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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Hidden Spin-Valley Locking Stabilizes Nanosecond Spin Polarization in 2D Perovskites

Room-temperature spin control in semiconductors is fundamental to spin-optoelectronics. Although inversion symmetry breaking offers one path for spin control in semiconductors, strong spin dephasing at elevated temperatures remains a persistent limitation. Hidden spin polarization without global symmetry breaking provides another promising material design strategy, but robust spin stabilization from this effect has yet to be experimentally realized. Here we show spin stabilization at room temperature in two-dimensional hybrid organic-inorganic perovskites through hidden spin-valley locking. We use functional non-primary ammonium cations to induce symmetry-breaking distortions in the metal-halide layers, producing giant local spin splitting while preserving global inversion symmetry. Time-resolved circular dichroism measurements reveal optically generated spin-polarized carriers that persist for 686 ps in (AzOH)2PbI4 and 4.7 ns in the lead-free analogue (AzOH)2SnI4 at room temperature, without an external magnetic field. First-principles calculations suggest that these long lifetimes arise from hidden spin valleys, enhanced dielectric screening and reduced spin-orbit-induced scattering in the Sn-based compound. Our design paradigm unlocks inversion-symmetric semiconductors with ultralong spin lifetimes, broadening the materials options for light-driven spin control.

14 SOLAR ENERGY↗
DOE OSTI2019

Broadband Optical Detection Using the Spin Seebeck Effect

The generation, control, and detection of spin currents in solid-state devices are critical for Joule-heating minimization, spin-based computation, and electrical-energy generation from thermal gradients. Although incorporation of spin functionality into technologically important architectures is still in its infancy, advantages over all-electric devices are increasingly becoming clear. Here, we utilize the spin Seebeck effect (SSE) in Pt/Y 3 Fe 5 O 12 devices to detect light from 390 to 2200 nm. We find that the device responsivity is remarkably flat across this technologically important wavelength range, closely following the Pt absorption coefficient. As expected from a SSE-generation mechanism, we observe that the photovoltage and Pt heating dynamics are in strong agreement. To precisely determine the optically created thermal gradient produced from a pointlike heat source, we introduce a field-modulation method for measuring the SSE. Furthermore, our results show that broadband optical detection can be performed with devices based solely on spin-current generation and detection.

36 MATERIALS SCIENCE↗
DOE OSTI2019

THz emission from Co/Pt bilayers with varied roughness, crystal structure, and interface intermixing

Ultrafast demagnetization of Co/Pt heterostructures induced by a femtosecond 800-nm laser pulse launches a spin current from Co to Pt and subsequent conversion of the spin current to a charge current in the Pt layer due to the inverse spin-Hall effect. At the same time, due to the spin-dependent photogalvanic effect, a circularly polarized femtosecond laser pulse also generates a photocurrent at the Co/Pt interface. Both ultrashort photocurrent pulses are effectively detected in a contactless way by measuring the THz radiation they emit. Here we aim to understand how the properties of the Co/Pt interface affect the photocurrents in the bilayers. By varying the interfacial roughness, crystal structure, and interfacial intermixing, as well as having an explicit focus on the cases when THz emissions from these two photocurrents reveal opposite trends, we identify which interface properties play a crucial role for the photocurrents. In particular, we show that by reducing the roughness, the THz emission due to the spin-dependent photogalvanic effect reduces to zero while the strength of the THz emission from the photocurrent associated with the inverse spin-Hall effect increases by a factor of 2. On the other hand, while intermixing strongly enhances the THz emission from the inverse spin-Hall effect by a factor of 4.2, THz emission related to the spin-dependent photogalvanic effect reveals the opposite trend. Furthermore, these findings indicate that microstructural properties of the Co-Pt interface play a decisive role in the generation of photocurrents.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Revealing giant internal magnetic fields due to spin fluctuations in magnetically doped colloidal nanocrystals

Strong quantum confinement in semiconductors can compress the wavefunctions of band electrons and holes to nanometre-scale volumes, significantly enhancing interactions between themselves and individual dopants. In magnetically doped semiconductors, where paramagnetic dopants (such as Mn 2+ , Co 2+ and so on) couple to band carriers via strong sp–d spin exchange, giant magneto-optical effects can therefore be realized in confined geometries using few or even single impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit. In nanoscale volumes, the statistical √N fluctuations of N spins are expected to generate giant effective magnetic fields B eff , which should dramatically impact carrier spin dynamics, even in the absence of any applied field. In this paper, we directly and unambiguously reveal the large B eff that exist in Mn 2+ -doped CdSe colloidal nanocrystals using ultrafast optical spectroscopy. At zero applied magnetic field, extremely rapid (300–600 GHz) spin precession of photoinjected electrons is observed, indicating B eff ~ 15-30 T for electrons. Precession frequencies exceed 2 THz in applied magnetic fields. Finally, these signals arise from electron precession about the random fields due to statistically incomplete cancellation of the embedded Mn 2+ moments, thereby revealing the initial coherent dynamics of magnetic polaron formation, and highlighting the importance of magnetization fluctuations on carrier spin dynamics in nanomaterials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Unconventional Spintronics from Chiral Perovskites

Spintronic devices typically employ heterostructures with ferromagnets which break time-reversal symmetry and have non-vanishing magnetization. With the growing class of materials that support spin-polarized carriers, current, and excitations, it is possible to envision emerging spintronic applications that are not limited to magnetoresistance. Here, chiral perovskites with no net magnetization are considered, where the space-inversion and mirror symmetries are broken to induce chiral structure. The known importance of these perovskites is further expanded by the demonstration of the chiral-induced spin selectivity (CISS). However, the generation of the spin-polarized carriers across the interface with these chiral perovskites remains to be fully understood. These first-principles studies for 2D PbBr4-based chiral perovskites provide their electronic structure and an orbital-based symmetry analysis, which allows to establish an effective Hamiltonian to elucidate the underlying origin of their chirality. The same analysis is used for the Edelstein effect, responsible for electrical generation of the nonequilibrium spin polarization in many materials, which in chiral perovskites can be a mechanism contributing to CISS. Furthermore, by examining optical properties of chiral perovskites and the opportunity to use them to realize tunable altermagnets, another class of zero-magnetization spintronic materials, a versatile materials platform is put forth for unconventional spintronics.

14 SOLAR ENERGY↗
DOE OSTI2018

Dynamics and Spin-Valley Locking Effects in Monolayer Transition Metal Dichalcogenides

Transition metal dichalcogenides have been the primary materials of interest in the field of valleytronics for their potential in information storage, yet the limiting factor has been achieving long valley decoherence times. We explore the dynamics of four monolayer TMDCs (MoS 2 , MoSe 2 , WS 2 , WSe 2 ) using ab initio calculations to describe electron–electron and electron–phonon interactions. By comparing calculations which both omit and include relativistic effects, we isolate the impact of spin-resolved spin–orbit coupling on transport properties. In our work, we find that spin–orbit coupling increases carrier lifetimes at the valence band edge by an order of magnitude due to spin-valley locking, with a proportional increase in the hole mobility at room temperature. At temperatures of 50 K, we find intervalley scattering times on the order of 100 ps, with a maximum value of ~140 ps in WSe 2 . Finally, we calculate excited-carrier generation profiles which indicate that direct transitions dominate across optical energies, even for WSe 2 which has an indirect band gap. Our results highlight the intriguing interplay between spin and valley degrees of freedom critical for valleytronic applications. Further, our work points toward interesting quantum properties on-demand in transition metal dichalcogenides that could be leveraged via driving spin, valley, and phonon degrees of freedom.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Spin current generation and relaxation in a quenched spin-orbit-coupled Bose-Einstein condensate

Understanding the effects of spin-orbit coupling (SOC) and many-body interactions on spin transport is important in condensed matter physics and spintronics. This topic has been intensively studied for spin carriers such as electrons but barely explored for charge-neutral bosonic quasiparticles (including their condensates), which hold promises for coherent spin transport over macroscopic distances. Here, we explore the effects of synthetic SOC (induced by optical Raman coupling) and atomic interactions on the spin transport in an atomic Bose-Einstein condensate (BEC), where the spin-dipole mode (SDM, actuated by quenching the Raman coupling) of two interacting spin components constitutes an alternating spin current. We experimentally observe that SOC significantly enhances the SDM damping while reducing the thermalization (the reduction of the condensate fraction). We also observe generation of BEC collective excitations such as shape oscillations. Our theory reveals that the SOC-modified interference, immiscibility, and interaction between the spin components can play crucial roles in spin transport.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Ultrafast Light-Induced Magnetoelectric Effect in van der Waals Magnetic Semiconductor Heterostructures

Atomic-scale heterostructures of van der Waals (vdW) magnets and semiconductors provide a unique environment for exploring magnetic dynamics. In contrast to typical photothermal excitation of precessional magnetization dynamics by a pump laser pulse, we find that ultrafast optical excitation of a WS2/CrGeTe3 (CGT) bilayer produces an opposite sign of magnetic torque compared to an isolated CGT film. Experimental observations by time-resolved magneto-optic Kerr effect (TR-MOKE) and theoretical analysis by density functional theory (DFT) and Landau-Lifshitz-Gilbert (LLG) simulations support a mechanism in which charge transfer of photoexcited carriers across the interface alters the perpendicular magnetic anisotropy, which in turn generates a torque on the magnetic layer to trigger precessional magnetization dynamics. These results provide new avenues for ultrafast manipulation of magnetization in vdW heterostructures with type-II band alignments. Lastly, we show that optically-generated spin currents from WS2 into CGT can also trigger precessional dynamics via angular momentum transfer.

Zhou, Wenyi [The Ohio State University]↗
DOE OSTI2023

Optically pumped gradient magnetometer

A method is provided for sensing a magnetic field in a magnetic gradiometer of the kind in which pump light and light constituting an optical carrier traverse first and second atomic vapor cells that contain host atoms and that are separated from each other by a known distance. According to such method, the host atoms are prepared in a coherent superposition of two quantum states that differ in energy by an amount that is sensitive to an ambient magnetic field. Modulation of the optical carrier in the respective cells gives rise to sidebands that interfere to generate a beat frequency indicative of the magnetic field gradient. The host atoms are prepared at least in a mode that allows measurement of ambient magnetic field components perpendicular to the axis of the pump light. In such mode, the host atoms are spin-polarized by pump light while subjected to a controlled magnetic field directed parallel to the pump beam, and then the controlled magnetic field is adiabatically extinguished.

47 OTHER INSTRUMENTATION↗
DOE OSTI2017

Trion valley coherence in monolayer semiconductors

The emerging field of valleytronics aims to exploit the valley pseudospin of electrons residing near Bloch band extrema as an information carrier. Recent experiments demonstrating optical generation and manipulation of exciton valley coherence (the superposition of electron-hole pairs at opposite valleys) in monolayer transition metal dichalcogenides (TMDs) provide a critical step towards control of this quantum degree of freedom. The charged exciton (trion) in TMDs is an intriguing alternative to the neutral exciton for control of valley pseudospin because of its long spontaneous recombination lifetime, its robust valley polarization, and its coupling to residual electronic spin. Trion valley coherence has however been unexplored due to experimental challenges in accessing it spectroscopically. In this work, we employ ultrafast two-dimensional coherent spectroscopy to resonantly generate and detect trion valley coherence in monolayer MoSe 2 demonstrating that it persists for a few-hundred femtoseconds. We conclude that the underlying mechanisms limiting trion valley coherence are fundamentally different from those applicable to exciton valley coherence.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2023

Neutral Silicon-Vacancy Centers in Diamond via Photoactivated Itinerant Carriers

Neutral silicon-vacancy (Si-V 0 ) centers in diamond are promising candidates for quantum network applications because of their exceptional optical properties and spin coherence. However, the stabilization of Si-V 0 centers requires careful Fermi-level engineering of the diamond host material, making further technological development challenging. Here, we show that Si-V 0 centers can be efficiently stabilized by photoactivated itinerant carriers. Even in this nonequilibrium configuration, the resulting Si-V 0 centers are stable enough to allow for resonant optical excitation and optically detected magnetic resonance. Furthermore, our results pave the way for on-demand generation of Si-V 0 centers as well as other emerging quantum defects in diamond.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Microscopic signature of insulator-to-metal transition in highly doped semicrystalline conducting polymers in ionic-liquid-gated transistors

Electronic state of charge carriers, in particular, in highly doped regions, in thin-film transistors of a semicrystalline conducting polymer poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene), has been studied by using field-induced electron spin resonance (ESR) spectroscopy. By adopting an ionic-liquid gate insulator, a gate-controlled reversible electrochemical hole-doping of the polymer backbone is achieved, as confirmed from the change of the optical absorption spectra. The edge-on molecular orientation in the pristine film is maintained even after the electrochemical doping, which is clarified from the angular dependence of the g value. As the doping level increases, spin 1/2 polarons transform into spinless bipolarons, which is demonstrated from the spin-charge relation showing a spin concentration peak around 1%, contrasting to the monotonic increase in the charge concentration. At high doping levels, a drastic change in the linewidth anisotropy due to the generation of conduction electrons is observed, indicating the onset of metallic state, which is also supported by the temperature dependence of the spin susceptibility and the ESR linewidth. Our results suggest that semicrystalline conducting polymers become metallic with retaining their molecular orientational order, when appropriate doping methods are chosen.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Strongly Surface State Carrier‐Dependent Spin–Orbit Torque in Magnetic Topological Insulators

Abstract The topological surface states (TSS) in topological insulators (TIs) can exert strong spin–orbit torque (SOT) on adjacent magnetization, offering great potential in implementing energy‐efficient magnetic memory devices. However, there are large discrepancies among the reported spin Hall angle values in TIs, and its temperature dependence still remains elusive. Here, the spin Hall angle in a modulation‐doped Cr‐Bi x Sb 2− x Te 3 (Cr‐BST) film is quantitatively determined via both transport and optic approaches, where consistent results are obtained. A large spin Hall angle of ≈90 in the modulation‐doped Cr‐BST film is demonstrated at 2.5 K, and the spin Hall angle drastically decreases to 0.3–0.5 as the temperature increases. Moreover, by tuning the top TSS carrier concentration, a competition between the top and bottom TSS in contributing to SOT is observed. The above phenomena can account for the large discrepancies among the previously reported spin Hall angle values and reveal the unique role of TSS in generating SOT.

Che, Xiaoyu↗
DOE OSTI2020

Bright trion emission from semiconductor nanoplatelets

The trion, a quasiparticle comprising one exciton and an additional charge carrier, offers unique opportunities for generating spin-photon interfaces that can be used in developing quantum networks. Trions are also actively sought after for integrated optoelectronic devices including photovoltaics, photodetectors, and spintronics. However, formation of trions in strongly confined low-dimensional materials is often deemed detrimental. This is because trion emission in such materials is typically prohibited due to the predominant nonradiative Auger recombination processes. Semiconductor nanoplatelets with their strong confinement in the thickness direction and extended lateral geometries exhibit large exciton coherence sizes and reduced carrier-carrier interactions that may enable unprecedented trion properties. In this paper, we perform optical spectroscopic studies of individual CdSe nanoplatelets at cryogenic temperatures and observe bright trion emission with intensities comparable to that of neutral exciton emission. We perform carrier dynamics studies of the nanoplatelets and find that due to their extended lateral geometry, the fast radiative decay rate of the nanoplatelets at cryogenic temperatures is comparable to the inhibited Auger recombination rate, leading to the bright trion emission. Our tight-binding theory further reveals distinct size-tunable trion emission in the nanoplatelets that is advantageous for efficient trion emission. These properties make semiconductor nanoplatelets potential candidates as photon sources for optoelectronic and quantum logic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Circular Photocurrent in Weyl Semimetals with Mirror Symmetry

We have considered theoretically Weyl’s semimetals, for which the point symmetry group contains reflection planes and which have equivalent valleys with opposite chiralities. These materials include the most often studied compounds, viz., monopnictides TaAs, NbAs, TaP, NbP of transition metals, as well as Bi{sub 1 ‒} {sub x}Sb{sub x} alloys. Circular photogalvanic current inverting its direction upon the sign reversal of the circular polarization has been calculated for light absorption under direct optical transitions near Weyl points. The total contribution of all valleys to the photocurrent differs from zero beyond the limits of Weyl’s model when the spin-independent tilt linear in wavevector k or spin-dependent terms cubic in k are taken into account additionally in the effective electron Hamiltonian. When the tilt of the energy dispersion curve in a Weyl semimetal of symmetry C{sub 4v} is considered, the photogalvanic current can be expressed in terms of the components of rank-two symmetric tensor determining the energy spectrum of carriers near a Weyl’s node. At low temperatures, this contribution to the photocurrent is excited in a certain limited frequency interval Δ. The photocurrent associated with cubic corrections in the optical absorption region is proportional to the square of light frequency and is generated both within window Δ and beyond its limits.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Effect of doping and chemical ordering on the optoelectronic properties of complex oxides: Fe 2 O 3 –V 2 O 3 solid solutions and hetero-structures

The electronic and optical properties of a-(Fe1xVx)2O3 at low (x = 0.04) and high (x = 0.5) doping levels are investigated using a combination of periodic and embedded cluster approaches, and time dependent density functional theory. At low V concentrations the onset of the optical absorption is B0.5 eV (i.e., nearly 1.6 eV lower than that in pure a-Fe2O3) and corresponds to the electron transitions from V 3d to Fe 3d* orbitals. At high V concentrations, optical absorption energies and intensities are sensitive to specific arrangements of Fe and V atoms and their spin configuration that determine Fe–V hybridization. The onset of the lowest inter-vanadium absorption band in the case of Fe2O3/V2O3 hetero-structures is as low as B0.3 eV and the corresponding peak is at B0.7 eV. In contrast, in the case of solid solutions this peak has lower intensity and is shifted to higher energy (B1.2 eV). Analysis of the orbital character of electronic excitation suggests that Fe2O3/V2O3 hetero-structures absorb light much more effectively than random alloys, thus promoting efficient photo-induced carrier generation. These predictions can be tested in a-(Fe1xVx)2O3 thin films synthesized with well-controlled spatial distribution of Fe and V species.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2025

Electronic structure and exchange interactions in altermagnetic MnGeP 2 in the quasiparticle self-consistent G W approach

The quasiparticle self-consistent 𝐺⁡𝑊 method is used to study the electronic band structure, optical dielectric function, and exchange interactions in chalcopyrite, 𝐼⁢ ‾ 4 ⁢2⁢𝑑, structure MnGeP 2 . The material is found to be an antiferromagnetic semiconductor with a lowest direct gap of 2.44 eV at the Γ point and a lower indirect gap of 1.87 eV from Γ to 𝑀. The material is an altermagnet because the two magnetic atoms of opposite spin are related by a twofold rotation operation perpendicular to the main fourfold rotation inversion axis. The spin splittings along a low symmetry line like 𝑃⁢𝑁 is sizable while at k points on the diagonal mirror planes or on the twofold symmetry axes the spin splitting is zero. The exchange interactions are calculated using a linear response approach. The antiferromagnetic exchange interaction between nearest neighbors in the primitive unit cell is dominating and found to be slightly decreasing upon carrier doping but not sufficiently to change the interaction to become ferromagnetic. The bare (noninteracting) and interacting transverse spin susceptibilities, which provide interatomic site exchange interactions after averaging over the muffin-tin spheres, are calculated from the 𝐺⁡𝑊 band structure and wave functions. From these exchange interactions, the spin wave spectra are obtained along the high symmetry lines and the Néel temperature is calculated using the mean-field and Tyablikov estimations. The dielectric function and the optical absorption spectra are calculated including excitonic effects using the Bethe Salpeter equation. The exchange interactions around Mn Ge defect sites is also studied. While we find it can generate ferromagnetic interactions with neighboring spins, we did not find direct evidence of producing an overall ferromagnetic phase. First, if Mn antisites are introduced by exchanging Mn with a nearby Ge, the interactions stay largely antiferromagnetic. Second, when we add additional Mn, in other words in Mn-rich stoichiometry, the Mn Ge antisites produce a strong ferromagnetic interaction primarily with the Mn in the same basal plane but weaker ferromagnetic interaction with adjacent plane Mn. The interactions between regular lattice Mn stay antiferromagnetic as before and thus favor keeping the antiferromagnetic order along the [001] direction. Adding Mn antisites, however, does lead to a metallic band structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Mechanism of Optical Charge Exchange of Magnetic Centers in BSO : Fe

High-resistance photosensitive crystals of Bi{sub 12}SiO{sub 20} (BSO) doped with iron ions were studied. X-ray diffraction analysis reveals the compression of a unit cell in a BSO : Fe crystal with increasing impurity concentration. Electron paramagnetic resonance demonstrates a decrease in the intensity of the EPR signal when the BSO: Fe crystal is exposed to light that generates photocarriers. It is found that the characteristic time of the EPR signal decrease is close to the value of the Maxwellian relaxation time measured with the help of the longitudinal electrooptical effect. A physical model of the mechanism of optical charge exchange of magnetic iron centers is discussed, based on the statement that the nature of the crystal bonds of the iron ion with ligands without structural modification of the crystal lattice changes during the photogeneration of carriers. A physical model is proposed, according to which a trivalent Fe{sup 3+} ion transforms into a divalent state of Fe{sup 2+} with a change in the total spin from 5/2 to 2. The compression of the unit cell with increasing iron ion concentration in the framework of the model under discussion is due to the transformation of atomic orbitals upon replacement of silicon ions by iron ions. The transformation process affects the cells unoccupied by iron, which is proved by the absence of a bifurcation of X-ray reflections and indicates the long-range nature of the intracrystalline interactions in sillenites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗