Magnetic Field Shift Research Articles

Spin mixing between subbands and extraordinary Landau-level shift in wide HgTe quantum wells

We present both the experimental and theoretical investigation of a non-trivial electron Landau levels shift in magnetic field in wide ~20 nm HgTe quantum wells: Landau levels split under magnetic fields but become degenerate again when magnetic field increases. We reproduced this behavior qualitatively within an isotropic 6-band Kane model, then using semiclassical calculations we showed this behavior is due to the mixing of the conduction band with total spin 3/2 with the next well subband with spin 1/2 which reduces the average vertical spin from 3/2 to around 1. This change of the average spin changes the Berry phase explaining the evolution of Landau levels under magnetic field.

Sensitivity of Tc to pressure and magnetic field in the cuprate superconductor YBa2Cu3Oy : Evidence of charge-order suppression by pressure

Cuprate superconductors have a universal tendency to form charge density-wave (CDW) order which competes with superconductivity and is strongest at a doping $p \simeq 0.12$. Here we show that in the archetypal cuprate YBa$_{2}$Cu$_{3}$O$_{y}$ (YBCO) pressure suppresses charge order, but does not affect the pseudogap phase. This is based on transport measurements under pressure, which reveal that the onset of the pseudogap at $T^*$ is independent of pressure, while the negative Hall effect, a clear signature of CDW order in YBCO, is suppressed by pressure. We also find that pressure and magnetic field shift the superconducting transition temperature $T_{\rm c}$ of YBCO in the same way as a function of doping - but in opposite directions - and most effectively at $p \simeq 0.12$. This shows that the competition between superconductivity and CDW order can be tuned in two ways, either by suppressing superconductivity with field or suppressing CDW order by pressure. Based on existing high-pressure data and our own work, we observe that when CDW order is fully suppressed at high pressure, the so-called "1/8 anomaly" in the superconducting dome vanishes, revealing a smooth $T_{\rm c}$ dome which now peaks at $p \simeq 0.13$. We propose that this $T_{\rm c}$ dome is shaped by the competing effects of the pseudogap phase below its critical point $p^{\star} \sim 0.19$ and spin order at low doping.

Microstructural, Optical, and Magnetic Properties of Vanadium-Substituted Nickel Spinel Nanoferrites

The current study investigates the impact of vanadium substitution on the structural, magnetic, and optical properties of NiFe2−xVxO4 (x ≤ 0.3) nanoparticles (NPs) produced by the cost-effective sol-gel route. The as-prepared spinel ceramic powders were examined by X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (DRS), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and vibrating sample magnetometry (VSM). The functional groups, spinel phase, and crystal structure were confirmed by XRD and FT-IR, respectively. The crystallites size decreased from 45.24 to 36.56 nm as the doping process increases. The plots of Tauc were drawn to determine optical band gap magnitudes of 1.291, 1.302, and 1.312 eV for x = 0.0, 0.2, and 0.3, respectively. The estimated saturation magnetization is maximum for pristine NiFe2O4 NPs and decreases to minimum for NiFe1.7V0.3O4 NPs. The σ-H hysteresis loops have finite coercivity (between 125 and 169 Oe) and retentivity (between 9.36 and 14.04 emu/g) values. The calculated σr/σs ratios are lower than 0.500, assigning the uniaxial anisotropy for NiFe2−xVxO4. The effective anisotropy constants (Keff) are in the range of 0.824 × 105 and 1.303 × 105 Erg/g. The magnetocrystalline anisotropy field (Ha) values are around 5.0 kOe. The characteristics of hysteresis (σ-H) curves and the order of magnetic data reveal the soft ferrimagnetic feature of as-prepared nanoparticle samples. From Mossbauer analysis, the variations in hyperfine magnetic field, quadrupole splitting, line width, and isomer shift have been evaluated. The distribution of cations showed that the octahedral B sites are occupied by all the ions of V3+. Mossbauer spectra are composed of four Zeeman sextets and one doublet.

Magnetic-field-induced crossover from the inverse Faraday effect to the optical orientation in EuTe

A time-resolved optical pump-probe technique has been applied for studying the ultrafast dynamics in the magnetic semiconductor EuTe near the absorption band gap. We show that application of external magnetic field up to 6 T results in crossover from the inverse Faraday effect taking place on the femtosecond time scale to the optical orientation phenomenon with an evolution in the picosecond time domain. We propose a model which includes both these processes, possessing different spectral and temporal properties. The circularly polarized optical pumping induces the electronic transition 4f75d0→4f65d1 forming the absorption band gap in EuTe. The observed crossover is related to a strong magnetic-field shift of the band gap in EuTe at low temperatures. It was found that manipulation of spin states on intrinsic defect levels takes place on a time scale of 19 ps in the applied magnetic field of 6 T.

Ferromagnetic Josephson junctions based on epitaxial NbN/Ni60Cu40/NbN trilayer

We fabricated and characterized a weakly coupled magnetic Josephson junction based on epitaxial NbN/Ni60Cu40/NbN trilayer heterostructures on single crystal MgO (100) substrates. X-ray diffraction and cross-sectional scanning transmission electron microscopy were used to verify the epitaxial growth of NbN/Ni60Cu40/NbN trilayer, while the ferromagnetic properties of NiCu on NbN film were recorded by plotting the magnetization as a function of both the temperature and the magnetic field strength. The NbN/Ni60Cu40/NbN junctions demonstrated typical Josephson effect with a nonlinear overdamped current–voltage characteristic and a characteristic voltage of 9.1 μV. The magnetic field dependence of the junction critical current showed a nearly ideal Fraunhofer-like pattern with a magnetic field shift of about 12 Oe due to the remanent magnetization of Ni60Cu40 layer.

Ce-Y co-substituted strontium nanohexaferrites: AC susceptibility and Mossbauer studies

Ce-Y co-substituted SrFe12O19, SrYxCexFe12–2xO19 (0.0 ≤ x ≤ 0.5), nanohexaferrites were synthesized via citrate sol-gel combustion route. X–ray diffractometry (XRD), Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), Energy dispersive spectroscope (EDXS) and Elemental mapping were used to study the structure, composition and morphology of the products. Examination through XRD confirms the formation of a pure M phase for all products. Hexagonal plate-like morphology was observed for all samples. EDX and elemental mapping analyses also validated the absence of impurity in the products. The variations in the values of line width, hyperfine magnetic field, quadrupole splitting and isomer shift on Ce and Y substitutions have been investigated through the data of 57Fe Mossbauer spectroscopy. The Mossbauer study indicated that Ce3+ and Y3+ ions are preferentially entering the 4f2 and 2b sites, respectively. The AC magnetic susceptibility experiments show dependency of the magnetic responses with applied frequency, revealing strongly magnetic interactions between the nano-sized particles. Moreover, it is explored that the magnetic interactions are strengthened in substituted nanohexaferrites. This is owed to the replacement of Fe3+ ions by both Ce3+ and Y3+ ions.

Phenomenological Modeling of Magnetocaloric Effect in La0.7SrxMnO3−δ

The magnetocaloric effect (MCE) is investigated for La0.7SrxMnO3−δ (0.1 ≤ x ≤ 0.3) system closes to a phase transition from a ferromagnetic to a paramagnetic state. The results recommend that La0.7SrxMnO3−δ (LSMO) samples have potential application as a magnetic refrigerant (MR) in an extended temperature range. Consequently, it is hoped that LSMO samples can be MR materials in low magnetic field shifts.

Neutral and Charged Exciton Fine Structure in Single Lead Halide Perovskite Nanocrystals Revealed by Magneto-optical Spectroscopy.

Revealing the crystal structure of lead halide perovskite nanocrystals is essential for the optimization of stability of these emerging materials in applications such as solar cells, photodetectors, and light-emitting devices. We use magneto-photoluminescence spectroscopy of individual perovskite CsPbBr3 nanocrystals as a unique tool to determine their crystal structure, which imprints distinct signatures in the excitonic sublevels of charge complexes at low temperatures. At zero magnetic field, the identification of two classes of photoluminescence spectra, displaying either two or three sublevels in their exciton fine structure, shows evidence for the existence of two crystalline structures, namely tetragonal D4h and orthorhombic D2h phases. Magnetic field shifts, splitting, and coupling of the sublevels provide a determination of the diamagnetic coefficient and valuable information on the exciton g-factor and its anisotropic character. Moreover, this spectroscopic study reveals the optical properties of charged excitons and allows the extraction of the electron and hole g-factors for perovskite systems.

Dosimetric Impact of Using a Virtual Couch Shift for Online Correction of Setup Errors for Brain Patients on an Integrated High-Field Magnetic Resonance Imaging Linear Accelerator

To quantify the dosimetric impact of using virtual couch shift (VCS) for correcting setup errors in glioblastoma multiforme (GBM) patients treated on a magnetic resonance imaging (MRI)-linac. Six GBM patients treated with 60Gy (30 fractions) were selected for this simulation study. For each case, 2 reference plans were generated in the MRL treatment planning system: With (WIB) and with no (NOB) MRI B field present. Subsequently, 2-mm, 4-mm, and 6-mm translational errors were simulated and corrected for using a VCS method based on shift-only, warm start segment weight (SWO), and segment weight and shape (SSO) optimization. The resulting distributions were compared with the reference plan using planning target volume (PTV) homogeneity index (HI), conformity index (CI), organs at risk (OAR) maximum dose (D0.01cc), and OAR median dose (D50). A simulated 30-fraction treatment was constructed to evaluate the cumulative effect of daily corrections. Feasibility and workflow for correcting rotations were also assessed. All reference plans were deemed clinically acceptable with respect to PTV and OAR objectives. The difference in HI (ΔHI) between corrected and reference was not statistically significant between WIB and NOB (P=.89). The average ΔHI was +0.8%, -0.1%, and -1.0% for shift-only, SWO, and SSO, respectively, with a statistically significant difference (P<.001) for shift-only versus SWO and SSO. The CI remained unchanged (mean ΔCI=-0.01) between the corrected and reference plans, with no statistically significant dependence on magnetic field presence, correction method, or shift magnitude or orientation. The brainstem D50 on average decreased with SWO and SSO; however, D0.01cc increased by a median value of 1.2%, 1.9%, and 2.0% for shift-only, SWO, and SSO, respectively. For other OARs, D0.01cc decreased using SWO or SSO. For the simulated treatment and rotational corrections, similar trends were measured. For translational errors in brain MRI-linac radiation therapy, the VCS method is an acceptable correction strategy, but caution must be used in particular for serial organs where maximum doses are most relevant. The effect of the magnetic field on relative changes between corrected versus reference plans is not clinically relevant.

Single-crystalline M-type Sr Hexaferrites studied by 57Fe Mössbauer spectroscopy

The 57Fe Mossbauer spectra of the single crystalline and the finely ground Sr1−xLaxFe12−yCoyO19 (x = 0 : y = 0, x = 0.192 : y = 0.152 and x = 0.456 : y = 0.225) samples have been measured to investigate the La-Co substitution effects. All observed spectra at 150 K were well fitted using the five subspectra which correspond to the five crystallographical nonequivalent Fe sites in the M-type hexaferrite, indicating that the valence changes to Fe2+ ions in the Fe3+ ions were not observed in our Sr1−xLaxFe12−yCoyO19 samples. In SrFe12O19, the relative absorption intensities in the five subspectra show the large anisotropies in the recoilless fractions at the five Fe sites whereas these anisotropies were not observed in Sr0.544La0.456Fe11.775Co0.225O19. These results indicate the chemical compositional dependence on the anisotropies of the recoilless fractions at the five Fe sites. The substitution of a Co2+ ion for the Fe3+ ion changes the center shifts of the Fe3+ ions near the Co2+ ion by the perturbation of the Fe-O-Co hybridizations. Therefore, the Co2+ ions occupy the 4f1 and the 4f2 sites due to the chemical compositional dependences of the refined magnetic hyperfine field and center shifts of the Fe3+ ions.

Preparation, optical and electrochemical properties, and molecular orbital calculations of tetraazaporphyrinato ruthenium (II) bis(4-methylpyridine) fused with one to four diphenylthiophene units

2,5-Diphenyl-3,4-dicyanothiophene (1) and phthalonitrile (2) were mixed and treated with ruthenium (III) trichloride, 4-methylpyridine, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in 2-ethoxyethanol at 135°C, to produce low-symmetrical tetraazaporphyrins (TAPs) (3), (4), (5), and (6) with one to three thiophene rings. Two thiophene-annelated tetraazaporphyrins were isolated as opposite and adjacent isomers 4 and 5. The structure of 3 was determined by X-ray crystallography, showing that the thiophene ring linked at the 3,4-positions on the tetraazaporphyrin scaffold deviates from the mean plane of the four central pyrrole nitrogen atoms (N1–N3–N5–N7). Optical and electrochemical properties of the products were examined by UV–vis and magnetic circular dichroism (MCD) spectroscopy, together with cyclic voltammetry. In the 1H NMR spectra, the signals of 4-methylpyridine coordinating to the central ruthenium atom appeared at a higher magnetic field than those of uncoordinated 4-methylpyridine itself due to the shielding effect of the TAP ring. Increasing the number of fused thiophene rings resulted in 1) lower magnetic field shifts of the signals of axially coordinated 4-methylpyridine in the 1H NMR spectra, 2) lower energy shifts of the Q band absorption in the UV–vis spectra, and 3) decreasing (cathodic shift) of the first oxidation potentials. The structures of simplified model compounds were optimized using the DFT method with the Gaussian 09 program at the B3LYP/LANL2DZ level for the Ru atom and the B3LYP/6-31G (d, p) level for the C, H, N, and S atoms. The optimized structures were utilized to calculate the NMR shielding constants, the HOMO and LUMO orbital energies, and the electronic absorption spectra.

Structural, magnetic and theoretical investigations on the magnetocaloric properties of La0.7Sr0.25K0.05MnO3 perovskite

Temperature dependence of magnetization for the La0.7Sr0.25K0.05MnO3 compound indifferent applied magnetic field shifts.

Mössbauer Studies Magnetic Properties of BaCo2−xZnxFe16O27

The BaCo2− x Zn x Fe16O27 ( x = 0, 0.5, 1) samples were prepared by solid-state reaction method. The prepared samples are single-phased, and their crystal structures are determined to be hexagonal with a space group of P63/mmc from Rietveld refinement analysis. The magnetic saturation values were increasing, and the values of coercivity were decreasing with increasing Zn contents. The experimentally measured temperature dependence of magnetization curves shows magnetic transitions. We have measured Mossbauer spectroscopy at various temperatures to investigate microscopic magnetic properties. We analyze the obtained spectra with the five magnetic sites of $4f_{\mathrm {VI}}$ , $6g$ + $4f_{\mathrm {VI}}$ , $4e_{\mathrm {IV}}$ , + $4f_{\mathrm {IV}}$ , $12k_{\mathrm {VI}}$ , and $2d_{\mathrm {V}}$ . In addition, from the Mossbauer spectra, we notice the changes in the magnetic hyperfine field and electric quadrupole shift. The substituted Zn2+ ions are located at tetrahedral site, and increase the $M_{s}$ . Curie temperatures were determined from zero-field-cooled magnetizations measurement.

Growth and properties of the MOVPE GaAs/InAs/GaAsSb quantum dot structures

This review paper summarizes some of results achieved during last years of our quantum dot (QD) research.We show that the QD shape (aspect ratio and elongation) significantly influences the QD photoluminescence (PL) spectrum. Magnetophotoluminescence can be used for determination of the anisotropy of QDs. While the calculated shifts in magnetic field of the energies of higher radiative transitions are found to be sensitive to the lateral elongation, the shift of the lowest transition is determined mainly by the exciton effective mass. This behavior can be used for determining both the effective mass and the elongation fairly reliably from the magnetophotoluminescence spectra displaying at least two resolved bands.Lateral shape of InAs/GaAs QDs in vertically correlated structures was also studied. We found the ways to control the QD elongation and consequently the energy difference between PL transitions by adjusting properly the spacer layer thickness.The main goal was to redshift QD PL emission towards telecommunication wavelengths of Metal Organic Vapor Phase Epitaxy prepared InAs/GaAs QDs using InGaAs or GaAsSb covering strain reducing layer (SRL). Our results proved that GaAsSb SRL improves the QD PL properties and the type I or type II band alignment can be controlled by both, GaAsSb composition and QD size. Maintaining the type I heterostructure is important for high luminescence efficiency and emission wavelength stability of QD structure. The simulation of electron structure in InAs QDs covered with GaAsSb SRL and experimental results reveal the importance of increasing QD size for obtaining a longer wavelength PL from the type I heterostructure.The type II structure of InAs/GaAs QDs covered by GaAsSb SRL with Sb content near 30% enabled us to achieve extremely long emission wavelength at 1.8µm. The high amount of antimony in the SRL causes the preservation of QD size, and increased QD size prolongs the PL wavelength. The type II heterostructures with ground state electrons confined in InAs QDs and ground state holes in GaAsSb SRL have a strong potential in detector and solar cell applications as is demonstrated by photocurrent measurement.The dependence of built in electric field on QD PL properties is presented since this knowledge is necessary for application of QDs in different kind of electronic structures.We have experimentally demonstrated that the type I/type II transition of band structure of InAs QD with GaAsSb SRL does not depend only on the composition of the SRL, but also on other structure parameters, like QD size or intensity of electric field.

Magneto-Optical Properties of CdMnTe Diluted Magnetic Semiconductor Nanoparticles Prepared by Physical Methods

Nanoparticles of Cd1-xMnxTe have been studied by optical absorption and photoluminescence in applied magnetic field. The pulsed laser deposition technique and ball milling were used for fabrication of the samples of diluted magnetic semiconductor nanocrystals. The performed transmission electron microscopy (TEM) analysis suggests of spherical shape of nanostructures. In optical absorption spectra of the Cd1-xMnxTe nanoparticles short wavelength shift of the absorption edge due to confinement effect has been observed. In magnetic field shifts of the absorption spectra and maximum of the peak of photoluminescence towards long wavelength was observed due to the strong spin-exchange interaction between band carriers and magnetic ions. The linear magnetic field dependence of the Zeeman shifts of the maximum of photoluminescence for nanoparticles with low mangan content suggest of increase of the role pairs and antiferromagnetic interaction between Mn2+ ions

LATE-TIME NEAR-INFRARED OBSERVATIONS OF SN 2005df

We present late-time ($200-400$ days) near-infrared spectral evolution for the Type Ia supernova SN 2005df. The spectra show numerous strong emission features of [CoII], [CoIII], and [FeII] throughout the $0.8-1.8$\mu m region. As the spectrum ages, the cobalt features fade as would be expected from the decay of $^{56}$Co to $^{56}$Fe. We show that the strong and isolated [FeII] emission line at $1.644$\mu m provides a unique tool to analyze near-infrared spectra of Type Ia supernovae. Normalization of spectra to this line allows separation of features produced by stable versus unstable isotopes of iron group elements. We develop a new method of determining the initial central density, $\rho_c$, and the magnetic field, $B$, of the white dwarf using the width of the $1.644$\mu m line. The line width is sensitive because of electron capture in the early stages of burning, which increases as a function of density. The sensitivity of the line width to $B$ increase with time and the effects of the magnetic field shift towards later times with decreasing $\rho_c$. The initial central density for SN 2005df is measured as $\rho_c=0.9(\pm0.2)$ (in $10^9$g/cm$^3$), which corresponds to a white dwarf close to the Chandrasekhar mass ($\rm M_{Ch}$) with $\rm M_{WD}=1.313(\pm0.034)$M$_{\odot}$ and systematic error less than $0.04$M$_{\odot}$. Within $\rm M_{Ch}$ explosions, however, the central density found for SN 2005df is very low for a H-accretor, possibly suggesting a helium star companion or a tidally-disrupted white dwarf companion. As an alternative, we suggest mixing of the central region. We find some support for high initial magnetic fields of strength $10^6$G for SN 2005df, however, $0$G cannot be ruled out because of noise in the spectra combined with low $\rho_c$.

Frequency-dependent polarization-angle-phase-shift in the microwave-induced magnetoresistance oscillations

Linear polarization angle, θ, dependent measurements of the microwave radiation-induced oscillatory magnetoresistance, Rxx, in high mobility GaAs/AlGaAs 2D electron devices have shown a θ dependence in the oscillatory amplitude along with magnetic field, frequency, and extrema-dependent phase shifts, θ0. Here, we suggest a microwave frequency dependence of θ0(f) using an analysis that averages over other smaller contributions, when those contributions are smaller than estimates of the experimental uncertainty.

Structural, Optical and Room Temperature Magnetic Study of Mn&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; Nanoparticles

In this work, Mn2O3 nanoparticles (NPs) are prepared by co-precipitation technique. The synthesized sample is characterized by X-ray powder diffraction (XRD). The XRD spectrum reveals the cubic structure of Mn2O3 NPs and the lattice parameter is calculated to be 9.4232 ?. Crystallite size (D) is estimated using Debye-Scherer’s formula and is found to be 17.3 nm. A micrograph for the NPs is obtained using Transmission Electron Microscopy (TEM). The Mn2O3 nanoparticles are viewed at 7500× magnification and their shape is spherical. D is also measured using TEM and it is 19.1 nm, which is very close to the one obtained from XRD. The elemental contents of the prepared samples are determined using particle induced X-ray emission (PIXE). In addition, the oxygencontent of the sample is obtained using non Rutherford backscattering spectroscopy (RBS) at 3 MeV proton beam. The sample shows high purity and the RBS technique is more accurate in determining the O-content. The presence of functional groups and the chemical bonding is confirmed by FTIR spectrum. The energy band gap (Eg) is calculated for the NPs using the UV-visible optical spectroscopy between 350 nm and 800 nm and found to be 1.24 eV. The sample shows high absorption in the visible range. The magnetization (VSM) is conducted to the sample and the saturation magnetization (Ms) is calculated as 2.642 emu/g. The hysteresis loop shows antiferromagnetic behavior. The EPR analysis is performed at room temperature for the NPs. The g-factor is calculated from the spectrum and found to be 1.985, and the magnetic field shift occurs at Bo = 350.5 mT. The intensity appeared to be high, which confirms the existence of Mn2+ ions on the surface.

Magnetooptical study of CdSe/ZnMnSe semimagnetic quantum-dot ensembles with n-type modulation doping

Magnetic and polarization investigations of the photoluminescence and resonant electron spin-flip Raman scattering in ensembles of self-organized CdSe/ZnMnSe semimagnetic quantum dots with n-type modulation doping are carried out. It is demonstrated that exciton transitions contribute to the photoluminescence band intensity, along with the transitions of trions in the singlet state. In the Hanle-effect measurements, negative circular polarization in zero magnetic field is observed, which is related to the optical orientation of a trion heavy hole. The lifetime and spin-relaxation time of a heavy hole are estimated as ≤3 and ≤1 ps, respectively. Such short times are assumed to be due to Auger recombination with the excitation of an intrinsic transition in a Mn2+ ion. Investigations of the photoluminescence-maximum intensity and shift in a longitudinal magnetic field at the σ−σ+ and σ−σ− polarizations reveal the pronounced spin polarization of electrons. Under resonant excitation conditions, a sharp increase in the photoluminescence-band maximum intensity at σ− excitation polarization over the σ+ one is observed. The Raman scattering peak at the electron spin-flip transition is observed upon resonant excitation in a transverse magnetic field in crossed linear polarizations. This peak is shown to be a Brillouin function of a magnetic field.

Numerical simulation of graphene in an external magnetic field

In this paper the results of numerical simulation of monolayer graphene in external magnetic field are presented. The numerical simulation is performed in the effective lattice field theory with noncompact $3 + 1$-dimensional Abelian lattice gauge fields and $2 + 1$-dimensional staggered lattice fermions. The dependences of fermion condensate and graphene conductivity on the dielectric permittivity of substrate for different values of external magnetic field are calculated. It is found that magnetic field shifts insulator-semimetal phase transition to larger values of the dielectric permittivity of substrate. The phase diagram of graphene in external magnetic field is drawn.