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At least 1,243 records · Page 69
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

NEXAFS Spectroscopy of P3HT and PBTTT at the Sulfur K-Edge

The sulfur K-edge near-edge X-ray absorption fine-structure (NEXAFS) spectra of the common conjugated polymers P3HT and PBTTT are studied from both experimental and theoretical perspectives. Experimental angle-resolved spectra are measured to characterize both the dominant peaks and the dichroism of the polymers. First-principles calculations using the density functional theory-based many-body X-ray absorption spectroscopy (MBXAS) method are performed for the two polymers as well as for the thiophene and thienothiophene units that make up the conjugated backbones of these polymers. Through this combined approach, we are able to confidently assign the observed peaks to specific molecular orbitals and identify the orientation of their transition dipole moments (TDMs) with respect to the coordinate frame of the polymer backbone. Here, in particular, we are able to establish the character and orthogonal nature of the three main low-energy peaks at: (i) 2473.5 eV, 1s → (S–C)­π* with TDM along the π-stacking direction; (ii) 2474.1 eV, 1s → (S–C)­σ* with TDM along the backbone; and (iii) 2475.4 eV, 1s → (S–C)­σ* with TDM perpendicular to the first two. By performing both gas-phase and solid-state simulations, and with reference to the NEXAFS spectra of thiophene and thienothiophene building blocks, the influences of polymerization and molecular packing are also explored.

Chantler, Paul Alexander [Monash University, Clayt↗
DOE OSTI2025

Mid-Infrared Photoluminescence from Tellurium Thin Films

Tellurium is an elemental semiconductor with a 0.34 eV optical band gap, showing promise for mid-infrared (MIR) optoelectronics including light-emitting diodes. However, quantitative measurement of the tellurium luminescence efficiency has not yet been investigated, and growing optically active films remain challenging. Here, we demonstrate the low-temperature growth of bright tellurium thin films by using physical vapor transport. The sample morphology is controlled by varying the growth temperature, yielding continuous thin films, microparticles, or nanowires. Additionally, we demonstrated patterned growth by using a seed layer for selective nucleation. Quantitative photoluminescence measurements at room temperature reveal an internal quantum yield of 2.0% at 0.34 eV for the as-grown tellurium films, comparable to the best-reported efficiencies of III-V and II-VI compound semiconductors with similar band gaps. Furthermore, we demonstrate band-gap tunability with Te-Se alloys, where up to 20% selenium incorporation gradually blue-shifts the band gap to 0.55 eV. The work demonstrates the potential use of tellurium for efficient MIR devices.

Wang, Shu↗
DOE OSTI2020

Sputtered p-Type Cu x Zn 1– x S Back Contact to CdTe Solar Cells

As thin-film cadmium telluride (CdTe) solar cells gain prominence, one particular challenge is optimizing contacts and their interfaces to transfer charge without losses in efficiency. Back contact recombination is still significant and will prevent CdTe solar technology from reaching its full potential in device efficiency, and transparent back contacts have not been developed for bifacial solar technology or multijunction solar cells. To address these challenges, here we investigate sputtered Cu x Zn 1– x S as a p-type semi-transparent back contact material to thin-film polycrystalline CdTe solar cells at Cu concentrations x = 0.30, 0.45, and 0.60. This material is selected for its high hole conductivity (160–2120 S cm –1 ), wide optical band gap (2.25–2.75 eV), and variable ionization potential (approximately 6–7 eV) that can be aligned to that of CdTe. We report that without device optimization, CdTe solar cells with these Cu x Zn 1– x S back contacts perform as well as control cells with standard ZnTe:Cu back contacts. We observe no reduction in external quantum efficiency, low contact barrier heights of approximately 0.3 eV, and carrier lifetimes on par with those of baseline CdTe. These cells are relatively stable over one year in air, with V OC and efficiency of the x = 0.30 cell decreasing by only 1 and 3%, respectively. Using scanning electron microscopy and scanning transmission electron microscopy to investigate the Cu x Zn 1– x S/CdTe interface, we demonstrate that the Cu x Zn 1– x S layer segregates into a bilayer of Cu-Te-S and Zn-Cd-S, and thermodynamic reaction calculations support these findings. Despite its bilayer formation, the back contact still functions well. This investigation explains some of the physical mechanisms governing the device stack, inspires future work to understand interfacial chemistry and charge transfer, and elicits optimization to achieve higher-efficiency CdTe cells.

14 SOLAR ENERGY↗
DOE OSTI2019

Wide-Bandgap Cu(In,Ga)S 2 Photocathodes Integrated on Transparent Conductive F:SnO 2 Substrates for Chalcopyrite-Based Water Splitting Tandem Devices

Recently, whole-chalcopyrite-based tandem devices for photoelectrochemical (PEC) water splitting have emerged as a promising route for obtaining ~20% solar-to-hydrogen efficiencies. Here we pursue this approach by demonstrating integration of the top cell wide-bandgap (E G ) chalcopyrite onto a transparent conductor, which is a critical step in the realization of tandem devices. We report specifically on our efforts to synthesize photoactive Cu(In,Ga)S 2 thin films on transparent conductive F:SnO 2 (FTO), while preserving the optoelectronic properties of the FTO substrate and preventing the formation of a resistive SnS x interfacial layer. We demonstrate that such attributes can be achieved via close-space sulfurization (CSS) of lower EG Cu(In,Ga)Se 2 precursors, co-evaporated on FTO at low-temperature. Depending on Cu(In,Ga)Se 2 precursors’ Ga and In content, the resulting Cu(In,Ga)S 2 solar absorbers have E G energies spanning 2.05 eV to 2.45 eV. The CSS process, which includes a low-temperature annealing in sulfur vapor followed by a high-temperature crystallization under inert atmosphere, allowed for up to 95% Se substitution with S in the chalcopyrite lattice, tuning both E G and band edge positions that impact PEC performance. Photoelectrochemical measurements performed under AM1.5G illumination in 0.5M H 2 SO 4 on the 2.05 eV CuInGaS 2 photocathode revealed a saturation photocurrent density (J SAT ) of -5.25 mA/cm 2 , a value corresponding to 38% of the absorber’s optical limit. We further concluded that such low J SAT originates from sub-par optical absorption of Cu(In,Ga)S 2 absorbers. Further enhancements of the CSS process are expected to improve material quality towards our end goal of achieving whole-chalcopyrite tandem PEC devices.

08 HYDROGEN↗
DOE OSTI2022

Li 8 MnO 6 : A Novel Cathode Material with Only Anionic Redox

In Li-excess transition-metal-oxide cathode materials, anionic oxygen redox can offer high capacity and high voltages, although peroxo and superoxo species may cause oxygen loss, poor cycling performance, and capacity fading. Previous work showed that undesirable formation of peroxide and superoxide bonds is controlled to some extent by Mn substitution, and the present work uses density functional calculations to examine the reasons for this by studying the anionic redox mechanism Li 8 MnO 6 . This material is obtained by substituting Mn for Sn in Li 8 SnO 6 or for Zr in Li 8 ZrO 6 , and we also compare to previous work on those materials. The calculations predict that Li 8 MnO 6 is stable at room temperature (with a band gap of 3.19 eV as calculated HSE06 and 1.82 eV as calculated with the less reliable with PBE+U), and they elucidate the chemical and structural effects involved in the inhibition of oxygen release in this cathode. Throughout the whole delithiation process, only O 2- ions are oxidized. The directional Mn-O bonds formed from unfilled 3d orbitals effectively inhibit the formation of O-O bonds, and the layered structure is maintained even after removing 3 Li per Li 8 MnO 6 formula unit. The calculated average voltage for removal of 3 Li is 3.69 V by HSE06, and the corresponding capacity is 389 mAh/g. The high voltage of oxygen anionic redox and the high capacity result in a high energy density of 1436 Wh/kg. The Li-ion diffusion barrier for the dominant interlayer diffusion path along the c-axis is 0.57 eV by PBE+U. Finally, these results help us to understand the oxygen redox mechanism in a new lithium-rich Li 8 MnO 6 cathode material and contribute to the design of high-energy-density lithium-ion-battery cathode materials with favorable electrochemical properties based on anionic oxygen redox.

Li8MnO6↗
DOE OSTI2026

A Chemist’s Guide to Solid-State Qubits: Diamond Color Centers as Embedded Vacancy-Centered Molecules

Diamond point defects or color centers are at the crux of solid-state quantum technology applications, from computing to sensing. Modeling molecular qubits has advanced by leaps and bounds mainly due to the advanced quantum mechanical (QM) tools in the arsenal of quantum chemists for modeling (isolated) molecular systems, which should be easily translatable for analogous systems in the solid state. Here, we simulate a series of diamond defects: a carbon vacancy (V C ) and second row substitutions that are adjacent to a V C or D C V C , with D = B, N, and O, to evaluate their magnetic and optical properties. We applied molecular orbital principles, an embedded cluster method, and a multiconfigurational QM theory to computationally characterize the above-mentioned defects for a multitude of spin, charge, and excited states. Our computational approach delivered high-quality QM insights that compare well with experiments. Furthermore, with this framework, we confirm the spin-triplets B C V C – and O C V C 0 to be analogous to N C V C – , with calculated bright transitions that respectively correspond to UV (3.80 eV) and blue (2.65 eV) light absorption and emission lifetimes (τ) of 2.3 and 5.6 ns, which are higher in energy and more short-lived (and thus brighter) than that for N C V C – : green (2.26 eV), τ = 9.5 ns.

Martirez, John Mark P. [Princeton Plasma Physics L↗
DOE OSTI2026

Radiative Defects in Chloride-Activated CdSe Thin Films

Defect recombination limits wide-gap Se-based chalcogenide devices, yet how chloride activation reshapes radiative pathways remains unclear. Here we show that a 40 min CdCl 2 anneal converts evaporated CdSe from porous nanograins into dense micrometer-scale polycrystals and sharpens the optical band edge, reducing the Urbach energy from 85 to 17 meV at 300 K. Combining temperature- and fluence-dependent photoluminescence (PL), time-resolved PL, hyperspectral mapping, and hybrid-DFT, we resolve three emissive channels and identify their mechanisms. The near-edge band is excitonic at low temperature and evolves into free-carrier emission at elevated temperature. A sub-gap band at E g −0.45 eV requires above-gap carriers and thermally quenches with a 0.16 eV activation energy. A broad ∼1.05 eV infrared band is excited by above- and below-gap photons and retains microsecond lifetimes at room temperature; patial mapping links it to edge-rich microstructure. Calculations suggest selenium-vacancy and cadmium-vacancy−chlorine complexes, pointing to routes to suppress defect-related losses in wide-gap chalcogenide devices.

Abudulimu, Abasi [Univ. of Toledo, OH (United Stat↗
DOE OSTI2025

Accelerated Discovery of Cost-Effective Photoabsorber Materials for Near-Infrared (λ = 1600 nm) Photodetector Applications

Current infrared sensing devices are based on costly materials with relatively few viable alternatives known. To identify promising candidate materials for infrared photodetection, we have developed a high-throughput screening methodology based on high-accuracy r 2 SCAN and HSE calculations in density functional theory. Using this method, we identify ten already synthesized materials between the inverse perovskite family, the barium silver pnictide family, the alkaline pnictide family, and ZnSnAs 2 as top candidates. Among these, ZnSnAs 2 emerges as the most promising candidate due to its experimentally verified band gap of 0.74 eV at 0 K and its cost-effective synthesis through Bridgman growth. BaAgP also shows potential with an HSE-calculated band gap of 0.64 eV, although further experimental validation is required. Lastly, we discover an additional material, Ca 3 BiP, which has not been previously synthesized, but exhibits a promising optical spectra and a band gap of 0.56 eV. The method applied in this work is sufficiently general to screen wider bandgap materials in high-throughput and now extended to narrow-band gap materials.

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

Bulk Assembly of Zero-Dimensional Organic Lead Bromide Hybrid with Efficient Blue Emission

Zero-dimensional (0D) organic metal halide hybrids are an emerging class of light emitting materials with exceptional photoluminescence quantum efficiencies (PLQEs), thanks to their perfect “host–guest” structures with light emitting metal halide species periodically “embedded” in a wide band gap organic cationic matrix through ionic bonds. However, achieving efficient blue emissions is challenging for this class of materials, as structural distortions of metal halides often lead to large Stokes shifts. Here we report a highly luminescent blue emitting 0D organic lead bromide, (C 13 H 19 N 4 ) 2 PbBr 4 , with a peak emission of 460 nm (2.70 eV), a full width at half maximum (FWHM) of 66 nm (0.40 eV), a Stokes shift of 111 nm (0.85 eV), and a PLQE of ~40%. Single crystal structure analysis shows that individual PbBr 4 2– species adopt a near-seesaw structure, which are coordinated to benzyl-hexamethylenetetrammonium (C 13 H 19 N 4 + ) organic cations. The relatively small Stokes shift as compared to those of previously reported 0D organic metal halide hybrids are attributed to the low chemical reactivity of Pb 6s 2 lone pairs and the rigid organic cationic matrix. Finally, (C 13 H 19 N 4 ) 2 PbBr 4 also shows exceptional stability in air with little-to-no change of properties for more than a year in ambient conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Realization of AlSb in the Double-Layer Honeycomb Structure: A Robust Class of Two-Dimensional Material

Exploring new two-dimensional (2D) van der Waals (vdW) systems is at the forefront of materials physics. Here, through molecular beam epitaxy on graphene-covered SiC(0001), we report successful growth of AlSb in the double-layer honeycomb (DLHC) structure, a 2D vdW material which has no direct analogue to its 3D bulk and is predicted kinetically stable when freestanding. The structural morphology and electronic structure of the experimental 2D AlSb are characterized with spectroscopic imaging scanning tunneling microscopy and cross-sectional imaging scanning transmission electron microscopy, which compare well to the proposed DLHC structure. The 2D AlSb exhibits a bandgap of 0.93 eV versus the predicted 1.06 eV, which is substantially smaller than the 1.6 eV of bulk. We also attempt the less-stable InSb DLHC structure; however, it grows into bulk islands instead. Here, the successful growth of a DLHC material here opens the door for the realization of a large family of novel 2D DLHC traditional semiconductors with unique excitonic, topological, and electronic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Red-Shifted Absorptions of Cation-Defective and Surface-Functionalized Anatase with Enhanced Photoelectrochemical Properties

Manipulating the atomic structure of semiconductors is a fine way to tune their properties. The rationalization of their modified properties is, however, particularly challenging as defects locally disrupt the long-range structural ordering, and a deeper effort is required to fully describe their structure. In this work, we investigated the photoelectrochemical properties of an anatase-type structure featuring a high content of titanium vacancies stabilized by dual-oxide substitution by fluoride and hydroxide anions. Such atomic modification induces a slight red-shift band gap energy of 0.08 eV as compared to pure TiO 2 , which was assigned to changes in titanium–anion ionocovalent bonding. Under illumination, electron paramagnetic resonance spectroscopy revealed the formation of Ti III and O 2 – radicals which were not detected in defect-free TiO 2 . Consequently, the modified anatase shows higher ability to oxidize water with lower electron–hole recombination rate. To further increase the photoelectrochemical properties, we subsequently modified the compound by a surface functionalization with N-methyl-2-pyrrolidone (NMP). This treatment further modifies the chemical composition, which results in a red shift of the band gap energy to 3.03 eV. Moreover, the interaction of the NMP electron-donating molecules with the surface induces an absorption band in the visible region with an estimated band gap energy of 2.25–2.50 eV. Under illumination, the resulting core–shell structure produces a high concentration of reduced Ti III and O 2 – , suggesting an effective charge carrier separation which is confirmed by high photoelectrochemical properties. This work provides new opportunities to better understand the structural features that affect the photogenerated charge carriers.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Platinum Graphene Catalytic Condenser for Millisecond Programmable Metal Surfaces

Accelerating catalytic chemistry and tuning surface reactions require precise control of the electron density of metal atoms. In this work, nanoclusters of platinum were supported on a graphene sheet within a catalytic condenser device that facilitated electron or hole accumulation in the platinum active sites with negative or positive applied potential, respectively. The catalytic condenser was fabricated by depositing on top of a p-type Si wafer an amorphous HfO 2 dielectric (70 nm), on which was placed the active layer of 2–4 nm platinum nanoclusters on graphene. A potential of ±6 V applied to the Pt/graphene layer relative to the silicon electrode moved electrons into or out of the active sites of Pt, attaining charge densities more than 1% of an electron or hole per surface Pt atom. At a level of charge condensation of ±10% of an electron per surface atom, the binding energy of carbon monoxide to a Pt(111) surface was computed via density functional theory to change 24 kJ mol –1 (0.25 eV), which was consistent with the range of carbon monoxide binding energies determined from temperature-programmed desorption (ΔBE CO of 20 ± 1 kJ mol –1 or 0.19 eV) and equilibrium surface coverage measurements (ΔBE CO of 14 ± 1 kJ mol –1 or 0.14 eV). Impedance spectroscopy indicated that Pt/graphene condensers with potentials oscillating at 3000 Hz exhibited negligible loss in capacitance and charge accumulation, enabling programmable surface conditions at amplitudes and frequencies necessary to achieve catalytic resonance.

Inorganic carbon compounds↗
DOE OSTI2022

Reorganization Energies for Interfacial Proton-Coupled Electron Transfer to a Water Oxidation Catalyst

The reorganization energy (λ) for interfacial electron transfer (ET) and proton-coupled electron transfer (PCET) from a conductive metal oxide (In 2 O 3 :Sn, ITO) to a surface-bound water oxidation catalyst were extracted from kinetic data measured as a function of the thermodynamic driving force. Visible light excitation resulted in rapid excited-state injection (k inj > 10 8 s -1 ) to the ITO, which photo-initiated the two interfacial reactions of interest. The rate constants for both reactions increased with the driving force, −ΔG°, to a saturating limit, k max , with rate constants consistently larger for ET than for PCET. Marcus-Gerischer analysis of the kinetic data provided the reorganization energy for interfacial PCET (0.90 ± 0.02 eV) and ET (0.40 ± 0.02 eV), respectively. The magnitude of k max for PCET was found to decrease with pH, behavior that was absent for ET. Both the decrease in k max and the larger reorganization energy for an unwanted competing PCET reaction from the ITO to the oxidized catalyst showcases a significant kinetic advantage for driving solar water oxidation at high pH. Computational analysis revealed a larger inner-sphere reorganization energy contribution for PCET than for ET arising from a more significant change in the Ru–O bond length for PCET. Extending Marcus-Gerischer theory to PCET by including the excited electron-proton vibronic states and the proton donor-acceptor motion provided an apparent reorganization energy of 1.01 eV. Furthermore, this study demonstrates that the Marcus-Gerischer theory initially developed for ET can be reliably extended to PCET for quantifying and interpreting reorganization energies observed experimentally.

Catalysts Kinetic parameters↗
DOE OSTI2025

Combining Organic Cations of Different Sizes Grants Improved Control over Perovskitoid Dimensionality and Bandgap

Because mixed-halide wide-bandgap (1.6-2.0 eV) perovskite solar cells suffer from operating instability related to light-induced halide segregation, it is of interest to study alternative means of bandgap widening. Perovskitoids combine wide bandgaps and structural stability resulting from face- or edge-sharing octahedral connections in their crystal structures. Unfortunately, there existed no prior reports of three-dimensional (3D) perovskitoids having direct bandgaps with optical absorption edges less than 2.2 eV. As the most significant predictor of perovskitoid bandgaps is the fraction of corner-sharing in their crystal structures, we hypothesized that increasing the amount of corner-sharing would access lower bandgaps than previously reported. Here, we accomplished this by mixing a spacer cation within the size range for 3D perovskitoid formation with a smaller perovskite-forming cation. We explored three spacer cations of different sizes: ethylammonium (EA), cyclopropylammonium (c-C3A), and cyclobutylammonium (c-C4A), combining these with methylammonium (MA), and found that the middle cation, c-C3A, pairs with MA to form a 3D perovskitoid with the formula (c-C3A) 3 (MA) 3 Pb 5 I 16 and a direct bandgap with an optical absorption edge at 2.0 eV. Solution-processed films of this perovskitoid showed improved light stability over mixed-halide perovskites, and solar cells based on these films exhibit increased maximum power point operating stability compared to reference mixed-halide devices.

Gilley, Isaiah W. [Northwestern University, Evanst↗
DOE OSTI2025

Cerium Dimer Anion and the Contribution of 4f Electrons to Lanthanide Metal–Metal Bonds

Direct metal−metal bonding between lanthanide atoms has been challenging to observe. We report on the first spectroscopic characterization of the cerium dimer anion (Ce 2 − ) and its neutral analog (Ce 2 ) in the gas phase, achieved using photoelectron and ultrafast spectroscopy combined with high-level quantum chemistry calculations. The electron affinity of Ce 2 is 0.24 eV, from which a dissociation energy of 2.21 eV is derived for Ce 2 − . The wave-packet dynamics upon photodetachment are studied and yield vibrational frequencies for electronically excited Ce 2 . Ce 2 − exhibits a conventional metal−metal triple bond with minimal contribution from 4f electrons. However, evidence of 4f-electron participation in bonding is identified for the low-energy excited states only 0.1 eV higher. The results challenge the assumption of inert 4f electrons in metal−metal bonding, and we propose a promising strategy for forming stable lanthanide−lanthanide bonds involving significant 4f-electron contributions.

Anions↗
DOE OSTI2025

Driving Force Dependent Photoinduced Charge Transfer Dynamics in Polymer-Wrapped Semiconducting Single-Walled Carbon Nanotubes

Here, we investigate the thermodynamic driving-force dependences of photoinduced charge separation (CS) and subsequent charge transfer dynamics in single-walled carbon nanotube (SWNT)–perylenediimide (PDI) donor–acceptor (D–A) superstructures. Pump–probe spectroscopy reveals that [SWNT (•+)n ]-(PDI –• ) n CS states form on an ∼100 fs time scale following photoexcitation; these dynamics are invariant across an ∼400 mV driving force range, indicating that SWNT hole polaron formation time scales are determined by nanotube lattice and solvent relaxation. These CS states feature SWNT hole polarons adjacent to (geminate) and nearby (nongeminate) PDI radical anions. Analysis of the free energy dependence for charge recombination (CR) of [SWNT •+ ] geminate -(PDI –• ) CS states highlights an ∼2 meV value for D–A electronic coupling (HAB) and ∼0.93 eV for the total reorganization energy (λ T ). A corresponding driving force dependence of the CR dynamics for [SWNT •+ ] nongeminate -(PDI –• ) CS states indicates a diminished H AB value (∼0.6 meV) and a larger λ T (∼1.1 eV), consistent with larger transfer distances. SWNT excitons that persist following photoinduced CS drive photooxidation of PDI –• components of [SWNT (•+)n ]-(PDI –• ) n CS states ( 1 SWNT* + PDI •– → PDI + SWNT •– ); this reaction manifests a significantly reduced λ T value (∼0.67 eV) as the initially prepared SWNT reduced state bears the character of a conduction band injected electron ([SWNT •– ] CB ). This reaction thus gives rise to relaxed, nongeminate SWNT electron and hole polarons on the same nanotube; these polarons react on a 10 2 ps time scale independent of the electronic structure of these SWNT-PDI superstructures.

anions↗
DOE OSTI2025

Selective reduction in epitaxial SrFe 0.5 Co 0.5 O 2.5 and its reversibility

Oxygen-vacancy engineering in transition metal oxides enables programmable functionalities by modulating the valence states and local coordination of constituents. Here, we report the selective reduction of cobalt ions in epitaxial SrFe 0.5 Co 0.5 O 2.5 thin films under reducing gas environments, while iron ions remain unchanged. X-ray absorption spectroscopy reveals an absorption edge shift of 1.65 eV in the Co L-edge upon reduction, and multiplet simulations estimate a decrease in the average Co valence from Co 2.91+ to Co 2.00+ . This site- and element-specific reduction leads to the formation of a structurally distinct oxygen-deficient phase stabilized by oxygen vacancies at tetrahedral sites, as confirmed by density functional theory. Optical spectroscopy reveals an increase in the bandgap from 2.47 eV to 3.04 eV, accompanied by enhanced transparency. Furthermore, simultaneous in situ diffraction and transport measurements confirm fully reversible redox-driven transitions among three phases: reduced defective perovskite, brownmillerite, and oxygen-rich perovskite phases. These findings demonstrate that selective redox control in multi-cation oxides enables the realization of chemically and functionally distinct oxygen-deficient phases.

Fuel cells↗
DOE OSTI2020

Linear and nonlinear optical responses in the chiral multifold semimetal RhSi

Chiral topological semimetals are materials that break both inversion and mirror symmetries. They host interesting phenomena such as the quantized circular photogalvanic effect (CPGE) and the chiral magnetic effect. In this work, we report a comprehensive theoretical and experimental analysis of the linear and nonlinear optical responses of the chiral topological semimetal RhSi, which is known to host multifold fermions. We show that the characteristic features of the optical conductivity, which display two distinct quasi-linear regimes above and below 0.4 eV, can be linked to excitations of different kinds of multifold fermions. The characteristic features of the CPGE, which displays a sign change at 0.4 eV and a large non-quantized response peak of around 160 μA/V 2 at 0.7 eV, are explained by assuming that the chemical potential crosses a flat hole band at the Brillouin zone center. Our theory predicts that, in order to observe a quantized CPGE in RhSi, it is necessary to increase the chemical potential as well as the quasiparticle lifetime. More broadly, our methodology, especially the development of the broadband terahertz emission spectroscopy, could be widely applied to study photogalvanic effects in noncentrosymmetric materials and in topological insulators in a contact-less way and accelerate the technological development of efficient infrared detectors based on topological semimetals.

Materials Science↗