Recent Advances in High-Frequency Electron Spin Resonance Detection Using a Microcantilever

Our recent developments in highly sensitive high-frequency electron spin resonance (ESR) using a microcantilever are reviewed. ESR signals of a Co Tutton salt microcrystal (<1 μg) have been detected at low temperature at frequencies up to 315 GHz under a static magnetic field using a microcantilever and a modulation technique. The achieved sensitivity is about 10⁹ spins/G at 4.5 K. Moreover, we have shown that similar ESR detection using a microcantilever is possible up to 130 GHz under a pulsed magnetic field without using a modulation technique. The achieved sensitivity is about 10¹¹ spins/G at 1.7 K. These results suggest that the ESR detection using a microcantilever is promising for applications to high-resolution and high-sensitivity terahertz ESR.     

Study of phase transition on nanocrystalline (La, Sr)(Mn, Fe)O system using high-pressure Mössbauer spectroscopy and high-pressure X-ray diffraction

Pressure-induced structural changes on nano-crystalline La_0.8Sr_0.2Mn_0.8Fe_0.2O₃ were studied using high-pressure Mössbauer spectroscopy and high-pressure X-ray diffraction. Mössbauer measurements up to 10 GPa showed first order transition at 0.52 GPa indicating transformation of Fe^4?+? to high spin Fe^3?+?, followed by another subtle transition at 3.7 GPa due to the convergence of two different configurations of Fe into one. High-pressure X-ray diffraction measurements carried up to 4.3 GPa showed similar results at 0.6 GPa as well as 3.6 GPa. Attempts were made to explain the changes at 0.6 GPa by reorientation of grain/grain boundaries due to uniaxial stress generated on the application of pressure. Similarly variation at 3.6 GPa can be explained by orthorhombic to monoclinic transition.     

A low-temperature high-pressure apparatus with a temperature control system

Abstract

A low-temperature high-pressure apparatus was designed using commercial cryogenic equipment. Pressures up to 1 GPa and temperatures down to 40 K can be obtained in a volume of up to 30 cm³. The apparatus is of the piston-cylinder type with a piston diameter of 45 mm, and the pressure can be varied at all temperatures, An adaptive temperature control system keeps the temperature inside the pressure cylinder constant to within ±0.1 K.     

Development of Hybrid-Type Pressure Cell for High-Pressure and High-Field ESR Measurement

A hybrid-type piston-cylinder pressure cell for the electron spin resonance (ESR) measurement has been developed. The cylinder of this pressure cell consists of a NiCrAl inner cylinder and a CuBe outer sleeve, and all inner parts are made of zirconium oxide which has good transmittance to the millimeter and submillimeter waves. We confirmed that the pressure reaches 2.1 GPa. We have also developed a transmission-type high-field ESR system having two different modulation methods for this pressure cell. A test measurement without pressure cell for the two-dimensional orthogonal-dimer spin system of SrCu₂(BO₃)₂ has been done successfully in the wide frequency region. The combination of this electromagnetic wave transmission-type pressure cell and this high-field ESR system is a promising tool for the study of the pressure-induced phase transition of SrCu₂(BO₃)₂.     

FEM study on a double-beveled anvil and its application to synthetic diamonds

A new double-beveled anvil for the synthesis of high-quality diamonds has been described, which is used in a China-type large-volume, cubic-anvil, high-pressure apparatus (LV-CHPA, SPD-6X2000). Our results indicate that the pressure generation of a double-beveled anvil is more efficient than that of a single-beveled anvil. To gain the same cell pressure (5.5 GPa), the oil pressure of LV-CHPA using double-beveled anvils decreased by about 10%, compared to using single-beveled anvils. Furthermore, a double-beveled anvil can pressurize a cubic cell of 36 mm³ up to about 6.0 GPa, and simultaneously can increase the temperature up to 1360°C for routine operation. This provides considerable advantages to the synthesis of high-quality diamonds under ultra-high-pressure conditions with the same hydraulic rams.     

Continuous-wave far-infrared ESR spectrometer for high-pressure measurements

We present a newly-developed microwave probe for performing sensitive high-field/multi-frequency electron spin resonance (ESR) measurements under high hydrostatic pressures. The system consists of a BeCu-made pressure-resistant vessel, which accommodates the investigated sample and a diamond microwave coupling window. The probe’s interior is completely filled with a pressure-transmitting fluid. The setup operates in reflection mode and can easily be assembled with a standard oversized microwave circuitry. The probe-head withstands hydrostatic pressures up to 1.6 GPa and interfaces with our home-built quasi-optical high-field ESR facility, operating in a millimeter/submillimeter frequency range of 105–420 GHz and in magnetic fields up to 16 T. The overall performance of the probe was tested, while studying the pressure-induced changes in the spin-relaxation mechanisms of a quasi-1D conducting polymer, KC₆₀. The preliminary measurements revealed that the probe yields similar signal-to-noise ratio to that of commercially available low-frequency ESR spectrometers. Moreover, by observing the conduction electron spin resonance (CESR) linewidth broadening for KC₆₀ in an unprecedented microwave frequency range of 210–420 GHz and in the pressure range of up to 1.6 GPa, we demonstrate that a combination of high-pressure ESR probe and high-field/multi-frequency spectrometer allows us to measure the spin relaxation rates in conducting spin systems, like the quasi-1D conductor, KC₆₀.     

Electron Spin Resonance Imaging Probes and a Sample Holder for Thin and Long One-Dimensional Samples

Most electron spin resonance (ESR) experiments involve the use of samples that can be easily placed in millimeter-size tubes and measured efficiently in conventional resonators. However, in some cases, the samples must remain intact, due to which conventional commercial resonators may not be suitable to measure them. Here, we describe a set of three resonators, which can be combined and incorporated as part of a 1-D continuous wave ESR imaging probe to measure and image very thin (~50–500 μm) and very long (~10–30 mm) objects. The dielectric resonators we employ make it possible to greatly enhance spin sensitivity per unit of length––compared to the use of a rectangular ESR cavity, at ~9.3 GHz. In addition, a special sample holder was developed to facilitate the handling and measurement of such thin and long delicate objects, which in our case are the Arabidopsis roots. A detailed design of the resonators, imaging probe, and the sample holder is provided, along with experimental results for the resonator properties, its spin sensitivity, and imaging capability.     

High-pressure study of NaAlH4 by Raman spectroscopy up to 17 GPa

A high-pressure Raman study was carried out on NaAlH₄ up to 17 GPa using the diamond anvil cell method. In the pressure region 2–5 GPa, several of the original modes split. Although this might be a sign of some structural change, the spectral changes do not allow us to claim the existence of a clear phase transition in this pressure range. The spectra revert to their ambient pressure forms on decreasing pressure below<3.0–1.4 GPa. A phase transition to β-NaAlH₄ was found at 14–16 GPa. This phase transition is also reversible with an unusually strong hysteresis: the β-NaAlH₄ can be followed upon decompression down to 3.9 GPa. Analysis of Raman data shows that this phase transition is compatible with a theoretical prediction of a strong volume collapse.     

Room Temperature High-Field Spin Dynamics of NV Defects in Sintered Diamonds

Sintered oriented nanodiamond arrays with the extremely high concentrations of the nitrogen-vacancy (NV) centers (up to 10³ ppm) were investigated by the W-band (94 GHz) electron spin echo electron paramagnetic resonance techniques. The NV centers were fabricated by the high-pressure high-temperature sintering of detonation nanodiamonds (DND) without the post or prior irradiation of the samples. The processes of polarization and recovery of the equilibrium population of the spin sublevels by optical and microwave pulses have been examined at room temperature in high magnetic fields corresponding to the fine-structure transitions for the NV defects at 94 GHz (3,250–3,450 mT). A long spin coherence time of 1.6 μs and spin–lattice relaxation time of 1.7 ms were measured. The results were compared with those obtained on the NV centers fabricated by the irradiation and subsequent annealing of the commercially available bulk diamonds. It was shown that the relaxation characteristics of the NV defects were similar in the both types of the samples despite the extremely high concentrations of NV defects and isolated nitrogen donors in the sintered DND.     

Ab initio study of the structural,electronic, elastic and thermal conductivity properties of SrClF with pressure effects

Abstract

SrClF is an important optical crystal and can be used as pressure gauge in diamond anvil cell at high pressure. In this work, we performed a systematic study on the structural, electronic and elastic properties of SrClF under pressure, as well as its thermal conductivity, by first-principles calculation. Different exchange-correlation functionals were tested and PBESOL was finally chosen to study these properties of SrClF. Studies reveal that SrClF has a bulk modulus of about 56.2 GPa (by fitting equation of states) or 54.3 GPa (derived from elastic constants), which agree well with the experimental result. SrClF is mechanically and dynamically stable up to 50 GPa. Its elastic constants increase with the applied pressure, but its mechanical anisotropy deteriorates as the pressure increases. Investigation of its electronic properties reveals that SrClF is a direct band-gap insulator with a gap value of 5.73 eV at 0 GPa, which decreases with the increasing pressure and the reason is found by analysing the partial density of states. Based on the calculated phonon dispersion curves, thermal conductivity of SrClF is predicated. At ambient conditions, the predicted thermal conductivity is about 3.74 Wm^?1 K^?1, while that obtained using the simplified Slack model give a slightly larger value of 4.62 Wm^?1 K^?1.     

Multipurpose High-Frequency ESR Spectrometer for Condensed Matter Research

We describe a multifrequency quasi-optical electron spin resonance (ESR) spectrometer operating in the 75–225 GHz range and optimized at 222.4 GHz for general use in condensed matter physics and chemistry. The quasi-optical bridge detects the change of millimeter wave polarization at the ESR condition. A controllable reference arm maintains a millimeter wave bias at the detector. The sensitivity of 2 × 10¹⁰ spin/(G Hz^0.5), measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4 GHz and 300 K, is comparable to that of commercial high-sensitive X-band spectrometers. The spectrometer is robust, easy to use and may be operated by undergraduate students. Its performance is demonstrated by examples from various fields of condensed matter physics.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Small plastic piston-cylinder cell for pulsed magnetic field studies at cryogenic temperatures

A plastic piston-cylinder cell based on a thick wall test-tube has been designed for pulsed magnetic field studies. The small 12.7 mm diameter and overall height of 19.3 mm allow the cell to freely rotate in a cryostat with a diameter of 21.5 mm. Electrical leads, coax cable or microstrip transmission lines can be introduced into the pressure chamber for a variety of measurements such as electrical transport, de Haas–van Alphen, Shubnikov–de Haas and Hall effect. A fiber optic has been introduced for the purpose of calibrating the pressure via a ruby manometer. The fiber optic opens up additional experimental techniques such as photoluminescence, photoconductivity and, with use of a special fiber with a Bragg grating, magnetostriction and thermal expansion. Maximum pressures of 0.35 GPa at room temperature have been obtained.     

Phase transition of ZnS at high pressures and temperatures

The high-pressure behaviour of zinc sulphide, ZnS, has been investigated, using an in situ X-ray powder diffraction technique in a diamond anvil cell, at pressures and temperatures up to 35 GPa and 1000 K, respectively. The pressure-induced phase transition from a zincblende (B3) to a rocksalt (B1) structure was observed. This transition occurred at 13.4 GPa and at room temperature, and a negative dependence on temperature for this transition was confirmed. The transition boundary was determined to be P (GPa) = 14.4 ? 0.0033 × T (K).     

Variable Coupling Scheme for High-Frequency Electron Spin Resonance Resonators Using Asymmetric Meshes

The sensitivity of a high-frequency electron spin resonance (ESR) spectrometer depends strongly on the structure used to couple the incident millimeter wave to the sample that generates the ESR signal. Subsequent coupling of the ESR signal to the detection arm of the spectrometer is also a crucial consideration for achieving high spectrometer sensitivity. In previous work, we found that a means for continuously varying the coupling was necessary for attaining high sensitivity reliably and reproducibly. We report here on a novel asymmetric mesh structure that achieves continuously variable coupling by rotating the mesh in its own plane about the millimeter-wave transmission-line optical axis. We quantify the performance of this device with nitroxide spin label spectra in both a lossy aqueous solution and a low-loss solid-state system. These two systems have very different coupling requirements and are representative of the range of coupling achievable with this technique. Lossy systems, in particular, are a demanding test of the achievable sensitivity and allow us to assess the suitability of this approach for applying high-frequency ESR, e.g., to the study of biological systems at physiological conditions. The variable coupling technique reported on here allows us to readily achieve a factor of ca. 7 improvement in the signal-to-noise ratio at 170 GHz and a factor of ca. 5 at 95 GHz over what has previously been reported for lossy samples.     

Structural and electrical transport properties of FeH x under high pressures and low temperatures

We present the first successful in situ simultaneous measurement of the electrical resistance and X-ray diffraction of FeH _x (x～ 1) under high-pressure H₂ up to 25.5 GPa and low temperatures down to 9 K. The electrical resistivity ρ showed a sharp increase with the formation of iron-hydride FeH _x (x～ 1) at 3.5 GPa. The ?′-phase of FeH _x was found to be metallic up to 25.5 GPa. The ρ vs. T curves up to 16.5 GPa approximately follow Fermi-liquid law below 25 K. However, T ⁵ was found to be better fitting at 25.5 GPa. This change can be considered to be related to the previously reported ferromagnetism collapse at corresponding pressure.     

Diamond anvil cell syntheses and compressibility studies of the spinel-structured gallium oxonitride

The formation conditions of cubic spinel-structured gallium oxonitride have been investigated in situ under high-pressure/high-temperature conditions using a laser-heated diamond anvil cell. As starting materials, a mixture of the end members w-GaN/β-Ga₂O₃ in a molar ratio of 3:2 and a gallium oxonitride ceramic derived during pyrolysis from the metallo-organic precursor (Ga(O^tBu)₂NMe₂)₂ were used. In the mixture of the end members, spinel-structured gallium oxonitride starts crystallizing at a pressure of 3 GPa and at a temperature of about 1300 °C. The precursor-derived ceramic with predefined bondings reacted completely to the spinel phase, without by-products, at a pressure of 0.7 GPa. For the spinel-structured gallium oxonitride we determined a bulk modulus K of 216(7) GPa using a fixed value of 4 for K′. The spinel-structured gallium oxonitride exhibits a cell volume of 552.9(5) ų at ambient pressure.     

Study of partial melting at high-pressure using in situ X-ray diffraction

The high-pressure melting behavior of different iron alloys was investigated using the classical synchrotron-based in situ X-ray diffraction techniques. As they offer specific advantages and disadvantages, both energy-dispersive (EDX) and angle-dispersive (ADX) X-ray diffraction methods were performed at the BL04B1 beamline of SPring8 (Japan) and at the ID27-30 beamline of the ESRF (France), respectively. High-pressure vessels and pressure ranges investigated include the Paris–Edinburgh press from 2 to 17 GPa, the SPEED-1500 multi-anvil press from 10 to 27 GPa, and the laser-heated diamond anvil cell from 15 to 60 GPa. The onset of melting (at the solidus or eutectic temperature) can be easily detected using EDX because the grains start to rotate relative to the X-ray beam, which provokes rapid and drastic changes with time of the peak growth rate. Then, the degree of melting can be determined, using both EDX and ADX, from the intensity of diffuse X-ray scattering characteristic of the liquid phase. This diffuse contribution can be easily differentiated from the Compton diffusion of the pressure medium because they have different shapes in the diffraction patterns. Information about the composition and/or about the structure of the liquid phase can then be extracted from the shape of the diffuse X-ray scattering.     

Powder neutron diffraction of wüstite (Fe0.93O) to 12 GPa using large moissanite anvils

High-pressure powder neutron diffraction of wüstite-Fe_0.93O has been achieved to 12 GPa using a large gem-moissanite (SiC) anvil cell. The moissanite anvils are weakly absorbing and provide greater neutron fluxes to the sample than is possible with tungsten carbide anvils. There is minimal diffraction overlap from the single-crystal moissanite anvils compared to tungsten carbide or synthetic diamond anvils, providing cleaner background profiles. The required sample volume for high-pressure neutron diffraction is dramatically reduced to several cubic millimeters. High-quality powder diffraction patterns of wüstite were recorded at 90 min exposure times on the HIPPO diffractometer at LANSCE when the sample volume was in the range of ～10 mm³. This is about two orders of magnitude smaller than the necessary sample volume (～1.0 cm³) for the same kind of experiment with other high-pressure cells and nominal neutron fluxes.