The effect of 3-dimensional strain state on the magnetocaloric properties of epitaxial La0.8Ca0.2MnO3 (LCMO) thin films grown on two types of substrates, SrTiO3 (001) (STO) and LaAlO3 (001) (LAO) has been studied as a function of film thickness within the range of 25 to 300 nm. The STO substrate imposes an in-plane tensile biaxial strain while LAO substrate imposes an in-plane compressive biaxial strain. The in-plane biaxial strain on LCMO by STO substrate gets relaxed more rapidly than that by LAO substrate but both LCMO/STO and LCMO/LAO show a maximum entropy change of 12.1 J/Kg-K and 3.2 J/Kg-K, respectively at a critical thickness of 75 nm (at 6 T applied magnetic field). LCMO/LAO is found to exhibit a wider transition temperature region with full width at half maxima (FWHM) 40 K of the dM/dT vs T curve compared to LCMO/STO with FWHM 33 K of that curve. This broadening of the transition region indicates that the Table like magnetocaloric effect (MCE) is attainable by changing the strain type. The maximum Relative Cooling Power, 361 J/Kg of LCMO/STO and 339 J/Kg of LCMO/LAO is also observed at the thickness 75 nm. The Curie temperature varies with the thickness exploring the variation of ferromagnetic interaction strength due to strain relaxation. The film thickness and substrate induced lattice strain are proved to be the significant parameters for controlling MCE. The highest MCE response at a particular thickness shows the possibility of tuning MCE in other devices by optimizing thickness.
Large thermal changes driven by a magnetic field have been proposed for environmentally friendly energy efficient refrigeration, but only a few materials which suffer hysteresis show these giant magnetocaloric effects. Here we create giant and reversible extrinsic magnetocaloric effects in epitaxial films of the ferromagnetic manganite La0.7Ca0.3MnO3 using strain mediated feedback from BaTiO3 substrates near a first-order structural phase transition. Our findings should inspire the discovery of giant magnetocaloric effects in a wide range of magnetic materials, and the parallel development of …
Combining multiple degrees of freedom in strongly-correlated materials such as transition-metal oxides would lead to fascinating magnetic and magnetocaloric features. Herein, the strain effects are used to markedly tailor the magnetic and magnetocaloric properties of PrVO3 thin films. The selection of appropriate thickness and substrate enables us to dramatically decrease the coercive magnetic field from 2.4 T previously observed in sintered PVO3 bulk to 0.05 T for compressive thin films making from the PrVO3 compound a nearly soft magnet. This is associated with a marked enhancement of the ma…
Applying a magnetic field to a ferromagnetic Ni$_{50}$Mn$_{34}$In$_{16}$ alloy in the martensitic state induces a structural phase transition to the austenitic state. This is accompanied by a strain which recovers on removing the magnetic field giving the system a magnetically superelastic character. A further property of this alloy is that it also shows the inverse magnetocaloric effect. The magnetic superelasticity and the inverse magnetocaloric effect in Ni-Mn-In and their association with the first order structural transition is studied by magnetization, strain, and neutron diffraction stu…
Tuning functional properties of thin caloric films by mechanical stress is currently of high interest. In particular, a controllable magnetisation or transition temperature is desired for improved usability in magnetocaloric devices. Here, we present results of epitaxial magnetocaloric Ni-Mn-Ga-Co thin films on ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)$_{0.72}$Ti$_{0.28}$O$_3$ (PMN-PT) substrates. Utilizing X-ray diffraction measurements, we demonstrate that the strain induced in the substrate by application of an electric field can be transferred to the thin film, resulting in a change of the la…
La(Fe,Si)$_{13}$-based compounds are considered to be very promising magnetocaloric materials for magnetic refrigeration applications. Many studies have focused on this material family but only in bulk form. In this paper, we report the fabrication of thick films of La(Fe,Si)$_{13}$, both with and without post-hydriding. These films exhibit magnetic and structural properties comparable to those of bulk materials. We also observe that the ferromagnetic phase transition has a negative thermal hysteresis, a phenomenon not previously found in this material but which may have its origins in the ava…
Two-dimensional magnets could potentially revolutionize information technology, but their potential application to cooling technology and magnetocaloric effect (MCE) in a material down to the monolayer limit remain unexplored. Herein, we reveal through multiscale calculations the existence of giant MCE and its strain tunability in monolayer magnets such as CrX$_3$ (X=F, Cl, Br, I), CrAX (A=O, S, Se; X=F, Cl, Br, I), and Fe$_3$GeTe$_2$. The maximum adiabatic temperature change ($ΔT_\text{ad}^\text{max}$), maximum isothermal magnetic entropy change, and specific cooling power in monolayer CrF$_3…
The effect of the short milling times on R5(Si,Ge)4 R =Gd, Tb magnetocaloric material properties was investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (< 2.5h), a reduction of the Gd5Si1.3Ge2.7 and Tb5Si2Ge2 particles size was achieved down to approximately 3.5 DSm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd5Si1.3Ge2.7 case, a second-order phase transition emer…
Mechanical control of magnetic properties in magnetostrictive thin films offers the unexplored opportunity to employ surface wave acoustics in such a way that acoustic triggers dynamic magnetic effects. The strain-induced modulation of the magnetic anisotropy can play the role of a high frequency varying effective magnetic field leading to ultrasonic tuning of electronic and magnetic properties of nanostructured materials, eventually integrated in semiconductor technology. Here, we report about the opportunity to employ surface acoustic waves to trigger magnetocaloric effect in MnAs(100nm)/GaA…
The inverse magnetocaloric effect (MCE) in Ni-Mn-based Heusler compounds occurs during the magnetostructural transition between low-temperature, low-magnetization martensite and high-temperature, high-magnetization austenite. In this study, we analyze the metamagnetic transformation of a $Ni_{49.8}Mn_{35}In_{15.2}$ compound by simultaneous adiabatic temperature change and strain measurements in pulsed magnetic fields up to 10 T. We observe an adiabatic temperature change of -10 K and a strain of -0.22 % when the reverse martensitic transition is fully induced at a starting temperature of 285 K…
Significative enhance in magnetocaloric effect due to finite size and surface effects is reported in Terbium(Tb) thin films in the helimagnetic phase (corresponding to a temperature range from $T_C$=219 K to $T_N$=231 K), for external fields of the order of kOe. For a Tb thin film of 6 monolayers submitted to an applied field ($Δ$H= 30 kOe, $Δ$H= 50 kOe and $Δ$H= 70 kOe) we report a significative change in adiabatic temperature, $Δ$T/$Δ$H, near the Néel temperature, of the order ten times higher than that observed for Tb bulk. On the other hand, for small values of the magnetic field, large th…
We investigate MCA of CoFe(011) thin films as a function of strength of strain and film thickness has been studied. It is elucidated that perpendicular magnetocystalline anisotropy (MCA) energy (EMCA) is getting stronger with compressed xy-plane lattice constant while in-plane MCA is become an easy-axis by tensile strain on xy-plane. The reason of the EMCA behaviors can be explained by features of electronic structures.
A study investigating the effect of Sn substitution on the magnetocaloric properties of Mn$ _{3} $Ga$ _{(1-x)} $Sn$ _{x} $C compounds reveals that nature of the MCE has a strong dependence on the nature of the magnetic ordering. For small amounts of Sn ($ x\leq0.2 $) the MCE is of the inverse type wherein an increase in the applied field beyond 5T gives rise to a table like temperature dependence of the entropy due to a coupling between the first order FM - AFM transition and the field induced AFM - FM transition. Replacement of Ga by larger concentrations of Sn ($ x\geq0.71 $) $(ΔS_M)$ result…
We present the temperature- and thickness-dependent structural and morphological evolution of strain induced transformations in highly-strained epitaxial BiFeO3 films deposited on LaAlO3 (001) substrates. Using high-resolution X-ray diffraction and temperature-dependent scanning-probe-based studies we observe a complex temperature- and thickness-dependent evolution of phases in this system. A thickness-dependent transformation from a single monoclinically distorted tetragonal-like phase to a complex mixed-phase structure in films with thicknesses up to ~200 nm is the consequence of a strain-in…
The antiferromagnet Mn$_5$Si$_3$ has recently attracted attention because a noncollinear spin arrangement has been shown to produce a topological anomalous Hall effect and an inverse magnetocaloric effect. Here we synthesize single crystals of Mn$_5$Si$_3$ using flux growth. We determine the phase diagram through magnetization and measure the magnetoresistance and the Hall effect. We find the collinear and noncollinear antiferromagnetic phases at low temperatures and, in addition, a third magnetic phase, in between the two antiferromagnetic phases which has ferromagnetic character. The latter …
The structural stability, electronic structure, and thermal transport properties of one to six quintuple layers (QLs) of Bi2Se3 are investigated by van der Waals density functional theory and semi-classical Boltzmann theory. The bandgap amounts to 0.41 eV for a single QL and reduces to 0.23 eV when the number of QLs increases to six. A single QL has a significantly higher thermoelectric figure of merit (0.27) than the bulk material (0.10), which can be further enhanced to 0.30 by introducing 2.5% compressive strain. Positive phonon frequencies under strain indicate that the structural stabilit…
X-ray analysis of ferroelectric thin layers of Ba1/2Sr1/2TiO3 with different thickness reveals the presence of internal strain gradients across the film thickness and allows us to propose a functional form for the internal strain profile. We use this to calculate the direct influence of strain gradient, through flexoelectric coupling, on the degradation of the ferroelectric properties of thin films with decreasing thickness, in excellent agreement with the observed behaviour. This work highlights the link between strain relaxation and strain gradients in epitaxial films, and shows the pressing…
The discovery of ferroelectric polarization in HfO2-based ultrathin films has spawned a lot of interest due to their potential applications in data storage. Recently, a new R3m rhombohedral phase was proposed to be responsible for the emergence of ferroelectricity in the [111]-oriented Hf0.5Zr0.5O2 thin films, but the fundamental mechanism of ferroelectric polarization in such films remains poorly understood. In this paper, we employ density-functional-theory calculations to investigate structural and polarization properties of the R3m HfO2 phase. We find that the film thickness and in-plane c…
In the lifetime of a magnetic refrigerator the materials are subjected to millions of thermal magnetic cycles, so it is of the utmost importance to predict the behavior of such materials when subjected to related work conditions. Thus, in this work we investigated the influence of thermal cycling in the microstructure, magnetic phase transition and magnetic entropy change of a Gd5Si1.3Ge2.7 thin film up to 1000 cycles. In a first stage, till 450 cycles, the hysteresis area of the magnetization curves as a function of temperature was found to decrease 16% with thermal cycling, due to an arresti…
The ferromagnetic properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_3$ thin films have been studied. The magnetic transition is affected by both strain and finite thickness. We have used a series of films of different thickness and on different substrates in order to quantitatively determine the change in Curie temperature contributed by each effect. The phase diagram of T$_C$ versus in-plane strain suggests that the ferromagnetic transition temperature is suppressed by tensile strain and enhanced by compressive strain. The general method of separating strain and finite thickness effects should…
We report on the structural and magnetic characterization of (110) and (001) La2/3Ca1/3MnO3 (LCMO) epitaxial thin films simultaneously grown on (110) and (001)SrTiO3 substrates, with thicknesses t varying between 8 nm and 150 nm. It is found that while the in-plane interplanar distances of the (001) films are strongly clamped to those of the substrate and the films remain strained up to well above t=100 nm, the (110) films relax much earlier. Accurate determination of the in-plane and out-of-plane interplanar distances has allowed concluding that in all cases the unit cell volume of the mangan…
Magnetic properties of a series of (La0.7Sr0.3MnO3/SrRuO3) superlattices, where the SrRuO3 layer thickness is varying, are examined. A room-temperature magnetocaloric effect is obtained owing to the finite size effect which reduces the TC of La0.7Sr0.3MnO3 layers. While the working temperature ranges are enlarged,, -DeltaSmax values remains similar to the values in polycrystalline La0.7Sr0.3MnO3. Consequently, the relative cooling powers are significantly improved, the microscopic mechanism of which is related to the effect of the interfaces at La0.7Sr0.3MnO3/SrRuO3 and higher nanostructural d…
We demonstrate robust interface strain-induced perpendicular magnetic anisotropy in atomically flat ferrimagnetic insulator Tm3Fe5O12 films grown with pulsed laser deposition on substituted-Gd3Ga5O12 substrate which maximizes the tensile strain at the interface. In bilayers consisting of Pt and TIG, we observe large squared Hall hysteresis loops over a wide range of thicknesses of Pt at room temperature. When a thin Cu layer is inserted between Pt and TIG, the Hall hysteresis magnitude decays but stays finite as the thickness of Cu increases up to 5 nm. However, if the Cu layer is placed atop …
In this work we are investigating the effect of strain in ultrathin Mn-Ga thin films on the magnetic properties at room temperature. Two different Mn-Ga compositions of which one is furthermore doped with Co were grown on Cr buffered MgO (001) substrates. Films with a thickness below 12nm are highly strained and it was observed that the ratio c/a vs. thickness is depending on composition. Using c/a as an order parameter the PMA is shown to be drastically reduced with increasing strain. These findings should be considered when generalizing and downscaling results obtained from films >20nm. Furt…
We carried out matched experiments and molecular dynamics simulations of the compression of nanopillars prepared from Cu$\vert$Au nanolaminates with 25 nm layer thickness. The stress-strain behavior obtained from both techniques are in excellent agreement. Variation of the layer thickness in simulations reveals an increase of the strength with decreasing layer thickness. Pillars fail through the formation of shear bands whose nucleation we trace back to the existence of surface flaws. Our combined approach demonstrates the crucial role of contact geometry in controlling the deformation mode an…
We report a strong thickness dependence of the complex frequency-dependent optical dielectric function in epitaxial CaMnO3(001) thin films on SrTiO3(001), LaAlO3(001), and SrLaAlO4(001) substrates. A doubling of the peak value of the imaginary part of the dielectric function and spectral shifts of 0.5 eV for a given magnitude of absorption are observed. On the basis of the experimental data and first-principles density functional theory calculations of the dielectric function, its evolution with thickness from 4 to 63 nm has several regimes. In the thinnest, strain-coherent films, the response…
Recently, hexagonal boron nitride (hBN) layers have generated a lot of interest as ideal substrates for 2D stacked devices. Sapphire-supported thin hBN films of different thicknesses are grown using metalorganic vapour phase epitaxy technique by following a flow modulation scheme. Though these films of relatively large size are potential candidates to be employed in designing real devices, they exhibit wrinkling. The formation of wrinkles is a key signature of strain distribution in a film. Raman imaging has been utilized to study the residual strain distribution in these wrinkled hBN films. A…
We measure the remnant polarization of ferroelectric domains in BiFeO3 films down to 3.6 nm using low energy electron and photoelectron emission microscopy. The measured polarization decays strongly below a critical thickness of 5-7 nm predicted by continuous medium theory whereas the tetragonal distortion does not change. We resolve this apparent contradiction using first-principles-based effective Hamiltonian calculations. In ultra thin films the energetics of near open circuit electrical boundary conditions, i.e. unscreened depolarizing field, drive the system through a phase transition fro…
We propose a physical model which explains the existence of finite thickness wetting layers in epitaxially strained films. The finite wetting layer is shown to be stable due to the variation of the non-linear elastic free energy with film thickness. We show that anisotropic surface tension gives rise to a metastable enlarged wetting layer. The perturbation amplitude needed to destabilize this wetting layer decreases with increasing lattice mismatch. We observe the development of faceted islands in unstable films.
Cementite-type carbides are of interest for magnetocaloric applications owing to their temperature- or pressure-induced magnetic phase transition. Here, using first-principles calculations, we investigate the magnetism and the magnetic phase transition in iron carbide (Fe3C) with the substitution of Cr atoms at Fe sites with the strain effect. The presence of Cr atoms is found to give rise to a second-order magnetic phase transition from a ferromagnetic phase for Fe3C to a nonmagnetic phase in chromium carbide (Cr3C).While the ternary Fe2CrC and Cr2FeC compounds prefer the ferrimagnetic ground…
We present a synthesis/processing method for fabricating ferrimagnetic insulator (Bi-doped yttrium iron garnet) thin films with tunable magnetic anisotropy. Since the desired magnetic properties rely on controllable thickness and successful doping, we pay attention to the entire synthesis/processing procedure (nanopowder synthesis, nanocrystalline target preparation and pulsed laser deposition (PLD)). Atomically flat films were deposited by PLD on (111)-orientated yttrium aluminum garnet. We show a significant enhancement of perpendicular anisotropy in the films, caused by strain-induced aniso…
We investigate the effects of strain on a crystal surface close to the bulk melting temperature T_m, where surface melting usually sets in. Strain lowers the bulk melting point, so that at a fixed temperature below but close to T_m the thickness of the quasi-liquid film is expected to grow with strain, irrespective of sign. In addition, a strain-induced solid surface free energy increase/decrease takes place, favoring/disfavoring surface melting depending on the sign of strain relative to surface stress. In the latter case one can produce a strain-induced prewetting transition, where for incre…
While the unit cell volume of compounds belonging to the Mn$_3$Ga$_{1-x}$Sn$_x$C, (0 $ \le x \le $ 1) series shows a conformity with Vegard's law, their magnetic and magnetocaloric properties behave differently from those of parent compounds Mn$_3$GaC and Mn$_3$SnC. A correlation between the observed magnetic properties and underlying magnetic and local structure suggests that replacing Ga atoms by larger atoms of Sn results in the formation of Ga-rich and Sn-rich clusters. As a result, even though the long range structure appears to be cubic, Mn atoms find themselves in two different local en…
We study the formation and evolution of coherent islands on lattice mismatched epitaxially strained films. Faceted islands form in films with aniostropic surface tension. Under annealing, these islands ripen until a stable array is formed, with an island density which increases with film thickness. Under deposition, an island shape transition occurs, which leads to a bimodal island size distribution. In films with isotropic surface tension we observe continual ripening of islands above a certain film thickness. A stable wavy morphology is found in thinner films.
Ferromagnetic insulating La$_2$CoMnO$_{6-ε}$ (LCMO) epitaxial thin films grown on top of SrTiO$_3$ (001) substrates presents a strong magnetic anisotropy favoring the out of plane orientation of the magnetization with a strong anisotropy field ($\sim 70$ kOe for film thickness of about 15 nm) and with a coercive field of about 10 kOe. The anisotropy can be tuned by modifying the oxygen content of the film which indirectly has two effects on the unit cell: i) change of the orientation of the LCMO crystallographic axis over the substrate (from c in-plane to c out-of-plane) and ii) shrinkage of t…
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 report on the dielectric properties of epitaxial [001] and [100]-textured thin films of antiferromagnetic orthorhombic YMnO3 and their variation under compressive epitaxial strain. It is found that weakly strained YMnO3 films are ferroelectric with a polarization along c-axis switchable by 90 degrees by an external magnetic field. When reducing film thickness and increasing epitaxial strain, the ferroelectric order is progressively suppressed. Analysis of structural, dielectric and magnetic data indicates that suppression of ferroelectricity when reducing thickness is accompanied by an enla…
BiFeO3 thin films with various thicknesses were grown epitaxially on (001) LaSrAlO4 single crystal substrates using pulsed laser deposition. High resolution x-ray diffraction measurements revealed that a tetragonal-like phase with c-lattice constant ~4.65 Å is stabilized by a large misfit strain. Besides, a rhombohedral-like phase with c-lattice constant ~3.99 Å was also detected at film thickness of ~50 nm and above to relieve large misfit strains. In-plane piezoelectric force microscopy studies showed clear signals and self-assembled nanoscale stripe domain structure for the tetragonal-like …
The self-polarization of PbZr0.52Ti0.48O3 thin film is switched by changing film thickness through the competition between the strain relaxation-induced flexoelectric fields and the interfacial effects. Without an applied electric field, this reversal of self-polarization is exploited to control the magnetic properties of La0.67Sr0.33MnO3 by the competition/cooperation between the charge-mediated and the strain-mediated effects. Scanning transmission electron microscopy, polarized near edge x-ray absorption spectroscopy, and half-integer diffraction measurements are employed to decode this int…
Thin films of the ferromagnetic metal SrRuO3 (SRO) show a varying easy magnetization axis depending on the epitaxial strain and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on SrTiO3(001) substrates by soft x-ray magnetic circular dichroism (XMCD) at the Ru M2,3 edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to non-magnetic around 3monolayer thickness, consistent with previous magnetization and magneto-optical Kerr eff…
High quality epitaxial thin-films of EuTiO3 have been grown on the (001) surface of SrTiO3 using pulsed laser deposition. In situ x-ray reflectivity measurements reveal that the growth is two-dimensional and enable real-time monitoring of the film thickness and roughness during growth. The film thickness, surface mosaic, surface roughness, and strain were characterized in detail using ex situ x-ray diffraction. The thicnkess and composition were confirmed with Rutherford Backscattering. The EuTiO3 films grow two-dimensionally, epitaxially, pseudomorphically, with no measurable in-plane lattice…
We report first-principle atomistic simulations on the effect of local strain gradients on the nanoscale domain morphology of free-standing PbTiO$_3$ ultrathin films. First, the ferroelectric properties of free films at the atomic level are reviewed. For the explored thicknesses (10 to 23 unit cells), we find flux-closure domain structures whose morphology is thickness dependent. A critical value of 20 unit cells is observed: thinner films show structures with 90$^\circ$ domain loops, whereas thicker ones develop, in addition, 180$^\circ$ domain walls, giving rise to structures of the Landau-L…
Morphological properties of strained epitaxial films are examined through a mesoscopic approach developed to incorporate both the film crystalline structure and standard continuum theory. Film surface profiles and properties, such as surface energy, liquid-solid miscibility gap and interface thickness, are determined as a function of misfit strains and film elastic modulus. We analyze the stress-driven instability of film surface morphology that leads to the formation of strained islands. We find a universal scaling relationship between the island size and misfit strain which shows a crossover…
Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTiO$_3$ (Nb:STO) heterostructures with varying LSMO-layer thickness, in which the magnetic anisotropy strongly changes depending on the LSMO thickness due to the delicate balance between the strains originating from both the Nb:STO and LAO layers, usin…
We report measurements of the in-plane magnetoelastic coupling in ultra-thin Ta|CoFeB|MgO layers as a function of uniaxial strain, conducted using a four-point bending apparatus. For annealed samples, we observe a strong dependence on the thickness of the CoFeB layer in the range 1.3-2.0 nm, which can be modeled as arising from a combination of effective surface and volume contributions to the magnetoelastic coupling. We point out that if similar thickness dependence exists for magnetoelastic coupling in response to biaxial strain, then the standard Néel model for the magnetic anisotropy energ…
University of California, Irvine（加州大学尔湾分校）；University of California, Santa Barbara（加州大学圣塔芭芭拉分校）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）；Sandia National Laboratories（桑迪亚国家实验室）；Johns Hopkins University（约翰斯·霍普金斯大学）
We have studied the effect of the misfit on the equilibrium shape of three-dimensional pyramidal islands grown on a foreign substrate in the case of incomplete wetting (Volmer-Weber mode of growth). We have found that tensile islands have smaller aspect ratios compared with compressed islands owing to its better adhesion to the substrate. The average strains of consecutive layers decrease faster with thickness in compressed than in tensile islands. The strains decrease rapidly with thickness, with the consequence that above a certain height, the upper layers of the pyramid become practically u…
Typical Raman spectra of transition metal dichalcogenides (TMDs) display two prominent peaks, E2g and A1g, that are well separated from each other. We find that these modes are degenerate in bulk WSe2 yielding one single Raman peak. As the dimensionality is lowered, the observed peak splits in two as a result of broken degeneracy. In contrast to our experimental findings, our phonon dispersion calculations reveal that these modes remain degenerate independent of the number of layers. Interestingly, for minuscule biaxial strain the degeneracy is preserved but once the crystal symmetry is broken…
We present a segregrated strain model that describes the thickness-dependent dielectric properties of ferroelectric films. Using a phenomenological Landau approach, we present results for two specific materials, making comparison with experiment and with first-principles calculations whenever possible. We also suggest a "smoking gun" benchtop probe to test our elastic scenario.
NiFe$_2$O$_4$ thin films with varying thickness were grown on SrTiO$_3$(001) by reactive molecular beam epitaxy. Soft and hard x-ray photoelectron spectroscopy measurements reveal a homogeneous cation distribution throughout the whole film with stoichiometric Ni:Fe ratios of 1:2 independent of the film thickness. Low energy electron diffraction and high resolution (grazing incidence) x-ray diffraction in addition to x-ray reflectivity experiments were conducted to obtain information of the film surface and bulk structure, respectively. For ultrathin films up to 7.3 nm, lateral tensile and vert…
