We identify a nonequilibrium route for generating altermagnetic spin splitting in antiferromagnet by ultrafast light. Unlike existing strategies, this route does not require relativistic angular-momentum transfer, static symmetry breaking, or auxiliary external fields. Using real-time time-dependent density functional theory, we demonstrate in the antiferromagnetic perovskite KNiF3 that linearly polarized light can induce momentum-dependent altermagnetic spin splitting by breaking the effective time-reversal symmetry through photoexcited charge redistribution and the resulting lattice distortion. We provide a general symmetry selection rule for this route. These results establish a mechanism for ultrafast control of altermagnetism and extend its material realization into the nonequilibrium regime.

Quantum control allows a wide range of quantum operations employed in molecular physics, nuclear magnetic resonance and quantum information processing. Thanks to the existing microelectronics industry, semiconducting qubits, where quantum information is encoded in spin or charge degree freedom of electrons or nuclei in semiconductor quantum dots, constitute a highly competitive candidate for scalable solid-state quantum technologies. In quantum information processing, advanced control techniques are needed to realize quantum manipulations with both high precision and noise resilience. In this review, we first introduce the basics of various widely-used control methods, including resonant excitation, adabatic passage, shortcuts to adiabaticity, composite pulses, and quantum optimal control. Then we review the practical aspects in applying these methods to realize accurate and robust quantum gates for single semiconductor qubits, such as Loss-DiVincenzo spin qubit, spinglet-triplet qubit, exchange-only qubit and charge qubit.

Planar Germanium has emerged as a promising platform to build spin-based large scale quantum computers. By exploiting the anisotropic hyperfine interaction of holes in Ge, qubits with long T2* have been recently realized. While the performance of single qubits is still more or less limited by 73Ge nuclear spin fluctuations, the site-to-site variation of qubit sweet spot becomes obstacles to maintaining high fidelity of each qubit across the whole wafer. To achieve high performance Ge-based quantum circuit, it is therefore essential to eliminate the origin source of hyperfine noise. In its Silicon counterparts, reduction of 29Si abundance enables exceptional high-fidelity operation. In contrast, hole qubits based on isotopically purified Ge have not been demonstrated. Here, we report the synthesis of high quality 2-dimensional hole gas (2DHG) with enriched 70GeH4 precursor. Due to the suppression of nonzero spin nucleus, the qubits' T2* on the sweet spot is moderately extended beyond 20 us, surpassing the previous best reported Ge hole qubits. More importantly, the qubits' T2* off the sweet spot is enhanced to above 3 us, enabling single qubit gate fidelity exceeding 99.9

Ultrafast laser pulses can selectively induce either wide-sized coherent or localized incoherent dynamics in solids, yet harnessing these complex dynamics to achieve controlled phase transitions remains challenging. Here, we use real-time time-dependent density functional theory (rt-TDDFT) to investigate a double-pulse laser scheme in VO 2 , elucidating the roles of these two distinct types of dynamics and proposing two more energy-efficient phase transition routes. In the first scenario, a weak initial pulse induces coherent oscillations of V-V dimers. When the second pulse is applied at the dimer stretching time, transient bandgap narrowing enhances carrier excitation, thereby reducing the total laser fluence required for the phase transition. In the second scenario, a stronger but subthreshold first pulse activates localized structural distortions resembling photoinduced polarons. A subsequent long-wavelength pulse promotes accumulation and spatial propagation of the polarons, ultimately triggering a global phase transition. Our findings establish multipulse excitation as an energy-efficient and general approach for controlling structural phase transitions and disentangling distinct ultrafast dynamics in solids.

Optimizing the harmonic yield remains an important yet unresolved challenge in solid-state high-order harmonic generation (HHG), particularly under complex driving fields. In gases, the harmonic yield driven by counter-rotating bicircular (CRB) pulses peaks at a universal "magic" field ratio E2 omega : E omega approximate to 1.5 : 1. Here, using time-dependent density-functional theory (TDDFT), we show that this universality breaks in two-dimensional semiconductors where HHG becomes strongly material-dependent. We identify the material dependence from a real-space trajectory model (RTM) as the interplay between anisotropic Bloch velocity and anomalous Hall velocity. The RTM quantitatively reproduces the TDDFT results while requiring only band structure and Berry curvature. These insights provide a practical guidance for establishing harmonic yield of solid-state HHG, and a transferable and lightweight tool to screen candidate materials.

Abstract Remanent polarization and coercive field in ferroelectrics are often predicted to be high, yet experimentally observed to be much lower-an inconsistency that hinders the rational design of functional materials and devices. We identify a hidden mechanism underlying this discrepancy: the interaction between polarization domain walls (PDWs) and lattice domain walls (LDWs) that standard models omit. Using κ-Ga2O3 as a representative ferroelectric, we develop a machine-learning potential trained on ab initio molecular-dynamics data to capture realistic polarization switching. Our simulations reveal that PDWs become topologically blocked at 120° LDWs, stabilizing residual domain-wall networks that suppress remanent polarization while enabling rapid, low-field switching by bypassing slow nucleation. The blocking strengthens as lattice domains shrink, offering a new strategy for tuning ferroelectric performance through lattice-domain engineering. The mechanism not only reconciles theoretical with experimental results but also provides a practical approach for improving ferroelectric performance.

Two-dimensional (2D) electron/hole gases confined in semiconductor heterostructures suffer from weak Rashba spin-orbit coupling (SOC) for manipulating spin degreee of freedom via an electric rather than a magnetic field. Here, we show that complementary metal-oxide-semiconductor technology-accessible strain could substantially enhance the linear Rashba SOC of the top hole subband in Ge/SiGe quantum wells (QWs) to a level comparable to that of 2D Rashba materials through enhancing the mixture of the light-hole and heavy-hole bands. We further show that strongly enhanced Rashba SOC boosts the Rabi frequency of hole spin qubits confined in Ge/SiGe QWs by two orders of magnitude to an unprecedented 40 GHz, more than one order of magnitude faster than other qubit platforms. We also demonstrate that the hole spin rotation with Rabi frequency > 25 GHz enters a new regime being immune to gate control-induced electric noise, opening a new avenue to simultaneously improve the gate speed and gate fidelity. Our findings provide a new routine to substantially enhance the Rashba SOC in 2D semiconductor hole gases to a level that is great for spintronic applications.

We demonstrate that oxygen-related impurities significantly limit hole mobility in Ge quantum wells (QWs) and show that the use of purified precursors substantially mitigates this effect. Samples grown without precursor purification exhibit peak hole mobilities below 5.0×10 5 cm 2 /Vs, whereas those grown with purified precursors consistently achieve mobilities exceeding 1.0×10 6 cm 2 /Vs, with a maximum near 2.25×10 6 cm 2 /Vs. Precursor purification reduces the oxygen concentration in the QWs by roughly a factor of five. By fitting the density-dependent mobility with a model that includes remote-charge, background-charge, and interface-roughness scattering (following Huang and Sarma), we find that the background charge density N b is reduced by about one order of magnitude, from (5.7 ± 0.7)×10 14 to (4.9 ± 1.3)×10 13 cm -3 , when using the purified precursor. This reduction correlates with the observed mobility enhancement and identifies oxygen-related defects as the dominant source of background scattering. Although oxygen control is a key pathway to improve mobility in Ge QWs, interface-roughness and remote-charge scattering remain significant and together limit the achievable peak mobility.

Two-dimensional hole gases (2DHGs) confined in Ge/SiGe quantum wells offer a promising platform for exploring low-dimensional spintronic phenomena and valence band dynamics, due to their strong spin-orbit interaction and long spin coherence times. Biaxial compressive strain arising from the lattice mismatch between the Ge well and SiGe barriers defines the fundamental physical properties of the 2DHG. Meanwhile, practical device architectures introduce additional sources of strain. For instance, differential thermal contraction between gate electrodes and the quantum well leads to anisotropic strain, which in turn modulates the electronic structure and transport symmetry. This paper provides a systematic and quantitative investigation into the impact of deliberately in situ applied uniaxial strain on the properties of the 2DHG in Ge/SiGe quantum wells. Using a piezoelectric-based strain cell, uniaxial in-plane strain along the [110] crystallographic direction is applied and magnetotransport measurements are performed. The results reveal significant anisotropic modulation of the hole effective mass, mobility, and out-of-plane g-factor due to strain-induced deformation of the valence band. While the carrier density remains isotropic, the modification of mobility, effective mass, and the out-of-plane g-factor show pronounced anisotropy along different crystallographic directions. These findings highlight the critical role of uniaxial strain in shaping the fundamental transport coefficients of Ge/SiGe quantum wells and provide precise experimental benchmarks for strain engineering in future design of high-performance heterostructures and devices.

Photoinduced phase transitions (PIPTs) are typically described within Landau theory for group-subgroup systems. However, transitions between phases with unrelated symmetries-such as the diamond-to-graphite transformation from the cubic Fd-3m space group to the hexagonal P63/mmc space group-lie beyond its scope. Here, we uncover a two-step nucleation mechanism in carbon: the system first passes through a metastable intermediate, which evolves into graphite under weak excitation or into an amorphous state under strong excitation. Real-time mapping of carriers and bond distortions reveals that directional redistribution of photoexcited carriers generates atomic driving forces, breaking C-C bonds along specific directions and steering the lattice toward graphite. In contrast, Si and Ge lack stable pi bonds, leading only to amorphization under the same photoexcitation. These findings not only establish a unified framework for understanding PIPT between distinct symmetry groups but also resolve longstanding questions about the distinct phase transition pathways in diamond-based materials.

ABSTRACT Recent experiments suggest that atomic disordering dynamics are more universal than coherent processes in photoinduced phase transitions (PIPTs), yet their microscopic origin remains elusive. Using real‐time time‐dependent density functional theory (rt‐TDDFT), we reveal that at low photoexcitation, higher lattice temperatures accelerate disordering and reduce the phase transition threshold by thermally exciting phonons that randomize lattice vibrations in VO 2 . Beyond the threshold fluence, however, the timescale and degree of disordering become temperature‐independent due to similar non‐uniform distributions of photoexcited holes across different temperatures. We further uncover anisotropic disordering: photoexcitation first elongates V─V dimers, followed by O‐mediated rotations with correlated displacements along z ‐axis. This correlated motion constrains disordering along z ‐axis, while disorder along x ‐axis remains more pronounced — a process we term “correlated disorder”. The correlated dynamic drives a transient monoclinic metallic phase, characterized by V‐atom motion primarily along x ‐axis. These findings establish a microscopic framework for anisotropic disordering pathways in VO 2 and provide new insights into disordered PIPT mechanisms.

The absence of an efficient light source compatible with silicon complementary metal-oxide-semiconductor technology remains a pivotal bottleneck in integrated photonics. Recently, the hexagonal diamond phase of germanium (2H-Ge) has emerged as a promising alternative in light of the direct nature of its bandgap, yet its light emission efficiency falls behind that of III-V semiconductors. Here, by performing theoretical calculations using an atomistic semi-empirical pseudopotential method, we systematically investigate the electronic structure and interband optical gain of [0001]-oriented 2H-Ge nanowires (NWs). We show that quantum confinement in pure 2H-Ge NWs enables diameter-tunable bandgaps across the infrared spectrum, but retains a pseudodirect character with weak near-band-edge optical transitions at the Brillouin zone center. Interestingly, we demonstrate that a moderate uniaxial tensile strain can induce a conduction band inversion, which dramatically enhances the optical gain by over two orders of magnitude and switches the dominant polarization of the emission. We illustrate such an enhancement by correlating the gain characteristics with the energy ordering of the active conduction band states. Our results thus provide essential theoretical guidance and optimization strategies to realize high-performance, polarized light emitters based on 2H-Ge NWs for integrated photonic applications.

Magnetic field sensing is essential for applications in communication, environmental monitoring, and biomedical diagnostics. Quantum sensors based on solid-state spin defects, such as nitrogen-vacancy centers in diamond or boron vacancies in single-crystal hexagonal boron nitride (hBN), typically require precise alignment between the external magnetic field and the defect's spin quantization axis to achieve reliable sensing. This alignment constraint complicates device integration and hinders scalability. Here, we demonstrate room-temperature optically detected magnetic resonance (ODMR) from negatively charged boron vacancies (VB-) in commercially available hot-pressed polycrystalline hBN. The random grain orientation inherently samples a broad range of spin quantization axes, enabling alignment-free magnetic field detection. Numerical modeling further confirms that sensing remains feasible despite anisotropic sensitivity, establishing hot-pressed hBN as a robust and practical platform for quantum magnetometry. This approach paves the way toward low-cost, scalable, and mechanically stable quantum magnetic field sensors suitable for real-world deployment.

The Ge1-xSnx alloy is a promising candidate for silicon-based optoelectronic devices, yet few experimental studies have investigated its full indirect-to-direct bandgap transition behavior. The photoluminescence (PL) characteristics of Ge1-xSnx films with Sn compositions of 3.3 %, 6.5 %, 10.3 %, and 12.8 % were investigated by a high-resolution and wide spectrum (1 similar to 5.5 mu m) Fourier Transform Infrared Spectrometer PL system. Owing to the wide spectrum measurement capability, the PL spectra of these Ge1-xSnx films can be compared in a single system. It can be seen that the PL spectrum of the high-Sn content GeSn film (12.8 %) exhibits a high-energy shoulder peak, which is different from the low-Sn content GeSn film (3.3 %) with a low-energy shoulder peak. By extracting the transition peak position through Gaussian fitting, we observed for the first time that the direct bandgap energy of the high-Sn content GeSn is lower than the indirect bandgap energy, resulting in the negative energy separation. Furthermore, as the Sn content increases, a clear indirect-to-direct transition is observed, which leads to opposite temperature-dependent PL-intensity trends between the high- and low-component GeSn films. Finally, the insensitivity to temperature was demonstrated in GeSn with Sn compositions of 6.5 %, indicating a balance between radiative and non-radiative recombination. These findings provide further evidence of T-and L-valley inversion and quasi-direct bandgap GeSn materials, indicating great potential for direct bandgap GeSn and GeSn based optoelectronic devices grown on Si.

Germanium (Ge) has long been regarded as a promising laser material for silicon photonics due to its quasi-direct bandgap to make up for the deficiency of indirect bandgap silicon, but its energy band structure under pressure remains puzzling. Here, we study the pressure-dependent photoluminescence (PL) of Ge compressed in a diamond anvil cell to reveal its energy band structure up to 3.89 GPa. Unlike the earlier reported results studied by absorption, the effect of high pressure on bandgaps can be studied for the same sample and the determinations of bandgap positions are not influenced by the sample thickness and the interference pattern. The PL peak related to X-Gamma bandgap transition was observed with pressure coefficient of -12.4 +/- 2.1 meV/GPa. We unambiguously show that the L- and Gamma-valleys move upward while the X-valley moves downward in energy with increasing pressure, with a Gamma-X crossover observed at an onset pressure of 0.74 GPa, and then a L-X crossover takes place near 2.85 GPa. These findings provide experimental evidence for identification of the band structure of Ge, deepening the understanding of pressure-induced bandgap modification and conduction band valley crossover.

A newly analytic occupation-resolved theory capturing the temporal structure of attosecond pulses (APs) is derived. We validate it with real-time time-dependent density functional theory and show remarkable temporal confinement of APs with laser intensity in solid state. Using a simplified field-driven electron excitation together with a generalized pre-acceleration picture, the interband emission timing demonstrate intrinsically temporal mismatched with field-synchronous intraband radiation, leading to a nonmonotonic dependence of attosecond pulse width on laser intensity. Our findings not only shed light on the microscopic mechanisms behind solid-state high harmonic generation (HHG), but also establish the fundamental time-domain constraint on solid-state APs independent of material damage thresholds.

Due to the lack of efficient light sources compatible with complementary metal oxide semiconductor technology, the development of silicon-based photonic integrated circuits has been restricted. Germanium (Ge), with its small bandgap difference between the direct and indirect valleys, becomes a promising candidate for light emission when tensile strain is applied to modify its band structure. However, achieving high and uniform strain in electrically active devices remains a challenge. In this work, we present a biaxially tensile strained Ge light-emitting diode with a vertical p-i-n junction, fabricated on a germanium-on-insulator substrate. The energy difference between the Γ valley and the L valley is further reduced by introducing a biaxial tensile strain of ∼0.77% through the microbridge structure. A 1.7-fold enhancement is observed in the direct bandgap photoluminescence intensity at room temperature. Furthermore, the peak intensity of direct bandgap electroluminescence increases threefold at 400 K compared to room temperature. These results demonstrate the potential of biaxially strained Ge for efficient, Si-compatible light sources, advancing the integration of group-IV materials in silicon photonics.

The interplay between Coulomb interactions and kinetic energy underlies many exotic phases in condensed matter physics. In a two-dimensional electronic system, If Coulomb interaction dominates over kinetic energy, electrons condense into a crystalline phase which is referred as Wigner crystal. This ordered state manifests as Wigner molecule for few electrons at the microscopic scale. Observation of Wigner molecules has been reported in quantum dot and moire superlattice systems. Here we demonstrate hole Wigner molecules can be formed in a gate-defined germanium quantum dot with high tunability. By varying voltages applied to the quantum dot device, we can precisely tune the hole density by either changing the hole occupancy or the quantum dot size. For densities smaller than a certain critical value, Coulomb interaction localizes individual holes into ordered lattice sites, forming a Wigner molecule. By increasing the densities, melting process from a Wigner molecule to Fermi liquid-like particles is observed. An intermediate configuration which indicates the coexistence of ordered structure and disordered structure can be formed within a narrow effective density range. Our results provide a new platform for further exploration of the microscopic feature of strong correlated physics and open an avenue to exploit the application of Wigner molecules for quantum information in a very promising spin qubit platform.

The metal-induced gap states (MIGS) are commonly believed to cause the strong Fermi level pinning (FLP) in the metal-semiconductor contacts. Here, we reveal that the dangling-bond-induced surface states play a crucial role, even comparable with MIGS. The first-principles calculations show that metal-germanium (Ge) and metal-silicon (Si) contacts should possess a similar FLP strength if they adopt an identical interface bonding configuration: the reconstructed bonding configuration renders Si and Ge having pinning factors of 0.16 and 0.11, respectively, and the ideal non-reconstructed bonding configuration gives them pinning factors of 0.05 and 0, respectively. We illustrate that Si favors the reconstructed bonding configuration, and Ge favors the ideal non-reconstructed bonding configuration after metal deposition. The self-passivation of the dangling bonds reduces the interface gap states to give a much weaker FLP in the metal-Si contacts than in the metal-Ge contacts. We also demonstrate that the full passivation of the interface dangling bonds can further increase the pinning factor to 0.5 by further reducing the interface gap states. These findings shed light on alleviating the FLP to lower the contact resistance for Si and emerging materials towards advanced semiconductor technology.