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All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

A tutorial on high-order harmonic generation in atoms, molecules, and condensed matter

This tutorial introduces strong-field-driven high-order harmonics, their experimental generation and characterization techniques, and their main applications including attosecond pulse generation and ultrafast spectroscopy of the target material. We begin from the use of atomic targets, where the first high-order harmonic generation (HHG) experiments were realized in the late 1980s. Then, we briefly discuss the basics of the microscopic generation mechanism and how various steps of the mechanism were exploited in applications such as generating isolated attosecond pulses and probing molecular orbitals. We introduce and describe the standard experimental approaches for condensed phase HHG, where we discuss unique technical challenges of the use of solid-state materials, such as the mitigation of plasma formation and laser damage. We cover the fundamentals of high-harmonic spectroscopy in condensed matter systems, such as wide bandgap dielectrics, semiconductors, liquid media, and 2D-crystals. We provide some examples of rapidly emerging spectroscopic capabilities, such as for probing crystal symmetries, Berry phases, and associated non-trivial topological properties of the source material. Finally, we provide an overview of the research field, including some of the challenges, opportunities, and open questions.

Attosecond pulses↗
DOE OSTI2023

Manipulating topological Hall-like signatures by interface engineering in epitaxial ruthenate/manganite heterostructures

Topologically protected non-trivial spin textures (e.g. skyrmions) give rise to a novel phenomenon called the topological Hall effect (THE) and have promising implications in future energy-efficient nanoelectronic and spintronic devices. Here, we have studied the Hall effect in SrRuO 3 /La 0.42 Ca 0.58 MnO 3 (SRO/LCMO) bilayers. Our investigation suggests that pure SRO has hard and soft magnetic characteristics but the anomalous Hall effect (AHE) in SRO is governed by the high coercivity phase. Here, we have shown that the proximity effect of a soft magnetic LCMO on SRO plays a critical role in interfacial magnetic coupling and transport properties in SRO. Upon reducing the SRO thickness in the bilayer, the proximity effect becomes the dominant feature, enhancing the magnitude and temperature range of THE-like signatures. The THE-like features in bilayers can be explained by a diffusive Berry phase transition model in the presence of an emergent magnetic state due to interface coupling. This work provides an alternative understanding of THE-like signatures and their manipulation in SRO-based heterostructures, bilayers and superlattices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Dressed vs. pairwise states, and the geometric phase of monopoles and charges

We construct the Faddeev-Kulish dressed multiparticle states of electrically and magnetically charged particles, incorporating the effects of real and virtual soft photons. We calculate the properties of such dressed states under Lorentz transformations, and find that they can be identified with the pairwise multi-particle states that transform under the pairwise little group. The shifts in the dressing factors under Lorentz transformations are finite and have a simple geometric interpretation. Using the transformation properties of the dressed states we also present a novel, fully quantum field theoretic derivation of the geometric (Berry) phase obtained by an adiabatic rotation of the Dirac string, and also of the Dirac quantization condition. For half integer pairwise helicity, we show that these multiparticle states have flipped spin-statistics, reproducing the surprising fact that fermions can be made out of bosons.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Helical spin ordering in room-temperature metallic antiferromagnet Fe 3 Ga 4

Metallic Fe 3 Ga 4 displays a complex magnetic phase diagram that supports an intermediate antiferromagnetic (AFM) helical spin structure (HSS) state at room temperature which lies between two ferromagnetic (FM) phases. Here, magnetic measurements along the three crystallographic axes were performed in order to develop a model for the temperature and field dependence of the HSS state. These results show that the AFM state is a helically ordered spiral propagating along the c-axis with the magnetic moments rotating in the ab-plane. Under applied magnetic field, the AFM state exhibits a metamagnetic transition to conical ordering before entering a fully field-polarized FM state at high fields. The conical ordering in the AFM state is anisotropic even within the ab-plane and may gives rise to Berry phase effects in transport measurements. Metallic conductivity from density of states computations was confirmed through resistivity measurements and no anomalous behavior was observed through the various magnetic transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Phase-Slip Lines and Anomalous Josephson Effects in Tungsten Nanoscale Cluster-Based Topological Insulator Nanobridges: Implications for Topologically Protected Qubits and Quantum Sensors

Superconducting topological systems formed by a strong 3D topological insulator (TI) in proximity to a conventional s-wave superconductor (SC) have been intensely studied, as they may host Majorana zero modes. However, there are limited experimental realizations of TI-SC systems in which robust superconducting pairing is induced on the surface states of the TI and a topological superconducting state is established. Here, we fabricate a TI-SC system by depositing, via a focused ion beam, tungsten (W) nanoscale clusters on the surface of TI Bi 0.91 Sb 0.09 . We find that the resulting heterostructure supports phase-slip lines (PSLs) that act as effective Josephson junctions (JJs). Here, we probe the response of the system to microwave radiation. We find that for some ac frequencies, and powers, the resulting Shapiro steps’ structure of the voltage–current characteristic exhibits a missing first step and an unexpectedly wide second Shapiro step. The theoretical analysis of the measurements shows that the unusual Shapiro response arises from the interplay between a static JJ and a dynamic one and allows us to identify the conditions under which the missing first step can be attributed to the topological nature of the JJs formed by the PSLs. Our results suggest an approach to induce superconductivity in a TI, a route to realizing highly transparent topological JJs, and show how the response of superconducting systems to microwave radiation can be used to infer the dynamics of PSLs. Highly transparent topological junctions are promising candidates to realize vector field sensors with very high sensitivity. In addition, due to the nontrivial Berry phase of the TI’s surface states such junctions can be in a topological state which is ideal to create topologically protected qubits.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Winding θ and destructive interference of instantons

While the θ dependence of field theories is 2π periodic, the ground-state wavefunctions at θ and θ + 2π often belong to different classes of symmetry-protected topological states. When this is the case, a continuous change of the θ parameter can introduce an interface that supports a nontrivial field theory localized on the wall. We consider the 2d CP N-1 sigma model as an example and construct a weak-coupling setup of this interface theory by considering the small S1 compactification with nonzero winding θ parameter and a suitable symmetry-twisted boundary condition. This system has N classical vacua connected by fractional instantons, but the anomaly constraint tells us that the fractional-instanton amplitudes should vanish completely to have N-fold degeneracy at the quantum level. We show how this happens in this purely bosonic system, uncovering that the integration over the zero modes annihilates the fractional instanton amplitudes, in sharp contrast to what happens when the θ angle is constant. Moreover, we provide another explanation of this selection rule by showing that the N perturbative vacua acquire different charges under the global symmetry with the activation of the winding θ angle. We also demonstrate a similar destructive interference between instanton effects in the CP N-1 quantum mechanics with the Berry phase.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Topological Hall effect in magnetic topological insulator films

Geometric Berry phase can be induced either by spin–orbit coupling, giving rise to the anomalous Hall effect in ferromagnetic materials, or by chiral spin texture, such as skyrmions, leading to the topological Hall effect. Recent experiments have revealed that both phenomena can occur in topological insulator films with magnetic doping, thus providing us with an intriguing platform to study the interplay between these two phenomena. In this work, we report on a numerical simulation of the anomalous Hall and topological Hall effects in a four-band model that can properly describe the quantum well states in the magnetic topological insulator films by combining Landauer–Büttiker formula and the iterative Green’s function method. Our numerical results suggest that spin–orbit coupling in this model plays a different role in the quantum transport in the clean and disordered limits. In the clean limit, spin–orbit coupling mainly influences the longitudinal transport but does not have much effect on topological Hall conductance. In the disordered limit, the longitudinal transport is determined by disorder scattering and spin–orbit coupling is found to affect strongly the topological Hall conductance. Here, this sharp contrast unveils a dramatic interplay between spin–orbit coupling and disorder effect in topological Hall effect in magnetic topological insulator systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Predictive Design of Hybrid Improper Ferroelectric Double Perovskite Oxides

The computational design of suitable multiferroic double perovskite oxides requires finding materials that exhibit sizable polarization, magnetization, and coupling between them. Oxides with the chemical formula of AA'BB'O 6 with building blocks of ABO 3 single perovskite oxides in centrosymmetric Pnma symmetry are strong candidates that have been reported to satisfy such criteria. The system lowers to noncentrosymmetric, polar P2 1 symmetry if A/A' layered and B/B' rocksalt cation orderings are imposed. A detailed compositional search over a variety of chemical spaces followed by evaluating their polarization may lead to the identification of more of these compounds with ferroelectric ordering. The standard density functional theory practices to estimate polarization within the Berry phase formalism require the systems to be perfectly insulating. The number of compounds that can be evaluated using this method is therefore limited. In this work, we introduce a predictive learning strategy based on importance sampling to build a series of machine learning models using results from first-principles simulations to predict polarization and the corresponding switching barrier. The geometry-driven features related to charge states and cationic radii play key roles in predicting the switching barrier with complementary contributions from the key structural mode-based order parameters. These modes become important to draw reasonable predictions of polarization components from machine learning models. In conclusion, our predictive models identify candidates with high polarizations and low switching barriers from a pool of double perovskite oxides, suitable for future investigation for their potential applications in spintronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electronic transport properties of a lithium-decorated ZrTe 5 thin film

Through a combination of single crystal growth, experiments involving in situ deposition of surface adatoms, and complimentary modeling, we examine the electronic transport properties of lithiumdecorated ZrTe 5 thin films. We observe that the surface states in ZrTe 5 are robust against Li adsorption. Both the surface electron density and the associated Berry phase are remarkably robust to adsorption of Li atoms. Fitting to the Hall conductivity data reveals that there exist two types of bulk carriers: those for which the carrier density is insensitive to Li adsorption, and those whose density decreases during initial Li depositions and then saturates with further Li adsorption. We propose this dependence is due to the gating effect of a Li-adsorption-generated dipole layer at the ZrTe 5 surface.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Quantum-limit phenomena and band structure in the magnetic topological semimetal EuZn 2 As 2

The interplay between magnetism and electronic topology in the quantum limit is a forefront subject of condensed matter physics. Here, we show the electronic and magnetic properties of layered antiferromagnet EuZn 2 As 2 in pulsed magnetic fields up to 60 T and temperatures down to 0.6 K. By analyzing the quantum oscillations observed in the magnetoresistance (MR) and proximity detector oscillator (PDO) frequency, we find that (1) the oscillation frequency F = 46 ± 6 T for H // c and 42 ± 2 T for H // ab; (2) the corresponding Berry phase is close to π for H // c, implying a nontrivial topology; and (3) the large linear MR occurs beyond the first Landau level, without any sign of saturation. From these observations, we conclude that the linear MR can be understood by considering diffusing cyclotron centers due to compressed Landau wavefunction, an emergent behavior in the quantum limit.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetoresistance effects in cadmium arsenide thin films

In-plane magnetoresistance effects in the three-dimensional Dirac semimetal cadmium arsenide are studied as a function of film growth orientation, film thickness and surface Fermi level. For films with the Fermi level near the bulk nodes, which are gapped for very thin films, a pronounced planar Hall effect is observed due to the combination of magnetoresistance effects from the Berry phase and orbital magnetoresistance. At high Fermi levels, surface state transport produces weaker magnetoresistance effects. Furthermore, the results are discussed in the context of detecting signatures of the chiral anomaly.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo2O4 films

The inverse spinel ferrimagnetic NiCo2O4 possesses high magnetic Curie temperature TC, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo2O4 thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo2O4 films on MgAl2O4 substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while TC for 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity (σxy) and its scaling relation with the longitudinal conductivity (σxx) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo2O4.

Chen, Xuegang↗
DOE OSTI2021

Fermi surfaces of the topological semimetal CaSn 3 probed through de Haas van Alphen oscillations

Abstract In the search of topological superconductors, nailing down the Fermiology of the normal state is as crucial a prerequisite as unraveling the superconducting pairing symmetry. In particular, the number of time-reversal-invariant momenta (TRIM) in the Brillouin zone enclosed by Fermi surfaces is closely linked to the topological class of time-reversal-invariant systems, and can experimentally be investigated. We report here a detailed study of de Haas van Alphen quantum oscillations in single crystals of the topological semimetal CaSn 3 with torque magnetometry in high magnetic fields up to 35 T. In conjunction with density functional theory based calculations, the observed quantum oscillations frequencies indicate that the Fermi surfaces of CaSn 3 enclose an odd number of TRIM, satisfying one of the proposed criteria to realize topological superconductivity. Nonzero Berry phases extracted from the magnetic oscillations also support the nontrivial topological nature of CaSn 3 .

Physics↗
DOE OSTI2022

Unified theory of spin and charge in a ferromagnet

Here, we derive a unified theory of spin and charge degrees of freedom in a ferromagnet. The spin-transfer torque and spin electromotive force are examined from the coarse-grained perspective of collective coordinates. The resulting equations of motion reflect a balance of conservative, gyroscopic (Berry-phase), and dissipative forces. We then expand the space of collective coordinates by adding the electric charge. The adiabatic spin-transfer torque and spin electromotive force (emf) turn out to be a gyroscopic force; their nonadiabatic counterparts are a dissipative force.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Quantum oscillations evidence for topological bands in kagome metal ScV 6 Sn 6

Abstract Metals with kagome lattice provide bulk materials to host both the flat-band and Dirac electronic dispersions. A new family of kagome metals is recently discovered in A V 6 Sn 6 . The Dirac electronic structures of this material needs more experimental evidence to confirm. In the manuscript, we investigate this problem by resolving the quantum oscillations in both electrical transport and magnetization in ScV 6 Sn 6 . The revealed orbits are consistent with the electronic band structure models. Furthermore, the Berry phase of a dominating orbit is revealed to be around π , providing direct evidence for the topological band structure, which is consistent with calculations. Our results demonstrate a rich physics and shed light on the correlated topological ground state of this kagome metal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Magnetic-field-induced nontrivial electronic state in the Kondo-lattice semimetal CeSb

Synergic effect of electronic correlation and spin-orbit coupling is an emerging topic in topological materials. Central to this rapidly developing area are the prototypes of strongly correlated heavy-fermion systems. Recently, some Ce-based compounds are proposed to host intriguing topological nature, among which the electronic properties of CeSb are still under debate. Herein, we report a comprehensive study combining magnetic and electronic transport measurements, and electronic band-structure calculations of this compound to identify its topological nature. Quantum oscillations are clearly observed in both magnetization and magnetoresistance at high fields, from which one pocket with a nontrivial Berry phase is recognized. Angular-dependent magnetoresistance shows that this pocket is elongated in nature and corresponds to the electron pocket as observed in LaBi. Nontrivial electronic structure of CeSb is further confirmed by first-principle calculations, which arises from spin splitting in the fully polarized ferromagnetic state. These features indicate that magnetic field can induce nontrivial topological electronic states in this prototypical Kondo semimetal.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Anomalous Hall Effect in Kagome Ferrimagnet GdMn 6 Sn 6

We present magnetotransport data on the ferrimagnet GdMn 6 Sn 6 . From the temperature-dependent data we are able to extract a large intrinsic contribution to the anomalous Hall effect σ int xz ~ 32 Ω –1 cm –1 and σ int xz ~ 223 Ω –1 cm –1 , which is comparable to values found in other systems also containing kagome nets of transition metals. From our transport anisotropy, as well as our density functional theory calculations, we argue that the system is electronically best described as a three-dimensional system. Furthermore we show that reduced dimensionality is not a strong requirement for obtaining large Berry phase contributions to transport properties. In addition, the coexistence of rare-earth and transition-metal magnetism makes the hexagonal MgFe 6 Ge 6 structure type a promising system to tune the electronic and magnetic properties in future studies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Field-angle dependence of sound velocity in the Weyl semimetal TaAs

The elastic modulus c 44 of a single crystal of the Weyl semimetal TaAs was investigated by measuring relative changes in the sound velocity under application of a magnetic field up to 10 T. Using an ultrasonic pulsed-echo technique, we studied the shear response of the crystal when the angle between the sound wave propagation and the magnetic field is changed. We observe a broken tetragonal symmetry at fields above 6 T, an anisotropy that is likely related to a longitudinal negative magnetoresistance and therefore might provide evidence of the chiral anomaly, one of the main topological signatures of this class of materials. Furthermore, we also observe quantum oscillations in the sound velocity whose frequencies vary with magnetic field orientation. A fan diagram of Landau level indices reveals topological and trivial Berry phases, depending on the field orientation, indicating a sensitivity to different Fermi surface pockets that do or do not enclose Weyl nodes, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗