arXiv subjects

Klaus Krambrock

Publications and source records attributed to Klaus Krambrock.

5 recordsLinked to original sources

arXiv2026

Real-time ESR tracking for sub-micron 3D magnetic mapping with VB- quantum sensors in hexagonal boron nitride

The discovery of spin-dependent luminescent properties of negatively charged boron-vacancy centers ($V^-_B$) in hexagonal boron nitride (hBN) enabled a new platform for quantum sensing with van der Waals materials. Particularly, the possibility of performing optically detected magnetic resonance (ODMR) for determining the electron spin resonance (ESR) frequencies of hBN color centers became a strong tool for quantum sensing of magnetic fields with submicrometric resolution. However, due to low ODMR contrast, current techniques proposed for mapping DC magnetic fields require hours of integration to obtain a magnetic image of a micron-sized region. In this work, we report the implementation of a frequency-tracking approach for real-time monitoring of ESR frequencies of localized $V^-_B$ centers in hBN. With this technique, magnetic field monitoring was used to map the field pattern generated by a micron-sized conical magnetic tip in only a few minutes. By controlling the magnetic sample's position relative to the quantum sensor, three-dimensional magnetic mapping of the field was achieved with diffraction-limited resolution and shot-noise-limited sensitivity of 54 $μ$T$/\sqrt{\text{Hz}}$. Magnetic field gradients of 3.6 $\pm$ 0.2 $μ$T/nm were measured with our system, in which a maximum detected field rate of 6 mT/s was achieved. The results of this study establish spin resonance frequency-tracking as a viable technique and fast method for minute-scale magnetic imaging, reducing acquisition times by at least one order of magnitude if compared to conventional techniques.

physics.chem-ph↗

arXiv2026

Direct Measurement of the Singlet Lifetime and Photoexcitation Behavior of the Boron Vacancy Center in Hexagonal Boron Nitride

Optically active spin defects in van der Waals (vdW) materials are a promising platform for quantum sensing, potentially enabling shorter standoff distances than defects in diamond and thus improved measurement signal-to-noise ratio (SNR) and spatial resolution. The most studied such defect is the negatively charged boron vacancy center ($V^{-}_{B}$) in hexagonal boron nitride (hBN), yet many of its electronic and spin transition rates and branching ratios remain unknown. Here, we use time-resolved photoluminescence (PL) measurements with a nanosecond rise-time 515 nm laser to directly measure the singlet state lifetime of a $V^{-}_{B}$ ensemble in neutron-irradiated, sub-micron flakes of hBN. We perform this measurement on 16 flakes at room temperature and obtain an average lifetime of 15(3) ns. Additionally, we probe the PL dynamics of thermal and optically polarized electronic spin distributions of the $V^{-}_{B}$ ensemble in a sub-micron hBN flake, and fit our results to a 9-level model to extract electronic transition rates. Lastly, we present PL measurements that potentially indicate optically-induced conversion of $V^{-}_{B}$ to another electronic state, or possibly the neutral charge state ($V^{0}_{B}$), in neutron-irradiated hBN flakes of size $>$ 1 $μ$m.

quant-ph↗

arXiv2022

High throughput investigation of an emergent and naturally abundant 2D material: Clinochlore

Phyllosilicate minerals, which form a class of naturally occurring layered materials (LMs), have been recently considered as a low-cost source of two-dimensional (2D) materials. Clinochlore [Mg5Al(AlSi3)O10(OH)8] is one of the most abundant phyllosilicate minerals in nature, exhibiting the capability to be mechanically exfoliated down to a few layers. An important characteristic clinochlore is the natural occurrence of defects and impurities which can strongly affect their optoelectronic properties, possibly in technologically interesting ways. In the present work, we carry out a thorough investigation of the clinochlore structure on both bulk and 2D exfoliated forms, discussing its optical features and the influence of the insertion of impurities on its macroscopic properties. Several experimental techniques are employed, followed by theoretical first-principles calculations considering several types of naturally-ocurring transition metal impurities in the mineral lattice and their effect on electronic and optical properties. We demonstrate the existence of requirements concerning surface quality and insulating properties of clinochlore that are mandatory for its suitable application in nanoelectronic devices. The results presented in this work provide important informations for clinochlore potential applications and establish a basis for further works that intend to optimize its properties to relevant 2D technological applications through defect engineering.

cond-mat.mtrl-sci↗

arXiv2022

Exploring the structural and optoelectronic properties of natural insulating phlogopite in van der Waals heterostructures

Naturally occurring van der Waals crystals have brought unprecedented interest to nanomaterial researchers in recent years. So far, more than 1800 layered materials (LMs) have been identified but only a few insulating and naturally occurring LMs were deeply investigated. Phyllosilicate minerals, which are a class of natural and abundant LMs, have been recently considered as a low-cost source of insulating nanomaterials. Within this family an almost barely explored material emerges: phlogopite [KMg3(AlSi3)O10(OH)2]. Here we carry out a high throughput characterization of this LM by employing several experimental techniques, corroborating the major findings with first-principles calculations. We show that monolayers (1L) and few-layers of this material are air and temperature stable, as well as easily obtained by the standard mechanical exfoliation technique, have an atomically flat surface, and lower bandgap than its bulk counterpart, an unusual trend in LMs. We also systematically study the basic properties of ultrathin phlogopite and demonstrate that natural phlogopite presents iron impurities in its crystal lattice, which decreases its bandgap from about 7 eV to 3.6 eV. Finally, we combine phlogopite crystals with 1L-WS2 in ultrathin van der Waals heterostructures and present a photoluminescence study, revealing a significant enhancement on the 1L-WS2 optical quality (i.e., higher recombination efficiency through neutral excitons) similarly to that obtained on 1L-WS2/hBN heterostructures. Our proof-of-concept study shows that phlogopite should be regarded as a good and promising candidate for LM-based applications as a low-cost layered nanomaterial.

cond-mat.mtrl-sci↗

arXiv2019

Room Temperature Initialisation and Readout of Intrinsic Spin Defects in a Van der Waals Crystal

Optically addressable spins in widebandgap semiconductors have become one of the most prominent platforms for exploring fundamental quantum phenomena. While several candidates in 3D crystals including diamond and silicon carbide have been extensively studied, the identification of spindependent processes in atomically thin 2D materials has remained elusive. Although optically accessible spin states in hBN are theoretically predicted, they have not yet been observed experimentally. Here, employing rigorous electron paramagnetic resonance techniques and photoluminescence spectroscopy, we identify fluorescence lines in hexagonal boron nitride associated with a particular defect, the negatively charged boron vacancy and determine the parameters of its spin Hamiltonian. We show that the defect has a triplet ground state with a zero field splitting of 3.5 GHz and establish that the centre exhibits optically detected magnetic resonance at room temperature. We also demonstrate the spin polarization of this centre under optical pumping, which leads to optically induced population inversion of the spin ground state a prerequisite for coherent spin manipulation schemes. Our results constitute a leap forward in establishing two dimensional hBN as a prime platform for scalable quantum technologies, with extended potential for spin based quantum information and sensing applications, as our ODMR studies on hBN NV diamonds hybrid structures show.

physics.optics↗