arXiv subjects

Salva Salmani-Rezaie

Publications and source records attributed to Salva Salmani-Rezaie.

At least 19 recordsLinked to original sources

arXiv2026

Molecular Beam Epitaxy of AgTaO3

We report the first synthesis of single-crystal AgTaO3 thin films using molecular-beam epitaxy (MBE). High-quality epitaxial AgTaO3 films were grown on both (001)- and (111)-oriented SrTiO3 substrates using sequential deposition of atomic silver and TaO2 layers under an ozone/oxygen atmosphere (80 % O3 + 20 % O2). X-ray diffrac- tion and reciprocal space mapping demonstrate that the films are coherently strained to the SrTiO3 substrates with sharp rocking curves comparable to the substrates, indicating high structural perfection. High-angle annular dark-field scanning trans- mission electron microscopy (HAADF-STEM) confirms coherent, low defect growth for (001)pc-oriented films. For (111)pc-oriented films, initial coherent growth proceeds up to 10 nm before transitioning into a Ta-rich surface region. Energy-dispersive X-ray spectroscopy (EDX) reveals a narrow cation intermixing region at the sub- strate interface for both orientations. This work demonstrates an effective synthesis route for single-crystal AgTaO3 thin films, providing a platform to investigate strain engineering and emergent interfacial properties in silver-based tantalates.

cond-mat.mtrl-sci↗

arXiv2026

Solid-State Dealloying Enables Local Symmetry Breaking in Ternary Intermetallic Thin Films

Metastable quantum materials often occupy narrow composition windows with local symmetries distinct from competing equilibrium structures. These features open pathways for realizing qualitatively new electronic properties within a similar chemical subspace while at the same time complicating their deterministic synthesis. Here we illustrate a post-growth solid-state dealloying process using epitaxial ternary thin films in the La--Ag--Ge chemical space to realize a diffraction-averaged centrosymmetric superconducting structure. The process converts polar $P6_3mc$-LaAgGe into diffraction-averaged centrosymmetric AlB$_2$-type $P6/mmm$ La-Ag-Ge through net Ag loss during annealing at 800--900 $^\circ$C. Atomic-resolution electron microscopy reveals local Ag--Ge displacements of both signs relative to the planar configuration, consistent with local inversion-symmetry-breaking distortions. The converted films exhibit composition-dependent superconductivity with critical temperatures below 1 K and in-plane upper critical fields that exceed the weak-coupling Pauli-field estimate by a factor of approximately 6 in the thinnest superconducting films. These results establish epitaxial solid-state dealloying as a route to phase-selective synthesis of metastable phases, offering control over atomic-scale structural configurations and their interplay with emergent electronic properties.

cond-mat.mtrl-sci↗

arXiv2026

Crystalline Germanium Josephson Junctions

Conventional superconducting quantum electronics rely on well-established Josephson junctions made of Al/AlO$_{x}$ where the weak link AlO$_{x}$ is amorphous and is believed to host two-level systems that limit coherence. Crystalline Josephson junctions exhibit atomically ordered interface quality but remain constrained by complex fabrication and intrinsic asymmetry of epitaxial growth. Here, we demonstrate a fully epitaxial approach based on superconductivity in gallium-doped germanium, enabling the realization of Josephson junctions entirely grown in situ by molecular beam epitaxy. These devices feature atomically sharp interfaces and crystalline weak links, resulting in strong Josephson coupling in the ultra-short regime. We observe an unconventional enhancement of the switching current under applied magnetic field, which we attribute to quasiparticle-assisted thermalization processes from the Al contacts. This platform combines structural coherence, fabrication simplicity, and scalability, offering a promising route toward low-disorder, CMOS-compatible superconducting qubits in a merged element transmon architecture.

cond-mat.mes-hall↗

arXiv2026

Transport Evidence of Magnetic Polarization in the Altermagnetic Candidate MnTe

The ability to precisely control magnetic properties is central to the development of future spin-based electronics. In this work, we report the successful growth of epitaxial α-MnTe thin films on InP(111) substrates using molecular beam epitaxy. Magneto-transport measurements at low temperatures reveal a distinct, hysteretic butterfly longitudinal magnetoresistance alongside a nonlinear transverse magneto-resistance response, suggesting the presence of a finite net magnetic polarization in the films. To understand the origin of this behavior, density functional theory (DFT) calculations were performed. While pristine bulk MnTe is a compensated antiferromagnet, our computational results suggest multiple pathways through which a finite magnetization can emerge in thin-film geometries, including interface-induced symmetry breaking and point defects. These findings demonstrate an epitaxial route for engineering magnetic responses in thin films.

cond-mat.mtrl-sci↗

arXiv2026

Above Room Temperature Ferroelectricity in Epitaxially Strained KTaO3

Epitaxial strain is a powerful means to engineer emergent phenomena in thin films and heterostructures. Here, we demonstrate that KTaO3, a cubic perovskite in bulk form, can be epitaxially strained into a highly tunable ferroelectric. KTaO3 films grown commensurate to SrTiO3 (001) substrates experience an in-plane strain of -2.1 % that transforms the cubic structure into a tetragonal polar phase with transition temperature of 475 K, consistent with our thermodynamic calculations. We show that the Curie temperature and the spontaneous electric polarization can be system- atically controlled with epitaxial strain. Scanning transmission electron microscopy reveals cooperative polar displacements of the potassium columns with respect to the neighboring tantalum columns at room temperature. Optical second-harmonic generation results are described by a tetragonal polar point group (4mm), indicating the emergence of a global polar ground state. We observe a ferroelectric hysteresis response, using metal-insulator-metal capacitor test structures. The results demon- strate a robust intrinsic ferroelectric state in epitaxially strained KTaO3 thin films.

cond-mat.mtrl-sci↗

arXiv2025

Superconductivity in Substitutional Ga-Hyperdoped Ge Epitaxial Thin Films

Doping-induced superconductivity in group IV elements may enable quantum functionalities in material systems accessible with well-established semiconductor technologies. Non-equilibrium hyperdoping of group III atoms into C, Si, or Ge can yield superconductivity; however, its origin is obscured by structural disorder and dopant clustering. Here, we report the epitaxial growth of hyperdoped Ga:Ge films and trilayer heterostructures by molecular beam epitaxy with extreme hole concentrations ($n_\textup{h} = 4.15 \times 10^{21}$~cm$^{-3}$, ~17.9\% Ga substitution) that yield superconductivity with a critical temperature of $T_{\textup{c}} = 3.5$~K and an out-of-plane critical field of 1~T at 270~mK. Synchrotron-based X-ray absorption and scattering methods reveal that Ga dopants are substitutionally incorporated within the Ge lattice, introducing a tetragonal distortion to the crystal unit cell. Our findings, corroborated by first-principles calculations, suggest that the structural order of Ga dopants creates a narrow band for the emergence of superconductivity in Ge, establishing hyperdoped Ga:Ge as a low-disorder, epitaxial superconductor-semiconductor platform.

cond-mat.mes-hall↗

arXiv2025

Metalorganic Chemical Vapor Deposition of AlScN Thin Films and AlScN/AlN/GaN Heterostructures

AlScN thin films were grown via metalorganic chemical vapor deposition (MOCVD), showing controllable incorporation of scandium (Sc) into the AlN lattices. Systematic variation of growth parameters demonstrated an obvious influence on Sc incorporation, with X-ray photoelectron spectroscopy (XPS) analysis indicating Sc composition up to $\sim$13\% when (MCp)$_2$ScCl was used as the precursor. AlScN/AlN/GaN heterostructures grown on GaN templates exhibited the formation of a two-dimensional electron gas (2DEG) channel at the AlScN/AlN--GaN interface, confirming their potential use in high electron mobility transistor (HEMT) device technologies. Variation in AlScN/AlN barrier thickness within the heterostructures showed that thicker barriers yield higher sheet charge densities from both Hall and capacitance-voltage (C--V) measurements. With an AlScN/AlN barrier thickness of $\sim$30~nm, a sheet charge density of $5.22\times10^{12}$~cm$^{-2}$ was extracted from C--V. High-resolution scanning transmission electron microscopy (S/TEM) further confirmed Sc incorporation and revealed the wurtzite crystalline structure of the films and heterostructures. These results establish MOCVD growth of AlScN as a promising and compatible material for advancing III-nitride heterostructures in high-performance electronics and potentially ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2025

Large phonon-drag thermopower polarity reversal in Ba-doped KTaO3

This study reports the observation of phonon-drag thermopower polarity reversal in Ba-doped KTaO3 thin films, mediated by electron-phonon Umklapp scattering. Epitaxial films with distinct carrier concentrations (3.7 x 10^20 cm^-3 and 4.9 x 10^19 cm^-3) were grown via molecular-beam epitaxy. In heavily doped samples, where the Fermi surface spans 80% of the Brillouin zone, the Umklapp condition is satisfied, reversing electron momentum. This manifests as a sign-reversal in the thermopower around 80 K upon cooling despite the sample having only n-type carriers. On the other hand, the lightly doped sample (4.9 x 10^19 cm^-3) exhibits only a negative thermopower down to 2 K. These results advance the understanding of Umklapp electron-phonon drag in oxides and highlight KTaO3's potential for engineering unconventional thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2025

Electrostatic control of quantum phases in KTaO3-based planar constrictions

Two-dimensional electron gases (2DEGs) formed at complex oxide interfaces offer a unique platform to engineer quantum nanostructures. However, scalable fabrication of locally addressable devices in these materials remains challenging. Here, we demonstrate an efficient fabrication approach by patterning narrow constrictions in a superconducting KTaO3-based heterostructure. The constrictions are individually tunable via the coplanar side gates formed within the same 2DEG plane. Our technique leverages the high dielectric permittivity of KTaO3 (epsilon_r ~ 5000) to achieve strong electrostatic modulation of the superconducting 2DEG. Transport measurements through the constriction reveal a range of transport regimes: Within the superconducting state, we demonstrate efficient modulation of the critical current and Berezinskii Kosterlitz Thouless (BKT) transition temperature at the weak link. Further tuning of the gate voltage reveals an unexpectedly regular Coulomb blockade pattern. All of these states are achievable with a side gate voltage |V_SG| < 1 V. The fabrication process is scalable and versatile, enabling a platform both to make superconducting field-effect transistors and to study a wide array of physical phenomena present at complex oxide interfaces.

cond-mat.supr-con↗

arXiv2025

Superconducting vacancy-ordered rock-salt NbO films

We report molecular beam epitaxy synthesis of vacancy-ordered rocksalt NbO thin films which display superconductivity. A comparative study of substrates identifies Al$_2$O$_3$ (0001) as the optimal platform for realizing high-quality, single-phase films when growing at temperatures exceeding 1000 $^\circ$C. The controlled NbO films exhibit superconductivity with critical temperatures up to $T_\mathrm{c}$ = 1.37 K, comparable to bulk single crystals. This work addresses the fundamental bottlenecks encountered in the high-temperature epitaxy of compounds with uncommon oxidation states, while expanding the scope of available thin-film superconductors.

cond-mat.supr-con↗

arXiv2025

Superconductivity in tin telluride films grown by molecular beam epitaxy

The intersection of superconductivity and ferroelectricity hosts a wide range of exotic quantum phenomena. Here, we report on the observation of superconductivity in high-quality tin telluride films grown by molecular beam epitaxy. Unintentionally doped tin telluride undergoes a ferroelectric transition at ~100 K. The critical temperature of superconductivity increases monotonically with indium concentration. The critical field of superconductivity, however, does not follow the same behavior as critical temperature with indium concentration and exhibits a carrier-density-dependent violation of the Pauli limit. The electron-phonon coupling, from the McMillan formula, exhibits a systematic enhancement with indium concentration, suggesting a potential violation of BCS weak coupling at high indium concentrations.

cond-mat.supr-con↗

arXiv2025

Violation of Pauli Limit at KTaO3(110) Interfaces

The superconducting order parameter at the KTaO3 interfaces and its dependence on interface orientation remains a subject of debate. The superconductivity at these interfaces exhibits strong resilience against in-plane magnetic field and violates Pauli limit. The interface orientation dependence of critical field and violation of Pauli limit, however, have not been investigated. To address this problem, we grew epitaxial LaMnO3/KTaO3 heterostructures using molecular beam epitaxy. We show that superconductivity is extremely robust against the in-plane magnetic field. Our results indicate that the interface orientation, despite impacting the critical temperature, does not affect the ratio of critical field to the Pauli limiting field. These results offer opportunities to engineer superconductors which are resilient against magnetic field.

cond-mat.supr-con↗

arXiv2025

Surface and Bulk Two-Level System Losses in Lithium Niobate Acoustic Resonators

Lithium niobate (LN) is a promising material for building acoustic resonators used in quantum applications, but its performance is limited by poorly understood material defects called two-level systems (TLS). In this work, we fabricate high-performance acoustic resonators from LN with quality factors up to $6\times10^7$ and use them to separate bulk and surface contributions to TLS loss. By comparing these bulk acoustic wave (BAW) resonators with previous surface acoustic wave and phononic crystal studies, we show that devices with high surface participation ratios are limited by surface TLS, while our BAW devices reveal an intrinsic bulk TLS limit. Through systematic surface treatments and microscopy, we demonstrate that BAW resonator performance remains unchanged despite surface modifications, confirming operation in a bulk-limited regime. Our work establishes quantitative bounds on both surface and bulk TLS losses in LN, within the context of material growth and fabrication approaches we have pursued, and provides guidance for future device engineering and materials development.

cond-mat.mes-hall↗

arXiv2024

High temperature diffusion enabled epitaxy of the Ti-O system

High temperatures promote kinetic processes which can drive crystal synthesis towards ideal thermodynamic conditions, thereby realizing samples of superior quality. While accessing very high temperatures in thin-film epitaxy is becoming increasingly accessible through laser-based heating methods, demonstrations of such utility are still emerging. Here we realize a novel self-regulated growth mode in the Ti-O system by relying on thermally activated diffusion of oxygen from an oxide substrate. We demonstrate oxidation selectivity of single phase films with superior crystallinity to conventional approaches as evidenced by structural and electronic measurements. The diffusion-enabled mode is potentially of wide use in the growth of transition metal oxides, opening up new opportunities for ultra-high purity epitaxial platforms based on d -orbital systems.

cond-mat.mtrl-sci↗

arXiv2024

Monoclinic LaSb$_2$ Superconducting Thin Films

Rare-earth diantimondes exhibit coupling between structural and electronic orders which are tunable under pressure and temperature. Here we present the discovery of a new polymorph of LaSb$_2$ stabilized in thin films synthesized using molecular beam epitaxy. Using diffraction, electron microscopy, and first principles calculations we identify a YbSb$_2$-type monoclinic lattice as a yet-uncharacterized stacking configuration. The material hosts superconductivity with a $T_\mathrm{c}$ = 2 K, which is enhanced relative to the bulk ambient phase, and a long superconducting coherence length of 140 nm. This result highlights the potential thin film growth has in stabilizing novel stacking configurations in quasi-two dimensional compounds with competing layered structures.

cond-mat.mtrl-sci↗

arXiv2023

Supercell formation in epitaxial rare-earth ditelluride thin films

Square net tellurides host an array of electronic ground states and commonly exhibit charge-density-wave ordering. Here we report the epitaxy of DyTe$_{2-δ}$ on atomically flat MgO (001) using molecular beam epitaxy. The films are single phase and highly oriented as evidenced by transmission electron microscopy and X-ray diffraction measurements. Epitaxial strain is evident in films and is relieved as the thickness increases up to a value of approximately 20 unit cells. Diffraction features associated with a supercell in the films are resolved which is coupled with Te-deficiency. First principles calculations attribute the formation of this defect lattice to nesting conditions in the Fermi surface, which produce a periodic occupancy of the conducting Te square-net, and opens a band gap at the chemical potential. This work establishes the groundwork for exploring the role of strain in tuning electronic and structural phases of epitaxial square-net tellurides and related compounds.

cond-mat.mtrl-sci↗

arXiv2023

Enhanced Critical Field of Superconductivity at an Oxide Interface

The nature of superconductivity and its interplay with strong spin-orbit coupling at the KTaO3(111) interfaces remains a subject of debate. To address this problem, we grew epitaxial LaMnO3/KTaO3(111) heterostructures. We show that superconductivity is robust against the in-plane magnetic field, with the critical field of superconductivity reaching 25 T in optimally doped heterostructures. The superconducting order parameter is highly sensitive to carrier density. We argue that spin-orbit coupling drives the formation of anomalous quasiparticles with vanishing magnetic moment, providing the condensate significant immunity against magnetic fields beyond the Pauli paramagnetic limit. These results offer design opportunities for superconductors with extreme resilience against magnetic field.

cond-mat.supr-con↗

arXiv2022

Tracking ultrafast change of multiterahertz broadband response functions in a photoexcited Dirac semimetal Cd$_3$As$_2$ thin film

The electromagnetic response of Dirac semimetals in the infrared and terahertz frequency ranges is attracting growing interest for potential applications in optoelectronics and nonlinear optics. The interplay between the free-carrier response and interband transitions in the gapless, linear dispersion relation plays a key role in enabling novel functionalities. Here we investigate ultrafast dynamics in thin films of a photoexcited Dirac semimetal Cd$_3$As$_2$ by probing the broadband response functions as complex quantities in the multiterahertz region (10-45 THz, 40-180 meV, or 7-30 $μ$m), which covers the crossover between the inter and intraband response. We resolve dynamics of the photoexcited nonthermal electrons which merge with originally existing carriers to form a single thermalized electron gas and how it is facilitated by high-density excitation. We also demonstrate that a large reduction of the refractive index by 80% dominates the nonequilibrium infrared response, which can be utilized for designing ultrafast switches in active optoelectronics.

cond-mat.mtrl-sci↗