arXiv subjects

A. Gali

Publications and source records attributed to A. Gali.

At least 19 recordsLinked to original sources

arXiv2024

Experimental observation of spin defects in van der Waals material GeS$_2$

Spin defects in atomically thin two-dimensional (2D) materials such as hexagonal boron nitride (hBN) attract significant attention for their potential quantum applications. The layered host materials not only facilitate seamless integration with optoelectronic devices but also enable the formation of heterostructures with on-demand functionality. Furthermore, their atomic thickness renders them particularly suitable for sensing applications. However, the short coherence times of the spin defects in hBN limit them in quantum applications that require extended coherence time. One primary reason is that both boron and nitrogen atoms have non-zero nuclear spins. Here, we present another 2D material germanium disulfide ($\beta$-GeS$_2$) characterized by a wide bandgap and potential nuclear-spin-free lattice. This makes it as a promising host material for spin defects that possess long-coherence time. Our findings reveal the presence of more than two distinct types of spin defects in single-crystal $\beta$-GeS$_2$. Coherent control of one type defect has been successfully demonstrated at both 5 K and room temperature, and the coherence time $T_2$ can achieve tens of microseconds, 100-folds of that of negatively charged boron vacancy (V$_{\text{B}}^-$) in hBN, satisfying the minimal threshold required for metropolitan quantum networks--one of the important applications of spins. We entatively assign the observed optical signals come from substitution defects. Together with previous theoretical prediction, we believe the coherence time can be further improved with optimized lattice quality, indicating $\beta$-GeS$_2$ as a promising host material for long-coherence-time spins.

quant-ph↗

arXiv2024

Isotope substitution and polytype control for point defects identification: the case of the ultraviolet color center in hexagonal boron nitride

Defects in crystals can have a transformative effect on the properties and functionalities of solid-state systems. Dopants in semiconductors are core components in electronic and optoelectronic devices. The control of single color centers is at the basis of advanced applications for quantum technologies. Unintentional defects can also be detrimental to the crystalline structure and hinder the development of novel materials. Whatever the research perspective, the identification of defects is a key but complicated, and often long-standing issue. Here, we present a general methodology to identify point defects by combining isotope substitution and polytype control, with a systematic comparison between experiments and first-principles calculations. We apply this methodology to hexagonal boron nitride (hBN) and its ubiquitous color center emitting in the ultraviolet spectral range. From isotopic purification of the host hBN matrix, a local vibrational mode of the defect is uncovered, and isotope-selective carbon doping proves that this mode belongs to a carbon-based center. Then, by varying the stacking sequence of the host hBN matrix, we unveil different optical responses to hydrostatic pressure for the non-equivalent configurations of this ultraviolet color center. We conclude that this defect is a carbon dimer in the honeycomb lattice of hBN. Our results show that tuning the stacking sequence in different polytypes of a given crystal provides unique fingerprints contributing to the identification of defects in 2D materials.

cond-mat.mtrl-sci↗

arXiv2023

A physically motivated analytical expression for the temperature dependence of the zero-field splitting of the nitrogen-vacancy center in diamond

The temperature dependence of the zero-field splitting (ZFS) between the $|m_{s}=0\rangle$ and $|m_{s}=\pm 1\rangle$ levels of the nitrogen-vacancy (NV) center's electronic ground-state spin triplet can be used as a robust nanoscale thermometer in a broad range of environments. However, despite numerous measurements of this dependence in different temperature ranges, to our knowledge no analytical expression has been put forward that captures the scaling of the ZFS of the NV center across all relevant temperatures. Here we present a simple, analytical, and physically motivated expression for the temperature dependence of the NV center's ZFS that matches all experimental observations, in which the ZFS shifts in proportion to the occupation numbers of two representative phonon modes. In contrast to prior models our expression does not diverge outside the regions of fitting. We show that our model quantitatively matches experimental measurements of the ZFS from 15 to 500 K in single NV centers in ultra-pure bulk diamond, and we compare our model and measurements to prior models and experimental data.

cond-mat.mes-hall↗

arXiv2023

Optically-active spin defects in few-layer thick hexagonal boron nitride

Optically-active spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of two-dimensional quantum sensing units offering optimal proximity to the sample being probed. In this work, we first demonstrate that the electron spin resonance frequencies of boron vacancy centres (V$_\text{B}^-$) can be detected optically in the limit of few-atomic-layer thick hBN flakes despite the nanoscale proximity of the crystal surface that often leads to a degradation of the stability of solid-state spin defects. We then analyze the variations of the electronic spin properties of V$_\text{B}^-$ centres with the hBN thickness with a focus on (i) the zero-field splitting parameters, (ii) the optically-induced spin polarization rate and (iii) the longitudinal spin relaxation time. This work provides important insights into the properties of V$_\text{B}^-$ centres embedded in ultrathin hBN flakes, which are valuable for future developments of foil-based quantum sensing technologies.

quant-ph↗

arXiv2023

A planar defect spin sensor in a two-dimensional material susceptible to strain and electric fields

The boron-vacancy spin defect ($\text{V}_\text{B}^{-}$) in hexagonal boron nitride (hBN) has a great potential as a quantum sensor in a two-dimensional material that can directly probe various external perturbations in atomic-scale proximity to the quantum sensing layer. Here, we apply first principles calculations to determine the coupling of the $\text{V}_\text{B}^{-}$ electronic spin to strain and electric fields. Our work unravels the interplay between local piezoelectric and elastic effects contributing to the final response to the electric fields. The theoretical predictions are then used to analyse optically detected magnetic resonance (ODMR) spectra recorded on hBN crystals containing different densities of $\text{V}_\text{B}^{-}$ centres. We prove that the orthorhombic zero-field splitting parameter results from local electric fields produced by surrounding charge defects. By providing calculations of the spin-strain and spin-electric field couplings, this work paves the way towards applications of $\text{V}_\text{B}^{-}$ centres for quantitative electric field imaging and quantum sensing under pressure.

quant-ph↗

arXiv2022

Coherent dynamics of multi-spin $\rm V_B^-$ centers in hexagonal boron nitride

Hexagonal boron nitride (hBN) has recently been demonstrated to contain optically polarized and detected electron spins that can be utilized for implementing qubits and quantum sensors in nanolayered-devices. Understanding the coherent dynamics of microwave driven spins in hBN is of crucial importance for advancing these emerging new technologies. Here, we demonstrate and study the Rabi oscillation and related dynamical phenomena of the negatively charged boron vacancy ($\rm V_B^-$) spins in hBN. We report on different dynamics of the $\rm V_B^-$ spins at weak and strong magnetic fields. In the former case the defect behaves like a single electron spin system, while in the latter case it behaves like a multi-spin system exhibiting the multiple-frequency dynamical oscillation like clear beat in Ramsey fringes. We also carry out theoretical simulations for the spin dynamics of $\rm V_B^-$ and reveal that the nuclear spins can be driven via the strong electric-nuclear coupling existing in $\rm V_B^-$ center, which can be modulated by the magnetic field and microwave field.

quant-ph↗

arXiv2022

Temperature-dependent spin-lattice relaxation of the nitrogen-vacancy spin triplet in diamond

Spin-lattice relaxation within the nitrogen-vacancy (NV) center's electronic ground-state spin triplet limits its coherence times, and thereby impacts its performance in quantum applications. We report measurements of the relaxation rates on the NV center's $|m_{s}=0\rangle \leftrightarrow |m_{s}=\pm 1\rangle$ and $|m_{s}=-1\rangle \leftrightarrow |m_{s}=+1\rangle$ transitions as a function of temperature from 9 to 474 K in high-purity samples. We show that the temperature dependencies of the rates are reproduced by an ab initio theory of Raman scattering due to second-order spin-phonon interactions, and we discuss the applicability of the theory to other spin systems. Using a novel analytical model based on these results, we suggest that the high-temperature behavior of NV spin-lattice relaxation is dominated by interactions with two groups of quasilocalized phonons centered at 68.2(17) and 167(12) meV.

quant-ph↗

arXiv2022

Vacancy-related color centers in twodimensional silicon carbide monolayers

Basic vacancy defects in twodimensional silicon carbide (2D-SiC) are examined by means of density functional theory calculations to explore their magneto-optical properties as well as their potential in quantum technologies. In particular, the characteristic hyperfine tensors and optical excited states of carbon-vacancy, silicon-vacancy, and carbon antisite-vacancy pair defects in 2D-SiC are determined that are the key fingerprints of these defects that may be observed in electron paramagnetic resonance and photoluminescence experiments, respectively. Besides the fundamental characterization of the most basic native defects, we show that the negatively charged carbon antisite-vacancy defect is a promising candidate for realizing a near-infrared single-photon quantum emitter with spin doublet ground state, where the negative charge state may be provided by nitrogen doping of 2D-SiC. We find that the neutral carbon-vacancy with spin triplet ground state might be used for quantum sensing with a broad emission in the visible.

cond-mat.mtrl-sci↗

arXiv2022

Photoluminescence lineshapes and charge state control of divacancy qubits in silicon carbide

Divacancy in its neutral charge state (V$_\text{C}$V$_\text{Si}^0$) in 4H silicon carbide (SiC) is a leading quantum bit (qubit) contender. Owing to the lattice structure of 4H SiC four different V$_\text{C}$V$_\text{Si}$ configurations can be formed. Ground and optical excited states of V$_\text{C}$V$_\text{Si}^0$ exhibit $S$=1 spintriplet state and the corresponding transition energies are around $\approx 1.1$~eV falling in the near-infrared wavelength region. Recently, photoluminescence (PL) quenching has been experimentally observed for all V$_\text{C}$V$_\text{Si}$ configurations in 4H SiC, i.e. the corresponding zero-phonon lines (ZPLs) appear only at higher-than-ZPL photoexcitation energies (threshold energies). It has been shown that V$_\text{C}$V$_\text{Si}^0$ is converted to V$_\text{C}$V$_\text{Si}^-$ upon photoexcitation below the correspoding excitation threshold energies at cryogenic temperature, i.e. V$_\text{C}$V$_\text{Si}^-$ is the so-called dark state. In this study we demonstrate that the threshold energy for reinozation is temperature dependent. We further carry out density functional theory (DFT) calculations in order to investigate the temperature dependent reionization spectrum, i.e. the spectrum of the V$_\text{C}$V$_\text{Si}^- \rightarrow$ V$_\text{C}$V$_\text{Si}^0$ process and found that simultaneous reionization and qubit manipulation can be carried out at around room temperature ($\approx$300~K) by using the usually applied excitation wavelength. We also investigate the PL lineshape of V$_\text{C}$V$_\text{Si}^0$ by using the Huang-Rhys theory.

cond-mat.mes-hall↗

arXiv2021

Decoherence of V$_{\rm B}^{-}$ spin defects in monoisotopic hexagonal boron nitride

Spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of flexible two-dimensional quantum sensing platforms. Here we rely on hBN crystals isotopically enriched with either $^{10}$B or $^{11}$B to investigate the isotope-dependent properties of a spin defect featuring a broadband photoluminescence signal in the near infrared. By analyzing the hyperfine structure of the spin defect while changing the boron isotope, we first unambiguously confirm that it corresponds to the negatively-charged boron-vacancy center (${\rm V}_{\rm B}^-$). We then show that its spin coherence properties are slightly improved in $^{10}$B-enriched samples. This is supported by numerical simulations employing cluster correlation expansion methods, which reveal the importance of the hyperfine Fermi contact term for calculating the coherence time of point defects in hBN. Using cross-relaxation spectroscopy, we finally identify dark electron spin impurities as an additional source of decoherence. This work provides new insights into the properties of ${\rm V}_{\rm B}^-$ spin defects, which are valuable for the future development of hBN-based quantum sensing foils.

cond-mat.mtrl-sci↗

arXiv2021

Ultrahigh nitrogen-vacancy center concentration in diamond

High concentration of negatively charged nitrogen-vacancy ($\text{NV}^{-}$) centers was created in diamond single crystals containing approximately 100 ppm nitrogen using electron and neutron irradiation and subsequent thermal annealing in a stepwise manner. Continuous wave electron paramagnetic resonance (EPR) was used to determine the transformation efficiency from isolated N atoms to $\text{NV}^{-}$ centers in each production step and its highest value was as high as 17.5 %. Charged vacancies are formed after electron irradiation as shown by EPR spectra, but the thermal annealing restores the sample quality as the defect signal diminishes. We find that about 25 % of the vacancies form NVs during the annealing process. The large $\text{NV}^{-}$ concentration allows to observe orientation dependent spin-relaxation times and also the determination of the hyperfine and quadrupole coupling constants with high precision using electron spin echo (ESE) and electron-nuclear double resonance (ENDOR). We also observed the EPR signal associated with the so-called W16 centers, whose spectroscopic properties might imply a nitrogen dimer-vacancy center for its origin.

cond-mat.mtrl-sci↗

arXiv2021

Identification of silicon vacancy-related electron paramagnetic resonance centers in 4H SiC

The negatively charged silicon vacancy [V$_\text{Si}(-)$] in silicon carbide (SiC) is a paramagnetic and optically active defect in hexagonal SiC. V$_\text{Si}(-)$ defect possesses $S = 3/2$ spin with long spin coherence time and can be optically manipulated even at room temperature. Recently, electron spin resonance signals have been observed besides the signals associated with the V$_\text{Si}(-)$ defects in the 4H polytype of SiC. The corresponding centers share akin properties to those of the V$_\text{Si}(-)$ defects and thus they may be promising candidates for quantum technology applications. However, the exact origin of the new signals is unknown. In this paper we report V$_\text{Si}(-)$-related pair defect models as possible candidates for the unknown centers. We determine the corresponding electronic structures and magneto-optical properties as obtained by density functional theory (DFT) calculations. We propose models for the recently observed electron paramagnetic resonance centers with predicting their optical signals for identification in future experiments.

cond-mat.mtrl-sci↗

arXiv2020

Quantum well stabilized point defect spin qubits

Defect-based quantum systems in in wide bandgap semiconductors are strong candidates for scalable quantum-information technologies. However, these systems are often complicated by charge-state instabilities and interference by phonons, which can diminish spin-initialization fidelities and limit room-temperature operation. Here, we identify a pathway around these drawbacks by showing that an engineered quantum well can stabilize the charge state of a qubit. Using density-functional theory and experimental synchrotron x-ray diffraction studies, we construct a model for previously unattributed point defect centers in silicon carbide (SiC) as a near-stacking fault axial divacancy and show how this model explains these defect's robustness against photoionization and room temperature stability. These results provide a materials-based solution to the optical instability of color centers in semiconductors, paving the way for the development of robust single-photon sources and spin qubits.

cond-mat.mes-hall↗

arXiv2020

Theoretical study of quantum emitters in two-dimensional silicon carbide monolayer

The features of some potential single-photon sources in two-dimensional silicon carbide monolayers is studied via ab-initio calculation and group theory analysis. A few point defects in three charge states (negative, positive and neutral) are considered. By applying performance criteria, Stone-Wales defects without and with combination of antisite defects are studied in details. The formation energy calculations reveal that neutral and positive charge states of these defects are stable. We compute the zero-phonon-line energy, the Huang-Rhys (HR) factor and the photoluminescence spectrum for the available transitions in different charge states. The calculated HR values and the related Debye-Waller factors guarantee that the Stone-Wales defects have a high potential of performing as a promising single-photon emitter.

cond-mat.mtrl-sci↗

arXiv2019

Thermal evolution of silicon carbide electronic bands

Direct observation of temperature dependence of individual bands of semiconductors for a wide temperature region is not straightforward, in particular. However, this fundamental property is a prerequisite in understanding the electron-phonon coupling of semiconductors. Here we apply \emph{ab initio} many body perturbation theory to the electron-phonon coupling on hexagonal silicon carbide (SiC) crystals and determine the temperature dependence of the bands. We find a significant electron-phonon renormalization of the band gap at 0~K. Both the conduction and valence bands shift at elevated temperatures exhibiting a different behavior. We compare our theoretical results with the observed thermal evolution of SiC band edges, and discuss our findings in the light of high temperature SiC electronics and defect qubits operation.

cond-mat.mtrl-sci↗

arXiv2019

Optical Properties of Vanadium in 4H Silicon Carbide for Quantum Technology

We study the optical properties of tetravalent vanadium impurities in 4H silicon carbide (4H SiC). Emission from two crystalline sites is observed at wavelengths of 1.28 \mum and 1.33 \mum, with optical lifetimes of 163 ns and 43 ns. Group theory and ab initio density functional supercell calculations enable unequivocal site assignment and shed light on the spectral features of the defects. We conclude with a brief outlook on applications in quantum photonics.

quant-ph↗

arXiv2018

Identification of nickel-vacancy defect in the photocurrent spectrum of diamond by means of \emph{ab initio} calculations

There is a continuous search for solid-state spin qubits operating at room temperature with excitation in the IR communication bandwidth. Recently we have introduced the photoelectric detection of magnetic resonance (PDMR) to read the electron spin state of nitrogen-vacancy (NV) center in diamond, a technique which is promising for applications in quantum information technology. By measuring photoionization spectra on a diamond crystal we found two ionization thresholds that were not reported before. On the same sample we also observed absorption and photoluminescence signatures that were identified in literature as Ni associated defects. We performed \emph{ab initio} calculation of the photo-ionization cross-section of the nickel split vacancy complex (NiV) and N-related defects in their relevant charge states and fitted the concentration of these defects to the measured photocurrent spectrum, which led to a surprising match between experimental and calculated spectra. This study enabled to identify the two unknown ionization thresholds with the two acceptor levels of NiV. Because the excitation of NiV is in infrared, the photocurrent detected from the paramagnetic NiV color centers is a promising way towards designing a novel type of electrically readout qubits.

cond-mat.mtrl-sci↗

arXiv2017

Characterization and formation of NV centers in 3C, 4H and 6H SiC: an \emph{ab initio} study

Fluorescent paramagnetic defects in solids have become attractive systems for quantum information processing in the recent years. One of the leading contenders is the negatively charged nitrogen-vacancy defect in diamond with visible emission but alternative solution in technologically mature host is an immediate quest for many applications in this field. It has been recently found that various polytypes of silicon carbide (SiC), that are standard semiconductors with wafer scale technology, can host nitrogen-vacancy defect (NV) that could be an alternative qubit candidate with emission in the near infrared region. However, it is much less known about this defect than its counterpart in diamond. The inequivalent sites within a polytype and the polytype variations offer a family of NV defects. However, there is an insufficient knowledge on the magneto-optical properties of these configurations. Here we carry out density functional theory calculations, in order to characterize the numerous forms of NV defects in the most common polytypes of SiC including 3C, 4H and 6H, and we also provide new experimental data in 4H SiC. Our calculations mediate the identification of individual NV qubits in SiC polytypes. In addition, we discuss the formation of NV defects in SiC with providing detailed ionization energies of NV defect in SiC which reveals the critical optical excitation energies for ionizing this qubits in SiC. Our calculations unravel the challenges to produce NV defects in SiC with a desirable spin bath.

cond-mat.mtrl-sci↗