Method and apparatus for determining a spin resonance frequency

Abstract

The present invention relates to a method and a device for determining the spin resonance frequency of a material (1) with optically active spin centers (2), in which at least one optically active spin center (2) of the material (1) is excited to a higher energy state by a focused electron beam (4) emitted from an electron beam source (3) and subsequently returns to a lower energy state while emitting a signal, and in addition to the electron beam excitation from an external source (5), the material (1) is subjected to wave excitation (6). The frequency of the wave excitation (6) is varied over time, and a measuring device (8) determines the spin resonance frequency of the at least one optically active spin center (2) by measuring the signal, at which the signal exhibits an extremum.

Description

[0001] The present invention relates to a method and a device for determining a spin resonance frequency.

[0002] In magnetic resonance measurements, the optically detected magnetic resonance (ODMR) technique is often used, as described, for example, in US 2011 / 0062957A1. ODMR is frequently performed using laser excitation and a cantilever to increase spatial resolution, with a minimum spatial resolution of less than 50 nm. This is limited by the distance between the cantilever tip and the surface of the sample under investigation. To increase the spatial resolution, diamond cantilevers with exactly one nitrogen vacancy center (NV) at the tip are used. This vacancy, where a carbon atom should normally be located, provides spin and simultaneously enables light absorption and emission, which is referred to as the "spin center." Individual nitrogen vacancy centers are approximately...Nitrogen vacancy centers are implanted 10 nm away from the tip via ion implantation. The tips are more than 3 µm high and have a radius between 75 nm and 175 nm. Fabricating the cantilevers and embedding the nitrogen vacancy centers is difficult and complex. The distance between the cantilever tip and the sample surface is generally less than 50 nm, theoretically resulting in a maximum spatial resolution of 10 nm with full contact between the sample and cantilever tip, limited by the implantation depth of the nitrogen vacancy centers. A typical cantilever scan time across the sample is approximately 3 hours for a resolution of 1350 x 760 pixels. The measurement speed is generally limited by the mechanical scanning of the sample using nanometer positioning systems and the desired signal-to-noise ratio (SNR) or contrast of the optically detected magnetic resonance imaging (MRI) used. The resulting measurement resolution is therefore also sample-dependent.

[0003] Photoluminescence (PL) measurements are often used to detect spin resonance. The photon beam, focused by the cantilever, excites spin centers in the sample material to electronically excited energy states through laser absorption. These centers then return to lower energy states by emitting light, i.e., photoluminescence. The intensity of this light emission depends on the spin state. The spin state of the spin centers is manipulated by acoustic waves or microwaves. If a resonance occurs between the spin centers and the wave excitation, the intensity of the luminescence changes (whether it increases or decreases depends on the material system used and the specific spin center itself). Varying the excitation wave frequency results in a detected change in the luminescence (dPL) at the frequency corresponding to the resonance frequency of the spin center.The signal maxima or signal minima (dPL / PL) in the luminescence spectrum depend, for example, on the parameters magnetic field, temperature and pressure, which conversely makes it possible to determine an external magnetic field, pressure and temperature via the detection of spin resonance.

[0004] However, a disadvantage of the described methods is the comparatively high equipment cost and the resulting limitation of resolution due to the measuring apparatus. The spatial resolution is limited either by half the wavelength of the optical excitation (approx. 250 nm, optical resolution limit) or by the distance of the cantilever to the sample surface and the implantation depth of the nitrogen defect centers. Since the small distance between the cantilever and the sample surface must be kept constant, which requires a feedback system for each measurement point, the scanning speed and, consequently, the measurement speed are slow. Furthermore, the manufacturing process of the cantilever is very complex and expensive. In low-temperature applications, the laser-induced heating, the thermal and mechanical stability of the cantilever, and the experimental complexity further complicate matters.

[0005] The present invention is therefore based on the objective of proposing a method and a device by means of which a spin resonance or spin resonance frequency can be detected in a straightforward manner (e.g. with a simple measuring apparatus) with high spatial resolution and / or high measurement speed.

[0006] This problem is solved according to the invention by a method and a device according to the independent claims. Advantageous embodiments and further developments are described in the dependent claims.

[0007] In a method for determining the spin resonance frequency of a material with optically active spin centers, at least one of these optically active spin centers is excited to a higher energy state by a focused electron beam emitted from an electron beam source. Subsequently, the optically active spin center returns to a lower energy state by emitting a signal; that is, it relaxes to a more energetically favorable state, usually the ground state. It can be specifically provided that the spin center is excited from the ground state to an excited state by the electron beam and then relaxes back to the ground state. In addition to the electron beam excitation, the material is also subjected to wave excitation from a source. The frequency of the wave excitation is varied over time and determined by a measuring device.By detecting the signal at different frequencies or different times, a wave excitation frequency is determined at which the detected signal exhibits an extremum (i.e., a maximum or a minimum). Based on this, a resonance frequency of the at least one optically active spin center is determined. Thus, based on the determined wave excitation frequency at which the extremum of the detected signal occurs, a resonance frequency of the at least one optically active spin center is determined. The resonance frequency of the optically active center can, in particular, correspond to the wave excitation frequency at which the detected signal exhibits an extremum.

[0008] This method enables measurements with a simplified and straightforward setup, as the electron beam can be adjusted and focused relatively easily, thereby increasing spatial resolution and measurement speed. Furthermore, it offers high measurement reproducibility. In low-temperature applications, the thermal and mechanical stability of the electron beam plays a less significant role compared to atomic force microscopy. The source is typically an external source, meaning a source that, independent of the electron beam excitation, induces wave excitation in the material in addition to its thermal spectrum. The resonance frequency is defined as the frequency at which the signal strength reaches its extreme, i.e., its minimum or maximum.

[0009] It can be provided that the at least one optically active spin center emits a luminescence signal via electron beam excitation; that is, the optically active spin center emits luminescence radiation, which is detected as a luminescence signal, and the measurement of the luminescence signal is performed optically, thus enabling non-contact measurement. Alternatively or additionally, however, the at least one optically active spin center can generate an electrical signal from the excited state, and the measurement can also be performed electrically by measuring an electric current flowing through the material. This increases the variability and makes the measurement method adaptable to different circumstances.Since this latter variant of electrical measurement uses the spin-state-dependent photocurrent as the measured quantity, which results from the two-photon ionization of the spin center, a higher detection sensitivity compared to optical detection is also achieved.

[0010] Wave excitation can be achieved by subjecting at least one optically active spin center or the material to an acoustic wave, microwave radiation, and / or a spin wave (magnon). Depending on the measurement environment, this also results in a possible excitation in a variable manner.

[0011] It can be provided that the focus diameter of the electron beam has a value of 0.3 nm to 5 nm, preferably a value of 0.5 nm to 1.5 nm. These values ​​are, on the one hand, adjustable with reasonable effort, and on the other hand, already allow a significant increase in spatial resolution compared to previously used measurement methods. The theoretical limit here is an electron wavelength that is less than 0.3 nm for a cathode ray with an energy of 5 keV.

[0012] The electron beam can also be moved without problems during the measurement. In particular, it can be arranged that the electron beam is scanned across the material during the measurement.

[0013] The method can be carried out in particular without a cantilever, i.e. no cantilever is used, resulting in a significantly simplified measurement setup where only the electron beam is needed for excitation.

[0014] The material may in particular be selected from hexagonal boron nitride, silicon carbide and diamond, i.e. it may consist of the aforementioned materials or at least contain them.

[0015] A suitable measuring setup for determining the spin resonance frequency of a material with optically active spin centers comprises an electron beam source, a source, and a measuring device. The electron beam source is configured to emit a focused electron beam to excite at least one optically active spin center of the material to an excited state, whereby the optically active spin center subsequently returns to a lower energy state, typically the ground state, emitting a signal. In addition to the electron beam excitation, the source is configured to excite the material with a wave excitation. The source is also configured to vary the frequency of the wave excitation over time. The measuring device is configured to detect the signal and, based on this, to determine the resonance frequency of the at least one optically active spin center.In particular, the measuring device may be configured to determine a wave excitation frequency at which the detected signal exhibits an extremum, and based on this, to determine a resonance frequency of the at least one optically active spin center. The resonance frequency of the optically active center may, in particular, correspond to the wave excitation frequency at which the detected signal exhibits the extremum.

[0016] The described measuring arrangement is typically designed to carry out the described procedure, or the described procedure can be carried out with the described measuring arrangement.

[0017] The described method and / or measuring arrangement are generally used to measure an external parameter or an environmental parameter to which the material is exposed, preferably an (external) magnetic field, an (external) temperature and / or a pressure.

[0018] Exemplary embodiments of the invention are shown in the drawings and are described below with reference to the Fig. 1 and Fig. 2 explained.

[0019] They show: Fig. 1 a schematic view of a measurement setup and Fig. 2 exemplary measurement diagrams.

[0020] In Fig. Figure 1 shows a schematic side view of a measuring setup or arrangement. A sample 1, also referred to as a workpiece, is made of a material that has at least one optically active spin center 2, but usually a multitude of such centers 2. The sample 1 is held on a sample holder 10. Suitable materials include, in particular, 2D materials such as hexagonal boron nitride (hBN) or 3D materials such as silicon carbide (SiC) or diamond.

[0021] An electron beam source 3, positioned above the sample holder 10 and the sample 1, emits an electron beam 4 towards the sample 1. This beam can be focused, or is focused, onto a surface of the sample 1 or the spin center 2 by at least one electromagnetic lens (not shown for clarity) incorporating one or more magnets and / or one or more coils. The focus diameter is typically between 0.5 nm and 1.5 nm. The use of one or more electromagnetic lenses eliminates the need for moving parts and enables rapid two-dimensional scanning of the sample surface, for example, to determine the spatial magnetic field, temperature, and / or pressure distribution. The electron beam 4 can also scan the sample 1 to perform measurements at various points on the sample 1.

[0022] The electron beam 4 excites the spin center 2 to emit a luminescence signal 7 by cathodoluminescence, which is shown only as an example in Fig. The electron beam is shown in only one direction, but is generally emitted into the hemisphere above sample 1. In addition to the electron beam 4, sample 1 is also subjected to an acoustic wave 6, for example a microwave and / or a spin wave, from a source 5. Fig. For this purpose, the acoustic wave 6 is used. The source 5 can therefore be configured as a loudspeaker, microwave generator, spin wave source, or magnon source. The frequency ranges of the respective wave vary depending on the material and the strength of any applied magnetic field, but are typically in the range between 1 MHz and 10 GHz.

[0023] Typically, a control unit changes the frequency of the acoustic wave 6, usually from lower to higher frequencies or vice versa. At each frequency value, a measurement of the luminescence signal 7 is performed. The luminescence signal 7 is thereby measured in Fig. In the measuring arrangement shown in Figure 1, the electron beam is reflected by a parabolic mirror 9 towards a measuring device 8. The parabolic mirror 9 has a pass-through for the electron beam 4, resulting in a particularly compact measuring arrangement. An optical frequency filter 11, which can be configured as a high-pass, low-pass, or band-pass filter, may, but does not have to, be arranged in front of the measuring device 8. This filter allows only the frequency ranges of interest to pass through, thus spectrally selecting the signal of the spin center 2.

[0024] The measuring device 8, in turn, detects the intensity of the luminescence light or luminescence signal 7 at each frequency, and an extremum of the luminescence light 7, and accordingly a spin resonance, can be determined, typically by an evaluation unit that may be combined with the control unit and is usually in the form of a computer. The presence of this extremum is associated with a frequency that represents the spin resonance frequency. Generally, a cantilever is not used. The electron beam 4 is therefore primarily responsible for exciting the optically active spin center 2 and emitting the luminescence signal 7.

[0025] In Fig. Figure 1 shows an optical measurement; however, in further embodiments, the spin resonance frequency can also be determined by passing an electric current through sample 1 and measuring its current intensity as a function of the wave excitation frequency. In this case, the result would be an electrically detected magnetic resonance (EDMR) rather than an optically detected magnetic resonance (ODMR).

[0026] Since the spin resonance frequency depends on external parameters such as an applied external magnetic field, temperature, or ambient pressure, one or more of these parameters can be determined using the described measurement setup and a corresponding method. The present invention thus relates to a measurement setup and a measurement method in the field of quantum sensing for determining the spin resonance frequency and derived quantities of various materials with optically active spin centers by means of luminescence or fluorescence spectroscopy, and to a measurement setup and a measurement method for determining quantities derived from it (i.e., from the spin resonance) or environmental parameters such as magnetic field, temperature, and / or pressure.

[0027] In Fig. Figure 2 shows exemplary measurement spectra obtained using the described method under various external magnetic fields, employing a microwave for excitation. The microwave was guided from a microwave generator to sample 1 via a waveguide, which served as the antenna. The frequency was varied in the range of 10 MHz to 10 GHz. When the irradiated microwave frequency coincides with the spin resonance frequency of spin center 2 or spin centers 2, the intensity of the luminescence signal 7 changes. Accordingly, the diagrams of the Fig. 2. On the abscissa, the microwave frequency in Hz and on the ordinate, the ratio ΔPL / PL, i.e., the ratio of a change in the photoluminescence signal (ΔPL) to the luminescence signal (PL), in percent, is plotted.

[0028] As from Fig.As can be seen in section 2, the spectrum changes depending on the external magnetic field. While the upper spectrum was recorded at B=0 mT, the middle spectrum was recorded at B=6 mT and the lower spectrum at B=15 mT (where the magnetic field is perpendicular to the sample plane and the component B is accordingly higher). z (is specified). The local magnetic field B can be calculated directly from the frequency difference between the signal maxima, which is given by 2yB, where y=28 MHz / mT is the gyromagnetic ratio.

[0029] The described measurement method can also be carried out in a wide temperature range from room temperature (300 K) to helium temperatures (4 K).

[0030] In electrically detected magnetic resonance (EDMR) measurements, the spin resonance frequency is detected by measuring an electrical signal. This involves measuring a photocurrent in the material, for which electrical contacts made of conductive materials such as gold, silver, or graphite are applied to a sample surface. Excitation of spin center 2 results in changes in the electrical photocurrent and thus the electrical conductivity. The photocurrent itself is often very small (typically in the nanoampere or microampere range), which is why an operational amplifier or transimpedance amplifier is often necessary. Photocurrent measurement can, in principle, be performed with or without an applied magnetic field.

[0031] In summary, the described method involves the excitation of the optically active spin center 2 by the electron beam. Manipulation by wave excitation, in addition to the electron beam, can be achieved, for example, with microwaves, acoustic waves, or spin waves, i.e., magnons. Measurement or detection can be performed optically by detecting light emitted during the return from an excited state to a ground state. Alternatively, measurement can be performed electrically by detecting an electrical signal generated by the excited state.

Claims

[1] Method for determining a spin resonance frequency of a material (1) with optically active spin centers (2), in which at least one optically active spin center (2) of the material (1) is brought into an excited state by a focused electron beam (4) emitted from an electron beam source (3) and subsequently returns to a lower energy state by emitting a signal, and in addition to electron beam excitation from a source (5) the material (1) is subjected to wave excitation (6), wherein a frequency of the wave excitation (6) is changed over time and a frequency of the wave excitation (6) at which the signal has an extremum is determined by a measuring device (8) by measuring the signal, and based on this a spin resonance frequency of the at least one optically active spin center (2) is identified, wherein the at least An optically active spin center (2) generates an electrical signal from the excited state, and the measurement is carried out as an electrical measurement of an electric current flowing through the material (1). [2] Method according to claim 1, characterized by , that the at least one optically active spin center (2) emits a luminescence signal (7) as the signal by means of electron beam excitation and the measurement of the luminescence signal (7) is carried out as an optical measurement. [3] Method according to claim 1 or claim 2, characterized by , that the wave excitation (6) is carried out by applying an acoustic wave, a microwave radiation and / or a spin wave. [4] Method according to any one of the preceding claims, characterized by , that the focus diameter of the electron beam (4) is between 0.3-5 nm, preferably between 0.5-1.5 nm. [5] Method according to any one of the preceding claims, characterized by , that the electron beam (4) is guided scanning over the material (1) during the measurement. [6] Method according to any one of the preceding claims, characterized by that the procedure is carried out without a cantilever. [7] Method according to any one of the preceding claims, characterized by , that the material (1) is selected from hexagonal boron nitride, silicon carbide and diamond. [8] Measuring setup for determining a spin resonance frequency of a material (1) with optically active spin centers (2), with an electron beam source (3) configured to emit a focused electron beam (4) to convert at least one optically active spin center (2) of the material (1) into an excited state, wherein the optically active spin center (2) subsequently returns to a lower energy state by emitting a signal , with a source (5) which is configured to subject the material (1) to wave excitation (6) in addition to electron beam excitation, and to change the frequency of the wave excitation (6) over time, and with a measuring device (8) which is configured to detect the signal and to determine a frequency of the wave excitation (6) at which the signal has an extremum, and based on this to determine a spin resonance frequency of the at least one optically active spin center (2), wherein that at least one optically active spin center (2) generates an electrical signal from the excited state and the measurement is carried out as an electrical measurement of an electric current flowing through the material (1). [9] Use of a method according to any one of claims 1-7 and / or a device according to claim 8 for measuring an external parameter, in particular a magnetic field, a temperature and / or a pressure.

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