhanced erosion of the Pt film with the increase of potential energy of the Ar projectile.
We analyze X-ray diffraction data used to extract cell parameters of ultrathin films on closely matching substrates. We focus on epitaxial La2/3Sr1/3MnO3 films grown on (001) SrTiO3 single crystalline substrates. It will be shown that, due to extremely high structural similarity of film and substrate, data analysis must explicitly consider the distinct phase of the diffracted waves by substrate and films to extract reliable unit cell parameters. The implications of this finding for the understanding of strain effects in ultrathin films and interfaces will be underlined
We observe pronounced oscillations in differential reflectivity of 9 nm and 60 nm BaFe\textsubscript{2}As\textsubscript{2} (Ba-122) thin films using ultrafast optical spectroscopy. Our studies show that the oscillations result from propagating longitudinal acoustic (LA) phonon wave packets with strong thickness and temperature dependence. Particularly, the experimentally measured oscillation frequency approaches to 50 GHz for the ultra-thin film. Our calculations show that Young's modulus of 9 nm thin film is nearly four times as large as that of 60 nm thin film, consistent with the experiment…
We have successfully grown one of the higher manganese silicides, Mn4Si7 thin films on silicon (100) substrates using an ultra-high vacuum deposition with a base pressure of 1x10-9 torr. The thickness of the film was varied from 65-100 nm. These films exhibit a tetragonal crystal structure and display paramagnetic behavior as predicted for the stoichiometric Mn4Si7 system. They have a resistivity of 3.321 x 10-5 ohm-m at room temperature and show a semi-metallic nature.
We present a first-principles study on the dielectric properties of an NaCl crystal and ultrathin NaCl films under a finite external electric field. Our results show that the high-frequency dielectric constant of the films is not affected by the finite size effect from crystal surfaces and is close to that of the crystal, whereas the static one is sensitive to the thickness of the film due to the difference in the atomic configurations between the surface and inside of the film.
We report studies of ferroelectricity in ultra-thin perovskite films with realistic electrodes. The results reveal stable ferroelectric states in thin films less than 10 Åthick with polarization normal to the surface. Under short-circuit boundary conditions, the screening effect of realistic electrodes and the influence of real metal/oxide interfaces on thin film polarization are investigated. Our studies indicate that metallic screening from the electrodes is affected by the difference in work functions at oxide surfaces. We demonstrate this effect in ferroelectric PbTiO$_3$ and BaTiO$_3$ fil…
We study the ferroelectric stability and surface structural properties of an oxygen-terminated hexagonal YMnO$_3$ ultra-thin film using density functional theory. Under an open circuit boundary condition, the ferroelectric state with the spontaneous polarization normal to the (0001) surface, is found to be metastable in a single domain state despite the presence of a depolarizing field. We establish a connection between the result and the role of improper ferroelectric transition. Our results imply that improper ferroelectric ultrathin films can have rather unique properties that are distincti…
The ground-state polarization of PbTiO3 thin films is studied using a microscopic effective Hamiltonian with parameters obtained from first-principles calculations. Under short-circuit electrical boundary conditions, (001) films with thickness as low as three unit cells are found to have a perpendicularly polarized ferroelectric ground state with significant enhancement of the polarization at the surface.
Quantum manifestations of various properties of metallic thin films by quantum size effect (QSE) have been studied intensively. Here, using first-principles calculations, we show quantum manifestation in dielectric properties of Al(111) ultrathin films. The QSE on the dielectric function is revealed, which arises from size dependent contributions from both intraband and interband electronic transitions. More importantly, the in-plane interband transitions in the films thinner than 15 monolayers are found to be smaller than the bulk counterpart in the energy range from 1.5~eV to 2.5~eV. This in…
We investigate transport properties of ultrananocrystalline diamond films for a broad range of temperatures. Addition of nitrogen during plasma-assisted growth increases the conductivity of ultrananocrystalline diamond films by several orders of magnitude. We show that films produced at low concentration of nitrogen in the plasma are very resistive and electron transport occurs via a variable range hopping mechanism while in films produced at high nitrogen concentration the electron states become delocalized and the transport properties of ultrananocrystalline diamond films can be described us…
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…
To better understand the electronic and chemical properties of wide-gap oxide surfaces at the atomic scale, experimental work has focused on epitaxial films on metal substrates. Recent findings show that these films are considerably thinner than previously thought. This raises doubts about the transferability of the results to surface properties of thicker films and bulk crystals. By means of density-functional theory and approximate GW corrections for the electronic spectra we demonstrate for three characteristic wide-gap oxides (silica, alumina, and hafnia) the influence of the substrate and…
We have used the two-step growth technique, quench condensing followed by an anneal, to grow ultra thin films of silver on glass substrates. As has been seen with semiconductor substrates this process produces a metastable homogeneous covering of silver. By measuring the in situ resistance of the film during growth we are able to see that the low temperature growth onto substrates held at 100 Kelvin produces a precursor phase that is insulating until the film has been annealed. The transformation of the precursor phase into the final, metallic silver film occurs at a characteristic temperature…
Experimental efforts to stabilize ferromagnetism in ultrathin films of transition metal oxides have so far failed, despite expectations based on density functional theory (DFT) and DFT+U. Here, we investigate one of the most promising materials, SrRuO$_3$, and include correlation effects beyond DFT by means of dynamical mean field theory. In agreement with experiment we find an intrinsic thickness limitation for metallic ferromagnetism in SrRuO$_3$ thin films. Indeed, we demonstrate that the realization of ultrathin ferromagnetic films is out of reach of standard thin-film techniques. Proposin…
Ferromagnetism was observed in a Pt(100) ultrathin film deposited on a SrTiO3(100) substrate. The ferromagnetism, which appears in films with thicknesses of 2.2-4.4 nm, periodically changes with a period of approximately 1 nm (5-6 monolayers) depending on the film thickness. This is consistent with the period derived from the quantum-well states formed in the thin film. X-ray magnetic circular dichroism measurements show the evidence of appearance of intrinsic ferromagnetism in Pt(100) ultra-thin film. In addition, present results suggest a possibility that the orbital magnetic moment of pure …
We report model calculations of the magnetic phases of very thin Ho films in the temperature interval between 20K and 132K, and show that slab size, surface effects and magnetic field due to spin ordering may impact significantly the magnetic phase diagram. There is a relevant reduction of the external field strength required to saturate the magnetization and for ultra-thin films the helical state does not form. We explore the heat capacity and the susceptibility as auxiliary tools to discuss the nature of the phase transitions.
The mean-field Landau-type theory is used to analyze the polarization properties of epitaxial ferroelectric thin films grown on dissimilar cubic substrates, which induce biaxial compressive stress in the film plane. The intrinsic effect of the film surfaces on the spontaneous polarization is taken into account via the concept of the extrapolation length. The theory simultaneously allows for the influence of the misfit strain imposed on the film lattice by a thick substrate. Numerical calculations are performed for PbTiO3 and BaTiO3 films under an assumption of the polarization reduction in sur…
Previous studies showed that, as ferroelectric films become thinner, their Curie temperature (Tc) and polarization below Tc both typically decrease. In contrast, a recent experiment [Chang et al., Science 353, 274 (2016)] observed that atomic-thick SnTe films have a higher Tc than their bulk counterpart, which was attributed to extrinsic effects. Here, we find, using first-principles calculations, that the 0K energy barrier for the polarization switching (which is a quantity directly related to Tc) is higher in most investigated defect-free SnTe ultrathin films than that in bulk SnTe, and that…
The effect of high tensile strain and low dimensionality on the magnetic and electronic properties of CaMnO$_3$ ultrathin films, epitaxially grown on SrTiO$_3$ substrates, are experimentally studied and theoretically analyzed. By means of ab initio calculations, we find that, both, the high strain produced by the substrate and the presence of the free surface contribute to the stabilization of an in-plane ferromagnetic coupling, giving rise to a non-zero net magnetic moment in the ultrathin films. Coupled with this change in the magnetic order we find an insulator-metal transition triggered by…
We report on the heteroepitaxial stabilization of YCrO3 ultra-thin films on LSAT (001) substrate. Using a combination of resonant X-ray absorption spectroscopy (XAS) and atomic multiplet cluster calculation, the electronic structure of YCrO3 thin film was investigated. Polarization dependent Cr L3,2 edge XAS measurement reveal the presence of an anomalous orbital polarization uncharacteristic of a 3d3 electronic system. Atomic multiplet calculations demonstrate the critical importance of charge transfer energy, Coulomb correlation strength and hopping interaction in stabilizing this unusual or…
The effects of wet chemical processing conventionally employed in device fabrication standards are systematically studied on molybdenum oxide (MoOx) ultra-thin films. We have combined x-ray photoelectron spectroscopy (XPS), angle resolved XPS and x-ray reflectivity techniques to provide deep insights into the changes in composition, structure and electronic states upon treatment of films with different initial stoichiometry prepared by reactive sputtering. Our results show significant reduction effects associated with the development of gap states in MoOx, as well as changes in the composition…
We show that many two dimensional domain patterns observed in Monte Carlo simulations can be obtained from the many soliton solutions of the imaginary time Sine Gordon equation. This opens the door to analytic physical understanding of the micromagnetics in ultra-thin films.
We explore the potential of Time-Of-Flight Medium Energy Ion Scattering (TOFMEIS) for thin film analysis and analyze possible difficulties in evaluation of experimental spectra. Issues regarding different combinations of composition and stopping power as well as the influence of channeling are discussed. As a model system high-k material stacks made from ultra-thin films of HfO2 grown on a p-type Si (100) substrate with a 0.5 nm SiO2 interface layer have been investigated. By comparison of experimental spectra and computer simulations TOF-MEIS was employed to establish a depth profile of the f…
The influence of uniaxial anisotropy and the dipole interaction on the direction of the magnetization of ultra-thin ferromagnetic films in the ground-state is studied. The ground-state energy can be expressed in terms of anisotropy constants which are calculated in detail as function of the system parameters and the film thickness. In particular non-collinear spin arrangements are taken into account. Conditions for the appearance of a spin reorientation transition are given and analytic results for the width of the canted phase and its shift in applied magnetic fields associated with this tran…
A reduction of polarization in ultra-thin ferroelectric films appears to be fundamental to ferroelectricity at the nanoscale. For the model system PbTiO3 on SrTiO3, we report observation of the polarization vs. thickness relation. Distinct periodicity changes of ferroelectric domains obtained from x-ray diffraction and total energy calculations reveal a linear lowering of the polarization below a critical thickness of ~12 nm. Independent polarization and tetragonality measurements provide insight into the fundamental relation between polarization and tetragonality in nanoscale ferroelectrics.
Ferroelectric films in the paraelectric phase exhibit two undesirable properties: hysteresis in the voltage-capacitance characteristics and a significant relaxation time of the capacitance. Our experiments show that suppression of both of these is achieved by using ultraviolet (UV) radiation with wavelengths corresponding to the material forbidden gap. Experimentally we also observed UV radiation induced modulation of thin film permittivity without an applied electric field. The observed phenomena are believed to have the same origin: UV light causes generation of non-equilibrium charge carrie…
The coupling between the electrical transport properties of La2/3Sr1/3MnO3 (LSMO) thin films and structural phase transitions of SrTiO3 (STO) substrates at Ts = 105 K has been investigated. We found that the electrical resistivity of LSMO films exhibit a cusp at Ts, which is greatly amplified by tuning films to the verge of metallic and insulating phases, i.e., to the boundary of two delicate competing electronic states. Our results demonstrate that small amounts of strain can tip the subtle balance of competing interactions and tune the electronic properties in correlated electron materials.
Competition between electronic and atomic reconstruction is a constantly recurring theme in transition-metal oxides. We use density functional theory calculations to study this competition for a model system consisting of a thin film of the polar, infinite-layer structure ACuO2 (A=Ca, Sr, Ba) grown on a nonpolar, perovskite SrTiO3 substrate. A transition from the bulk planar structure to a chain-type thin film accompanied by substantial changes to the electronic structure is predicted for a SrCuO2 film fewer than five unit cells thick. An analytical model explains why atomic reconstruction bec…
We show that using maskless photolithography and the lift-off technique, patterned yttrium iron garnet thin films possessing ultra-low Gilbert damping can be accomplished. The films of 70 nm thickness were grown on (001)-oriented gadolinium gallium garnet by means of pulsed laser deposition, and they exhibit high crystalline quality, low surface roughness, and the effective magnetization of 127 emu/cm3. The Gilbert damping parameter is as low as 5x10-4. The obtained structures have well-defined sharp edges which along with good structural and magnetic film properties pave a path in the fabrica…
A detailed formulation for the effects of the surface integrity, i.e. surface topography, surface metallurgy, and surface mechanical properties, on the mechanics of ultra-thin films is proposed in the framework of linear elasticity. In this formulation, the ultra-thin film is modeled as a material bulk covered with two altered layers and two rough surfaces as distinct phases. Two versions of the proposed formulation are developed. In the first version, the governing equations are obtained depending on the general form of the surface topography. In the second version, the governing equations ar…
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…
To understand the near-interface magnetism in manganites, uniform, ultra-thin films of La_{0.67}Sr_{0.33}MnO_3 were grown epitaxially on single crystal (001) LaAlO_3 and (110) NdGaO_3 substrates. The temperature and magnetic field dependent film resistance is used to probe the film's structural and magnetic properties. A surface and/or interface related dead-layer is inferred from the thickness dependent resistance and magnetoresistance. The total thickness of the dead layer is estimated to be $\sim 30 Å$ for films on NdGaO_3 and $\sim 50 Å$ for films on LaAlO_3.
We develop the theory for stick-slip motion in ultra-thin liquid films confined between two moving atomically-flat surfaces. Our model is based on hydrodynamic equation for the flow coupled to the dynamic order parameter field describing the ``shear melting and freezing'' of the confined fluid. This model successfully accounts for observed phenomenology of friction in ultra-thin films, including periodic and chaotic sequences of slips, transitions from stick-slip motion to steady sliding.
The recent experimental fabrication of ultra stable glass films via vapour deposition [Science 315, 353 (2007)] and the observation of front-like response to the annealing of these films [Phys.Rev.Lett. 102, 065503 (2009)], have raised important questions about the possibility of manipulating the properties of glass films and addressing fundamental questions about kinetics and thermodynamics of amorphous materials. Central to both of these issues is the need to establish the essential physics that governs the formation of the ultra-stable films and their subsequent response. In this paper we d…
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…
A theoretical model for understanding the formation of interfacial thin films is presented, which combines density functional theory calculations for interface energies with thermodynamic modeling techniques for multicomponent bulk systems. The theory is applied to thin film formation in VC-doped WC-Co cemented carbides. It is predicted that ultrathin VC films may exist in WC/Co interfaces at the high temperature sintering conditions where most of the WC grain growth occurs, which provides an explanation of the grain growth inhibiting effect of VC additions in the WC-Co system.
The properties of ultrathin films have been studied within the framework of Ising model and the method of random-field interactions. It is shown that the Curie temperature is inversely proportional to the number of layers. Critical exponent $ν$ has been obtained and it is shown that it does not depend on the type of crystalline lattice.
We have measured the Anisotropic Magnetoresistance (AMR) of ultra thin (5nm) Ga(0.95)Mn(0.05)As films. We find that the sign of the AMR can be positive or negative, which may depend on the direction of the current with respect to the crystal. At low temperatures, transport measurements and SQUID magnetometry suggest that the magnetisation has a component pointing out of the plane of the film.
The reorientation transition of the magnetization of ferromagnetic films is studied on a microscopic basis within Heisenberg spin models. Analytic expressions for the temperature dependent anisotropy are derived from which it is seen that the reduced magnetization in the film surface at finite temperatures plays a crucial role for this transition. Detailed phase diagrams in the temperature-thickness plane are calculated.
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…
Polystyrene and polyvinyl chloride thin films are explored as sample supports for extreme ultraviolet (XUV) spectroscopy of molecular transition metal complexes. Thin polymer films prepared by slip-coating are flat, smooth, and transmit much more XUV light than silicon nitride windows. Analytes can be directly cast onto the polymer surface, or codeposited within it. The M-edge x-ray absorption near-edge (XANES) spectra (40-90 eV) of eight archetypal transition metal complexes (M=Mn, Fe, Co, Ni) are presented to demonstrate the versatility of this method. The films are suitable for pump/probe t…
We report on three key processes involving atomic step motion during the dewetting of thin solid films: (i) the growth of an isolated island nucleated far from a hole, (ii) the spreading of a monolayer rim, and (iii) the zipping of a monolayer island along a straight dewetting front. Kinetic Monte Carlo results are in good agreement with simple analytical models assuming diffusion-limited dynamics.
The nanometer-scale size-dependent electronic transport properties of crystalline (c-) and amorphous (a-) germanium telluride (GeTe) ultrathin films sandwiched by titanium nitride (TiN) electrodes are investigated using ab initio molecular dynamics (AIMD), density functional theory (DFT), and Green's function calculations. We find that a-GeTe ultrathin films scaled down to about 38 Angstrom (12 atomic layers) still shows a band gap and the electrical conductance is mainly due to electron transport via intra-gap states. If the ultrathin films are further scaled, the a-GeTe band gap disappears d…
Photothermoelectric materials have important applications in many fields. Here, we joined a silver nanostructure film (AgNSF) and a carbon nanotube film (CNTF) by van der Waals force to form a AgNSF/CNTF heterojunction, which shows excellent photothermal and photoelectric conversion properties. The local temperature difference and the output photovoltage increase rapidly when the heterojunction is irradiated by lasers with wavelengths ranging from ultraviolet to terahertz. The maximum of the local temperature difference reaches 205.9 K, which is significantly higher than that of other photothe…
Atomistic simulations are used to explore the effect of interfacial structure on residual radiation damage. Specifically, an ordered grain boundary is compared to a disordered amorphous intergranular film, to investigate how interface thickness and free volume impacts point defect recombination. The collision cascades induced by neutron bombardment are simulated and residual point defect populations are analyzed as a function of boundary type and primary knock on atom energy. While ordered grain boundaries easily absorb interstitials, these interfaces are inefficient vacancy sinks. Alternative…
Terahertz (THz) radiation has been observed from multiferroic BiFeO$_3$ thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO$_3$ thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. On the basis of our findings, we demonstrate potential approaches to ferroelectric nonvolatile random access memory with nondestructive readability and ferroelect…
Here we report a method to fabricate high quality Bi2Se3 thin films using molecular beam epitaxy with a radio frequency cracker cell as an atomic selenium source. With rates close to exact stoichiometry, optimal layer-by-layer growth of high quality Bi2Se3 thin films with smooth surfaces, has been achieved on CaF2(111) substrates and Si(111) substrates with a thin CaF2 buffer layer(CaF2/Si). Transport measurements show a characteristic weak antilocalization mangnetoresistance, with emergence of weak localization in the ultrathin film limit. Quantum Oscillations attributed to the topological su…
We perform linear and non-linear photon absorption calculations in topological insulator ultra-thin films on a substrate. Due to the unique band structure of the coupled topological surface states, novel features are observed for suitable photon frequencies, including a divergent edge singularity in one-photon absorption process and a significantly enhancement in two-photon absorption process. The resonanct frequencies can be controlled by tuning the energy difference and coupling of the top and bottom surface states. Such unique linear and nonlinear optical properties make ultra-thin films of…
