Field Modulation Technique Research Articles

Microwave technique for measurements of electron mobility in p-Cd xHg 1- xTe

A simple and practical microwave method for determining the electron mobility in p-type Cd x Hg 1- x Te ( x ≈ 0.2) at liquid nitrogen temperatures is described. The method relies on measurements of the microwave reflectivity ( f = 36 GHz) from the sample covering the waveguide and magnetic field modulation technique which permits one to detect a magnetoreflectance signal from a small number of electrons. The mobility of the electrons is determined from the maximum of the reflectance derivative d | γ | /d B using the simple formul μ = 1 / √3 B max.

New methods to measure the current perpendicular to the plane magnetoresistance of multilayers

The experimental devices presented here enable us to measure variations with magnetic field of electrical resistances in the range (0.1 nΩ, 1 μΩ), for applied magnetic fields up to 3 T, and for temperatures less than 9 K. The setups work with three measurement methods. Two provide direct access to R(H), one in constant current, the other in alternating current. Their absolute sensitivity is about 1 pΩ, and their relative resolution ΔR/R is limited to 0.3%. The third method is a sinusoidal field modulation technique giving the field derivative of the resistance: dR/dH. It is sensitive to variations of the resistance as small as 1 pΩ independent of the absolute resistance of the sample, thus the resulting resolution ΔR/R is, for example, 10−6 for a 1 μΩ sample. With these devices, the magnetoresistance of multilayers can be studied with the current perpendicular to the plane.

Optically-detected magnetic field effects on the Dl-D2-cyt b-559 complex of Photosystem II. Temperature dependence of kinetics and structure

The magnetic field effect (MFE) on photosynthetic reactions in the Dl-D2 reaction centre complex was studied using a new field modulation technique, and monitoring the absorbance from 1.2 to 200 K. The MFE recorded at low temperature (below 60 K) showed several features that cannot be due to the intrinsic properties of the primary donor triplet only. The temperature dependence of the amplitude of the MFE was simulated using a thermally activated charge-recombination rate from the radical-pair singlet state to the ground state and thermally activated charge recombination to the excited primary donor state. Due to repeated re-population of the excited state at high temperatures, triplet formation by intersystem crossing becomes important. The simulation yields upper limits for the activation energies of the two recombination processes of 26 meV and 70 meV for recombination to the singlet ground and excited state, respectively. From a Gaussian deconvolution of magnetic field-induced Triplet-minus-Singlet (T − S) spectra recorded at different temperatures in the range of 1.2 to 200 K, it is concluded that the conformation of one of the main components of the T − S spectra is transformed between 50 K and 70 K. Furthermore, going on to higher temperatures, an additional bleaching appears centered at 674.5 nm. The additional component could be due to delocalisation of the primary donor triplet. In the T − S spectra recorded above 50 K, also a bleaching at 545 nm was observed. The formation of the triplet giving rise to this bleaching (probably a pheophytin triplet) is related to triplet formation by the radical pair mechanism.

Frequency modulated non-resonant r.f. and microwave absorption in high-T c superconductors

It is shown that frequency modulation combined with phase sensitive detection provides a better (than magnetic field modulation) technique of recording intrinsic low-field dependent r.f./microwave response of the high- T c superconductors.

New frequency-domain electric birefringence spectrometer using an advanced digital lock-in technique

We have developed a frequency-domain electric birefringence spectrometer to measure stationary birefringence Δndc and alternating one Δn2ω induced by an applied sinusoidal electric field with an angular frequency ω. The new spectrometer is based on (1) the optimization of birefringence detection S/N ratio by taking into account the effects of optical noise, shot noise, optical leakage, and parasitic birefringence, (2) the electric field modulation technique, and (3) the advanced digital lock-in technique for further S/N enhancement. The electric field amplitude modulation at an angular frequency Ω shifts Δndc and Δn2ω signals to locate Ω and 2ω±Ω, which are demodulated with sinusoidal reference waves at Ω and 2ω and retrieved by digital data accumulation, i.e., the advanced digital lock-in technique. These techniques ensure the signal detection with extremely narrow bandwidth by removing the adverse effects of odd harmonic noises, drift, and 1/f noise in the measurement system. The detection sensitivity of 10−9 rad in a wide range of ω is accessible.

Low field transport studies of HgTe/CdTe superlattices: Quantum interference effects

Low field transport studies of HgTe/CdTe superlattices grown by laser assisted molecular beam epitaxy show weak localization effects for H < 30 Oe and T < 30 K. We have observed small amplitude fluctuations (d R/ R ∼ 10 −5) which are superimposed on the anti-weak localization background. Field modulation techniques have been used to enhance the structure and study its temperature and field dependence. We suggest that these small amplitude fluctuations in our macroscopic sample (∼4 mm 2) originate from quantum interference effects which are enhanced by the relatively large coherence length in this type III superlattice system.

Magnetophonon resonance in semiconductors

The study of the magnetophonon resonance effect in semiconductors carried out in Osaka University is reviewed, covering theories, experimental techniques and results obtained. Experimental techniques are briefly described, where emphasis is placed on the magnetic field modulation technique. The effective mass is determined in III–V semiconductors at a wide range of lattice temperatures and its temperature dependence is discussed. Magnetophonon resonance associated with impurity state, zone-edge phonons, and intervalley phonons are also reviewed. At high electric and high magnetic fields, the magnetophonon resonance extrema in the magnetoconductivity exhibit splitting, which is shown to be interpreted in terms of the inelastic inter-Landau level scattering. Magnetophonon resonance of two-dimensional electron gas in heterojunctions is discussed and the sheet electron density dependence of the magnetophonon resonance amplitudes is explained in terms of the Landau level broadening due to remote impurity scattering.

Frequency dependence of the ferromagnetic resonance linewidth of barium ferrite

Ferromagnetic resonance was measured in both a swept frequency mode of operation, in which the magnetic field was fixed, and a field swept mode, using field modulation techniques. Single-crystal spheres of 0.381, 0.305, and 0.483 mm in diameter were inserted in the waveguide and transmission was observed in the measurement. The g values for all the spheres averaged to 2.052±0.011 and the uniaxial anisotropy field was 16.4 kOe.Our measurements show that the linewidth of barium ferrite is nearly independent of resonant frequency from 48 to 105 GHz. This is in disagreement with the Kasuya–LeCraw two-magnon–one-phonon mechanism, which would predict a linewidth linear with the resonant frequency. Previous measurements [J. Magn. Magn. Mater. 54-57, 1141 (1986); IEEE Trans. Magn. MAG-22, 984 (1986)], however, show a strong temperature dependence, which rules out magnon scattering from static defects as the primary contributor to the intrinsic linewidth. These results are consistent with a single scattering process in which the uniform precession magnon is scattered into the continuum of magnons by time-dependent fluctuations of the trigonal symmetry ion between two equilibrium sites on either side of the mirror plane [J. Appl. Phys. 63, 3350 (1988)]. This process differs from the usual two-magnon scattering in that the scattering mechanism is time dependent, and, therefore, the magnon frequency need not be conserved in the process.

Quantum interference effects in the weak localisation regime in HgTe/CdTe superlattices grown by laser-assisted molecular beam epitaxy

Low-field transport studies of HgTe/CdTe superlattices grown by laser-assisted molecular beam epitaxy show weak localisation effects for H<30 Oe and T<30 K. The authors have observed small-amplitude fluctuations (dR/R approximately 10-5) which are superimposed on the anti-weak localisation background. Field modulation techniques have been used to enhance the structure and study its temperature and field dependence. They suggest that these small-amplitude fluctuations in their macroscopic sample ( approximately 4 mm2) originate from quantum interference effects which are enhanced by the relatively large coherence length in this type III superlattice system.

Shubnikov–de Haas effect in thin epitaxial films of gray tin

The transverse magnetoresistance and Hall effect have been studied for n-type gray tin epilayers grown on (001)CdTe substrates by the molecular beam epitaxy technique. Shubnikov–de Haas oscillations were observed in samples having Hall mobilities ≥104 cm2 /V s at low temperatures. Measurements were carried out using both the dc method and field modulation techniques in the temperature range from 1.2 to 10 K and in magnetic fields up to 10 T. Beat patterns were observed in the Shubnikov–de Haas spectra which we ascribe either to inhomogeneous doping, arising from the diffusion of Cd and Te from the substrate, or to quantization of the motion in the direction parallel to the film normal. The Shubnikov–de Haas carrier concentration of a 1210 Å film was determined to be nSdH =2.3×1017 cm−3, in good agreement with the Hall density.

Magnetoresistance in YBa 2Cu 3O 7-δ above Tc: Superconducting fluctuations and strong pair-breaking

Magnetoresistance above T c is studied on a sintered sample of YBa 2Cu 3O 7-δ up to 4T using a field-modulation technique. The magnetoresistance Δϱ near T c diverges as ε −5 2 , with ε ¬ ( T - T c )/ T c , and is quantitatively explained by the 3D behavior of Aslamasov-Larkin (AL) term in superconducting fluctuations. From the analysis, ξ c(0) = 2.0 ± 0.5 A ̊ and ξ ab(0) = 16.4 ± 0.2 A ̊ are derived for the inter and the intra layer coherence lengths. From the analysis of Maki-Thompson (MT) term, τ L ≲ 3 × 10 −14 s is derived for the phase-breaking time τ L , which implies a strong pair-breaking; ħ/τ L ⩾ 2.7 k T c . Several consequences of the stron pair-breaking are discussed. Temperature independent magnetoresistance Δϱ/ϱ ≅ 7 × 10 −6/(Tesla) 2 is also observed at higher temperatures, and its possible connection to an intrinsic transport mechanism is suggested.

REMANENT EFFECTS AND GRANULAR JOSEPHSON TUNNELLING IN 1:2:3 MICRO-BRIDGE JUNCTIONS

A study has been made of supercurrent in a 1:2:3 micro-bridge or slot-junction. It is shown that the maximum supercurrent through a slot-junction is strongly dependent on temperature, applied magnetic field, and magnetic history. In addition, inter-granular Josephson tunnelling has been observed using field-modulation techniques.

Infrared diode laser spectroscopy of the ν2 fundamental band of SH 3+

The high-resolution absorption spectrum of the ν 2 fundamental band of SH 3 + has been observed with the magnetic field modulation technique. The ions were generated in a hollow cathode discharge through a mixture of H 2 and H 2S. The molecular constants in the excited vibrational state are determined through a least-squares fit of the data to a standard symmetric top energy level expression. The anomalous sign of the centrifugal distortion constants is explained by the Coriolis interaction between the v 2 = 1 and v 4 = 1 states.

Shubnikov-de Haas Oscillations in CdSnAs2 Obseved by Magnetic Field-Modulation Technique

By use of the magnetic field-modulation technique, Shubnikov-de Haas oscillations have been observed in single crystals of CdSnAs 2 at temperatures between 4.5 and 31.0 K. The oscillations show clear beats, resulting from two components of oscillatory structures, and the Fourier transformation reveals an existence of two different kinds of carriers in the conduction bands, of which densities are n a =1.6×10 18 and n b =1.9×10 18 cm -3 . The effective electron masses have also been estimated from the temperature dependence of the oscillations as \(m_{a}^{*}{\cong}m_{b}^{*}{=}(0.040{\pm}0.025)m_{o}\).

Acoustic de Haas-van Alphen Effect of CeB6 and LaB6

All of the main orbits of LaB6 have been observed successfully by the ultrasonic measurement of dHvA oscillations of elastic constants without field modulation technique, where the high resolution important. Adopting the ultrasonic technique in a dilution refrigerator, the acoustic dHvA oscillation of CeB6 is obtained for the heavy fermion with effective cyclotron mass ∼7.2m0 of the extremal orbit 0.012 A-2(1.26MG)

Diode laser spectroscopy of the ν4 (HCN bend) band of HCNH +

The ν 4 (HCN bend) band of the HCNH + ion has been observed by diode laser spectroscopy using a magnetic field modulation technique. Twenty-three Q-, P-, and R-branch lines were observed and analyzed to yield the rotational, centrifugal distortion, and l-type doubling constants for the ν 4 state. The band origin was determined to be 801.5934(6) cm −1.

Pressure dependence of the hole Fermi surface anisotropy in antimony

The anisotropy of the hydrostatic pressure derivative of the cross-sectional area of the hole Fermi surface (FS) of pure antimony was determined at five different pressures up to 4.2 kbar by means of the de Haas-van Alphen (dHvA) effect. An extensive coverage of the hole FS was made possible by using the field modulation technique modified through utilization of the vector directions of the modulation field and the oscillatory magnetization. Appreciable changes in magnitude and sign of the pressure derivative of the FS area with crystal orientation were observed. The pressure dependence of the departure of the FS from a perfect ellipsoid was determined using the ellipsoidal approximation. The departure under pressure exceeds its value at standard atmospheric pressure. The tilt angle of the FS is independent of pressure within quoted experimental errors. The hole carrier concentration as a function of pressure was estimated by employing the ellipsoidal-parabolic approximation with measurements of the pressure dependence of FS areas.

Infrared diode laser study of the hydrogen bifluoride anion: FHF− and FDF−

The ν3 vibration–rotation transition of the hydrogen bifluoride anion in the X̃ 1Σ+g state has been detected for the first time by infrared diode laser spectroscopy using the magnetic field modulation technique. The anion was generated by a hollow cathode discharge in a mixture of H2 and a fluorine-contained molecule such as CF4, C2F4, and CHF3. The carbon compound was indispensable to produce the anion. Identification of the species was based on the spectral pattern showing intensity alternation, the magnitude of the rotational constant (B″=0.334 181 cm−1), and the ion drift velocity sign determined by the velocity modulation method. The observed ν3 frequencies of FHF−(1848.699 cm−1) and FDF−(1397.236 cm−1) are much different from the reported values in the solid phase. By contrast, the ν1 and ν2 frequencies estimated from the centrifugal distortion constant and the perturbation of Coriolis interaction between the ν1+ν2 and ν3 states are in good agreement with solid state measurements.

Magnetophonon Resonance in InxGa1-xAs (x=0.53)

Magnetophonon resonance effect of the transverse magnetoresistance is studied in InxGa1-xAs (x=0.53) using a magnetic field modulation technique at temperatures from 40 to 300 K. The resonance arises from GaAs-like phonons at higher temperatures and two mode resonance due to GaAs-like phonons and InAs-like phonons is observed at low temperatures. Band edge effective mass is determined to be 0.0409m0 at 77 K and 0.0388m0 at 300 K.

Magnetic Field Modulation Method for Measurements of Magnetophonon Effect

The first and the second derivatives of the transverse magnetoresistance are measured by the magnetic field modulation technique and are compared with each other. The results show that the second derivative is easily affected by the second harmonic component of the modulation magnetic field and that the peaks of the second derivative at high magnetic fields will not give accurate resonance positions. To avoid such effect and to measure the magnetophonon effect accurately, the first derivative method of the magnetoresistance is prorosed for the magnetophonon measurements as the best method. It is also shown that the oscillatory component of the magnetophonon resonance can be obtained by numerical integration of the first derivative signals. The effective mass of the conduction band edge is estimated to be 0.0664±0.0005 m for n-GaAs at 160 K and 0.0135±0.0002 m for n-InSb at 77 K.