arXiv subjects

D. A. Muller

Publications and source records attributed to D. A. Muller.

At least 19 recordsLinked to original sources

arXiv2025

Photoinduced twist and untwist of moiré superlattices in TMDC heterobilayers

Two-dimensional moiré materials are formed by artificially stacking atomically thin monolayers. A wealth of correlated and topological quantum phases can be engineered via precise choice of stacking geometry. These designer electronic properties depend crucially on interlayer coupling and atomic registry. An important open question is how atomic registry responds on ultrafast timescales to optical excitation and whether the moiré geometry can be dynamically reconfigured to tune emergent phenomena in real time. Here we show that femtosecond photoexcitation drives a coherent twist-untwist motion of the moiré superlattice in $2^\circ$ and $57^\circ$ twisted WSe$_2$/MoSe$_2$ heterobilayers, resolved directly by ultrafast electron diffraction. Upon above-band-gap photoexcitation, the moiré superlattice diffraction features are enhanced within 1 ps and subsequently suppressed several picoseconds after, deviating markedly from typical photoinduced lattice heating. Kinetic diffraction analysis, supported by simulations of the sample dynamics, indicates a peak-to-trough local twist angle modulation of $0.6^\circ$, correlated with a sub-THz frequency moiré phonon. This motion is driven by ultrafast charge transfer that transiently increases interlayer attraction. Our results could lead to ultrafast control of moiré periodic lattice distortions and, by extension, the local moiré potential that shapes excitons, polarons, and correlation-driven behaviors

cond-mat.mtrl-sci↗

arXiv2024

Superconductivity in the parent infinite-layer nickelate NdNiO$_2$

We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound NdNiO$_2$. A combination of oxide molecular-beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples with a variety of techniques, including electrical transport, scanning transmission electron microscopy, x-ray absorption spectroscopy, and resonant inelastic x-ray scattering, to investigate the possible origins of superconductivity. We propose that superconductivity could be intrinsic to the undoped infinite-layer nickelates but suppressed by disorder due to its nodal order parameter, a finding which would necessitate a reconsideration of the nickelate phase diagram. Another possible hypothesis is that the parent materials can be hole doped from randomly dispersed apical oxygen atoms, which would suggest an alternative pathway for achieving superconductivity.

cond-mat.supr-con↗

arXiv2023

Silicon Implantation and Annealing in $β$-Ga$_2$O$_3$: Role of Ambient, Temperature, and Time

Optimizing thermal anneals of Si-implanted $β$-Ga$_2$O$_3$ is critical for low resistance contacts and selective area doping. We report the impact of annealing ambient, temperature, and time on activation of room temperature ion-implanted Si in $β$-Ga$_2$O$_3$ at concentrations from 5x10$^{18}$ to 1x10$^{20}$ cm$^{-3}$, demonstrating full activation (>80% activation, mobilities >70 cm$^{2}$/Vs) with contact resistances below 0.29 $Ω$-mm. Homoepitaxial $β$-Ga$_2$O$_3$ films, grown by plasma assisted MBE on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5x10$^{18}$, 5x10$^{19}$, and 1x10$^{20}$ cm$^{-3}$. Anneals were performed in a UHV-compatible quartz furnace at 1 bar with well-controlled gas composition. To maintain $β$-Ga$_2$O$_3$ stability, $p_{O2}$ must be greater than 10$^{-9}$ bar. Anneals up to $p_{O2}$ = 1 bar achieve full activation at 5x10$^{18}$ cm$^{-3}$, while 5x10$^{19}$ cm$^{-3}$ must be annealed with $p_{O2}$ <10$^{-4}$ bar and 1x10$^{20}$ cm$^{-3}$ requires $p_{O2}$ <10$^{-6}$ bar. Water vapor prevents activation and must be maintained below 10$^{-8}$ bar. Activation is achieved for anneal temperatures as low as 850 °C with mobility increasing with anneal temperature up to 1050 °C, though Si diffusion has been reported above 950 °C. At 950 °C, activation is maximized between 5 and 20 minutes with longer times resulting in decreased carrier activation (over-annealing). This over-annealing is significant for concentrations above 5x10$^{19}$ cm$^{-3}$ and occurs rapidly at 1x10$^{20}$ cm$^{-3}$. RBS (channeling) suggests damage recovery is seeded from remnant aligned $β$-Ga$_2$O$_3$ that remains after implantation; this conclusion is also supported by STEM showing retention of the $β$-phase with inclusions that resemble the $γ$-phase.

cond-mat.mtrl-sci↗

arXiv2023

Multi-scale time-resolved electron diffraction: A case study in moiré materials

Ultrafast-optical-pump -- structural-probe measurements, including ultrafast electron and x-ray scattering, provide direct experimental access to the fundamental timescales of atomic motion, and are thus foundational techniques for studying matter out of equilibrium. High-performance detectors are needed in scattering experiments to obtain maximum scientific value from every probe particle. We deploy a hybrid pixel array direct electron detector to perform ultrafast electron diffraction experiments on a WSe$_2$/MoSe$_2$ 2D heterobilayer, resolving the weak features of diffuse scattering and moiré superlattice structure without saturating the zero order peak. Enabled by the detector's high frame rate, we show that a chopping technique provides diffraction difference images with signal-to-noise at the shot noise limit. Finally, we demonstrate that a fast detector frame rate coupled with a high repetition rate probe can provide continuous time resolution from femtoseconds to seconds, enabling us to perform a scanning ultrafast electron diffraction experiment that maps thermal transport in WSe$_2$/MoSe$_2$ and resolves distinct diffusion mechanisms in space and time.

physics.ins-det↗

arXiv2023

3D oxygen vacancy order and defect-property relations in multiferroic (LuFeO$_3$)$_9$/(LuFe$_2$O$_4$)$_1$ superlattices

Oxide heterostructures exhibit a vast variety of unique physical properties. Examples are unconventional superconductivity in layered nickelates and topological polar order in (PbTiO$_3$)$_n$/(SrTiO$_3$)$_n$ superlattices. Although it is clear that variations in oxygen content are crucial for the electronic correlation phenomena in oxides, it remains a major challenge to quantify their impact. Here, we measure the chemical composition in multiferroic (LuFeO$_3$)$_9$/(LuFe$_2$O$_4$)$_1$ superlattices, revealing a one-to-one correlation between the distribution of oxygen vacancies and the electric and magnetic properties. Using atom probe tomography, we observe oxygen vacancies arranging in a layered three-dimensional structure with a local density on the order of 10$^{14}$ cm$^{-2}$, congruent with the formula-unit-thick ferrimagnetic LuFe$_2$O$_4$ layers. The vacancy order is promoted by the locally reduced formation energy and plays a key role in stabilizing the ferroelectric domains and ferrimagnetism in the LuFeO$_3$ and LuFe$_2$O$_4$ layers, respectively. The results demonstrate the importance of oxygen vacancies for the room-temperature multiferroicity in this system and establish an approach for quantifying the oxygen defects with atomic-scale precision in 3D, giving new opportunities for deterministic defect-enabled property control in oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2023

Growth of $α-Ga_2O_3$ on $Al_2O_3$ by conventional molecular-beam epitaxy and metal-oxide-catalyzed epitaxy

We report the growth of $α-Ga_2O_3$ on $m$-plane $Al_2O_3$ by conventional plasma-assisted molecular-beam epitaxy (MBE) and In-mediated metal-oxide-catalyzed epitaxy (MOCATAXY). We report a growth-rate-diagram for $α-Ga_2O_3$ (10-10), and observe (i) a growth rate increase, (ii) an expanded growth window, and (iii) reduced out-of-lane mosaic spread when MOCATAXY is employed for the growth of $α-Ga_2O_3$. Through the use of In-mediated catalysis, growth rates over $0.2\,μ\text{m}\,\text{hr}^{-1}$ and rocking curves with full width at half maxima of $Δω\approx 0.45^{\circ}$ are achieved. Faceting is observed along the $α-Ga_2O_3$ film surface and is explored through scanning transmission electron microscopy.

cond-mat.mtrl-sci↗

arXiv2020

Transferring Orbital Angular Momentum to an Electron Beam Reveals Toroidal and Chiral Order

Orbital angular momentum and torque transfer play central roles in a wide range of magnetic textures and devices including skyrmions and spin-torque electronics(1-4). Analogous topological structures are now also being explored in ferroelectrics, including polarization vortex arrays in ferroelectric/dielectric superlattices(5). Unlike magnetic toroidal order, electric toroidal order does not couple directly to linear external fields. To develop a mechanism that can control switching in polarization vortices, we utilize a high-energy electron beam and show that transverse currents are generated by polar order in the ballistic limit. We find that the presence of an electric toroidal moment in a ferro-rotational phase transfers a measurable torque and orbital angular momentum to the electron beam. Furthermore, we find that the complex polarization patterns, observed in these heterostructures, are microscopically chiral with a non-trivial axial component of the polarization. This chirality opens the door for the coupling of ferroelectric and optical properties.

cond-mat.mtrl-sci↗

arXiv2020

Role of Dirac nodal lines and strain on the high spin Hall conductivity of epitaxial IrO2 thin films

Since the discovery of a 'giant' spin Hall effect (SHE) in certain heavy metal elements there has been an intense effort to identify and develop new and technologically viable, heavy-metal-based thin film materials that could generate spin currents with even greater efficiency to exert spin-orbit torques (SOT) on adjacent ferromagnetic nanostructures. In parallel, there have been wide ranging fundamental studies of the spin currents that can arise from robust, intrinsic spin-orbit interaction (SOI) effects in more exotic systems including topological insulators, transition metal dichalcogenides with broken crystalline symmetry, Weyl and Dirac semimetals where gapless electronic excitations are protected by topology and symmetry. Here we experimentally study strong SOT from the topological semimetal IrO2 in (001) and (110) normal films, which exhibit distinctly different SHE strengths. Angle resolved photoemission spectroscopy studies have shown IrO2 exhibits Dirac nodal lines (DNL) in the band structure, which could enable a very high spin Hall conductivity (SHC). The (001) films exhibit exceptionally high damping like torque efficiency ranging from 0.45 at 293 K to 0.65 at 30 K which sets the lower bound of SHC that is ten times higher and of opposite sign than the theoretical prediction. We observe a substantial reduction of SHC in anisotropically strained (110) films, which suggests that the DNLs that are present in the (001) films and contribute to SHC, are disrupted and gapped due to the large anisotropic strain in (110) films, which in turn significantly lowers SHC. Very large value of SHC at room temperature of this Dirac semimetal could be very promising for the practical application.

cond-mat.mes-hall↗

arXiv2020

Controlled introduction of defects to delafossite metals by electron irradiation

The delafossite metals PdCoO$_{2}$, PtCoO$_{2}$ and PdCrO$_{2}$ are among the highest conductivity materials known, with low temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure, or are a consequence of highly unusual levels of crystalline perfection. We report the results of experiments in which high energy electron irradiation was used to introduce point disorder to the Pd and Pt layers in which the conduction occurs. We obtain the cross-section for formation of Frenkel pairs in absolute units, and cross-check our analysis with first principles calculations of the relevant atomic displacement energies. We observe an increase of resistivity that is linear in defect density with a slope consistent with scattering in the unitary limit. Our results enable us to deduce that the as-grown crystals contain extremely low levels of in-plane defects of approximately $0.001\%$. This confirms that crystalline perfection is the most important factor in realizing the long mean free paths, and highlights how unusual these delafossite metals are in comparison with the vast majority of other multi-component oxides and alloys. We discuss the implications of our findings for future materials research.

cond-mat.mtrl-sci↗

arXiv2019

Ferromagnetism and conductivity in atomically thin SrRuO3

Atomically thin ferromagnetic and conducting electron systems are highly desired for spintronics, because they can be controlled with both magnetic and electric fields. We present (SrRuO3)1-(SrTiO3)5 superlattices and single-unit-cell-thick SrRuO3 samples that are capped with SrTiO3. We achieve samples of exceptional quality. In these samples, the electron systems comprise only a single RuO2 plane. We observe conductivity down to 50 mK, a ferromagnetic state with a Curie temperature of 25 K, and signals of magnetism persisting up to approximately 100 K.

cond-mat.mtrl-sci↗

arXiv2018

Exceptionally high, strongly temperature dependent, spin Hall conductivity of SrRuO3

Spin-orbit torques (SOT) in thin film heterostructures originate from strong spin-orbit interactions (SOI) that, in the bulk, generate a spin current as the result of extrinsic spin-dependent, skew or/and side-jump, scattering, or in the intrinsic case due to Berry curvature in the conduction band. While most SOT studies have focused on materials with heavy metal components, the oxide perovskite SrRuO3 has been predicted to have a pronounced Berry curvature. Through quantification of its spin current by the SOT exerted on an adjacent Co ferromagnetic layer, we determine that SrRuO3 has a strongly temperature (T) dependent spin Hall conductivity which becomes particularly high at low T, e.g. σ_{SH} \geqslant (\hbar/2e)3x10^{5} Ω^{-1}m^{-1} at 60 K. Below the SrRuO3 ferromagnetic transition, non-standard SOT components develop associated with the magnetic characteristics of the oxide, but these do not dominate as with spin currents from a conventional ferromagnet. Our results establish a new approach for the study of SOI in epitaxial conducting oxide heterostructures and confirm SrRuO3 as a promising candidate material for achieving new and enhanced spintronics functionalities.

cond-mat.mtrl-sci↗

arXiv2018

Electrical half-wave rectification at ferroelectric domain walls

Ferroelectric domain walls represent multifunctional 2D-elements with great potential for novel device paradigms at the nanoscale. Improper ferroelectrics display particularly promising types of domain walls, which, due to their unique robustness, are the ideal template for imposing specific electronic behavior. Chemical doping, for instance, induces p- or n-type characteristics and electric fields reversibly switch between resistive and conductive domain-wall states. Here, we demonstrate diode-like conversion of alternating-current (AC) into direct-current (DC) output based on neutral 180$^{\circ}$ domain walls in improper ferroelectric ErMnO$_3$. By combining scanning probe and dielectric spectroscopy, we show that the rectification occurs for frequencies at which the domain walls are fixed to their equilibrium position. The practical frequency regime and magnitude of the output is controlled by the bulk conductivity. Using density functional theory we attribute the transport behavior at the neutral walls to an accumulation of oxygen defects. Our study reveals domain walls acting as 2D half-wave rectifiers, extending domain-wall-based nanoelectronic applications into the realm of AC technology.

cond-mat.mtrl-sci↗

arXiv2017

Electron Accumulation and Emergent Magnetism in LaMnO3/SrTiO3 Heterostructures

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO3/SrTiO3 (001) heterostructures. Using a combination of element-specific X-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO3. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron over-accumulation. In turn, by controlling the doping of the LaMnO3, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO3 films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

cond-mat.mtrl-sci↗

arXiv2014

Epitaxial Growth of VO$_{2}$ by Periodic Annealing

We report the growth of ultrathin VO$_{2}$ films on rutile TiO$_{2}$ (001) substrates via reactive molecular-beam epitaxy. The films were formed by the cyclical deposition of amorphous vanadium and its subsequent oxidation and transformation to VO$_{2}$ via solid-phase epitaxy. Significant metal-insulator transitions were observed in films as thin as 2.3 nm, where a resistance change ΔR/R of 25 was measured. Low angle annular dark field scanning transmission electron microscopy was used in conjunction with electron energy loss spectroscopy to study the film/substrate interface and revealed the vanadium to be tetravalent and the titanium interdiffusion to be limited to 1.6 nm.

cond-mat.mtrl-sci↗

arXiv2013

LaAlO3 stoichiometry found key to electron liquid formation at LaAlO3/SrTiO3 interfaces

Emergent phenomena, including superconductivity and magnetism, found in the two-dimensional electron liquid (2-DEL) at the interface between the insulators LaAlO3 and SrTiO3 distinguish this rich system from conventional two-dimensional electron gases at compound semiconductor interfaces. The origin of this 2-DEL, however, is highly debated with focus on the role of defects in the SrTiO3 while the LaAlO3 has been assumed perfect. Our experiments and first principles calculations show that the cation stoichiometry of the nominal LaAlO3 layer is key to 2-DEL formation: only Al-rich LaAlO3 results in a 2-DEL. While extrinsic defects including oxygen deficiency are known to render LaAlO3/SrTiO3 samples conducting, our results show that in the absence of such extrinsic defects, an interface 2-DEL can form. Its origin is consistent with an intrinsic electronic reconstruction occurring to counteract a polarization catastrophe. This work provides a roadmap for identifying other interfaces where emergent behaviors await discovery.

cond-mat.mtrl-sci↗

arXiv2008

High-temperature interface superconductivity between metallic and insulating cuprates

High-temperature superconductivity confined to nanometer-size interfaces has been a long standing goal because of potential applications^{1,2} and the opportunity to study quantum phenomena in reduced dimensions^{3,4}. However, this is a challenging target: in conventional metals the high electron density restricts interface effects such as carrier depletion/accumulation to a region much narrower than the coherence length, the scale necessary for superconductivity to occur. In contrast, in copper oxides the carrier density is low while the critical temperature (T_c) is high and the coherence length very short; so, this provides a breakthrough opportunity but at a price: the interface must be atomically perfect. Here we report on superconductivity in bilayers consisting of an insulator (La_2CuO_4) and a metal (La_{1.55}Sr_{0.45}CuO_{4}), neither of which is superconducting in isolation. However, in bilayers T_c is either ~15 K or ~30 K, depending on the layering sequence. This highly robust phenomenon is confined within 2-3 nm from the interface. If such a bilayer is exposed to ozone, T_c exceeds 50 K and this enhanced superconductivity is also shown to originate from the interface layer about 1-2 unit cell thick. Enhancement of T_c in bilayer systems was observed previously^5 but the essential role of the interface was not recognized at the time. Our results demonstrate that engineering artificial heterostructures provides a novel, unconventional way to fabricate stable, quasi two-dimensional high T_c phases and to significantly enhance superconducting properties in known or new superconductors.

cond-mat.supr-con↗

arXiv2008

Negative Differential Resistance Induced by Mn Substitution at SrRuO3/Nb:SrTiO3 Schottky Interfaces

We observed a strong modulation in the current-voltage characteristics of SrRuO$_3$/Nb:SrTiO$_3$ Schottky junctions by Mn substitution in SrRuO$_3$, which induces a metal-insulator transition in bulk. The temperature dependence of the junction ideality factor indicates an increased spatial inhomogeneity of the interface potential with substitution. Furthermore, negative differential resistance was observed at low temperatures, indicating the formation of a resonant state by Mn substitution. By spatially varying the position of the Mn dopants across the interface with single unit cell control, we can isolate the origin of this resonant state to the interface SrRuO$_3$ layer. These results demonstrate a conceptually different approach to controlling interface states by utilizing the highly sensitive response of conducting perovskites to impurities.

cond-mat.str-el↗

arXiv2007

Nanometer scale electronic reconstruction at the interface between LaVO3 and LaVO4

Electrons at interfaces, driven to minimize their free energy, are distributed differently than in bulk. This can be dramatic at interfaces involving heterovalent compounds. Here we profile an abrupt interface between V 3d2 LaVO3 and V 3d0 LaVO4 using electron energy loss spectroscopy. Although no bulk phase of LaVOx with a V 3d1 configuration exists, we find a nanometer-wide region of V 3d1 at the LaVO3/LaVO4 interface, rather than a mixture of V 3d0 and V 3d2. The two-dimensional sheet of 3d1 electrons is a prototypical electronic reconstruction at an interface between competing ground states.

cond-mat.str-el↗