In-plane Magnetic Field Dependence Research Articles

Evidence of striped electronic phases in a structurally modulated superlattice.

The electronic properties of crystals can be manipulated by superimposing spatially periodic electric, magnetic or structural modulations. Long-wavelength modulations incommensurate with the atomic lattice are particularly interesting1, exemplified by recent advances in two-dimensional (2D) moiré materials2,3. Bulk van der Waals (vdW) superlattices4-8 hosting 2D interfaces between minimally disordered layers represent scalable bulk analogues of artificial vdW heterostructures and present a complementary venue to explore incommensurately modulated 2D states. Here we report the bulk vdW superlattice SrTa2S5 realizing an incommensurate one-dimensional (1D) structural modulation of 2D transition metal dichalcogenide (TMD) H-TaS2 layers. High-quality electronic transport in the H-TaS2 layers, evidenced by quantum oscillations, is made anisotropic by the modulation and exhibits commensurability oscillations paralleling lithographically modulated 2D systems9-11. We also find unconventional, clean-limit superconductivity in SrTa2S5 with a pronounced suppression of interlayer relative to intralayer coherence. The in-plane magnetic field dependence of interlayer critical current, together with electron diffraction from the structural modulation, suggests superconductivity12-14 in SrTa2S5 is spatially modulated and mismatched between adjacent TMD layers. With phenomenology suggestive of pair-density wave superconductivity15-17, SrTa2S5 may present a pathway for microscopic evaluation of this unconventional order18-21. More broadly, SrTa2S5 establishes bulk vdW superlattices as versatile platforms to address long-standing predictions surrounding modulated electronic phases in the form of nanoscale vdW devices12,13 to macroscopic crystals22,23.

Magnetic tuning of tunnel coupling between InAsP double quantum dots in InP nanowires

We study experimentally and theoretically the in-plane magnetic field dependence of the coupling between dots forming a vertically stacked double dot molecule. The InAsP molecule is grown epitaxially in an InP nanowire and interrogated optically at millikelvin temperatures. The strength of interdot tunneling, leading to the formation of the bonding-antibonding pair of molecular orbitals, is investigated by adjusting the sample geometry. For specific geometries, we show that the interdot coupling can be controlled in-situ using a magnetic field-mediated redistribution of interdot coupling strengths. This is an important milestone in the development of qubits required in future quantum information technologies.

Magnetic Field Effect on Topological Spin Excitations in CrI3

The search for topological spin excitations in recently discovered two-dimensional (2D) van der Waals (vdW) magnetic materials is important because of their potential applications in dissipation-less spintronics. In the 2D vdW ferromagnetic (FM) honeycomb lattice CrI$_3$(T$_C$= 61 K), acoustic and optical spin waves were found to be separated by a gap at the Dirac points. The presence of such a gap is a signature of topological spin excitations if it arises from the next nearest neighbor(NNN) Dzyaloshinskii-Moriya (DM) or bond-angle dependent Kitaev interactions within the Cr honeycomb lattice. Alternatively, the gap is suggested to arise from an electron correlation effect not associated with topological spin excitations. Here we use inelastic neutron scattering to conclusively demonstrate that the Kitaev interactions and electron correlation effects cannot describe spin waves, Dirac gap and their in-plane magnetic field dependence. Our results support the DM interactions being the microscopic origin of the observed Dirac gap. Moreover, we find that the nearest neighbor (NN) magnetic exchange interactions along the axis are antiferromagnetic (AF)and the NNN interactions are FM. Therefore, our results unveil the origin of the observedcaxisAF order in thin layers of CrI$_3$, firmly determine the microscopic spin interactions in bulk CrI$_3$, and provide a new understanding of topology-driven spin excitations in 2D vdW magnets.

Cascade of phase transitions and Dirac revivals in magic-angle graphene.

Twisted bilayer graphene near the magic angle1-4 exhibits rich electron-correlation physics, displaying insulating3-6, magnetic7,8 and superconducting phases4-6. The electronic bands of this system were predicted1,2 to narrow markedly9,10 near the magic angle, leading to a variety of possible symmetry-breaking ground states11-17. Here, using measurements of the local electronic compressibility, we show that these correlated phases originate from a high-energy state with an unusual sequence of band population. As carriers are added to the system, the four electronic 'flavours', which correspond to the spin and valley degrees of freedom, are not filled equally. Rather, they are populated through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moiré lattice. At each transition, a single spin/valley flavour takes all the carriers from its partially filled peers, 'resetting' them to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breaking. The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state thatwe report here, with its strongly broken electronic flavour symmetry and revived Dirac-like electronic character, is important in the physics of magic-angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.

Determination of perpendicular magnetic anisotropy based on the magnetic droplet nucleation

We propose an alternative method of determining the magnetic anisotropy field μ0HK in ferro-/ferrimagnets. On the basis of the droplet nucleation model, there exists linearity between domain-wall (DW) energy density and in-plane magnetic field. We find that the slope is simply represented by μ0HK and Dzyaloshinskii–Moriya interaction (DMI). By measuring the in-plane magnetic field dependence of the coercivity field, closely corresponding to the DW energy density, a robust value for μ0HK can be quantified. This robust value can be used to determine μ0HK over a wide range of values, overcoming the limitations caused by the small strength of the external magnetic field typically used in experiments.

Micromagnetic study of high-power spin–torque oscillator with perpendicular magnetization in half-metallic Heusler alloy spin valve nanopillar under external magnetic fields

We investigated the high-power spin–torque oscillator in a half-metallic Heusler alloy Co 2 MnSi spin-valve nanopillars with perpendicular magnetization under external magnetic field using micromagnetic simulations. Our simulations show that the narrow optimum current of magnetization precession in the Heusler-based spin valve is broadened by introducing the surface anisotropy. The linear decrease of frequency with the out-of-plane magnetic field is obtained in our simulation. Additionally, the in-plane magnetic field dependence of frequency shows a parabolic curve which is explained by the magnetization trajectory tilting. Furthermore, we also discussed the decrease of output power using the excitation of non-uniform magnetization precession in the in-plane magnetic fields. • We investigated spin–torque oscillator in Co 2 MnSi spin-valve under magnetic fields. • The narrow optimum current is broadened by introducing the surface anisotropy. • The frequency dependences of out-of-plane and in-plane magnetic fields show linear and parabola. • The results may give the guidance for designing Heusler-based spin–torque oscillator.

Origins of Nonlocality Near the Neutrality Point in Graphene

We present an experimental study of nonlocal electrical signals near the Dirac point in graphene. The in-plane magnetic field dependence of the nonlocal signal confirms the role of spin in this effect, as expected from recent predictions of the Zeeman spin Hall effect in graphene, but our experiments show that thermo-magneto-electric effects also contribute to nonlocality, and the effect is sometimes stronger than that due to spin. Thermal effects are seen to be very sensitive to sample details that do not influence other transport parameters.

In-plane magnetic field dependence of electric field-induced magnetization switching

Electric field-induced magnetization switching through magnetization precession is investigated as a function of in-plane component of external magnetic field for a CoFeB/MgO-based magnetic tunnel junction with perpendicular easy axis. The switching probability is an oscillatory function of the duration of voltage pulses and its magnitude and period depend on the magnitude of in-plane magnetic field. Experimental results are compared with simulated ones by using Landau-Lifshitz-Gilbert-Langevin equation, and possible factors determining the probability are discussed.

Tunable zero-field Kondo splitting in a quantum dot

We present detailed low-temperature transport measurements of a single quantum dot formed in an InGaAs/InP heterostructure with a strong tunnel coupling to the source and drain leads. The conventional spin-1/2 Kondo effect is observed for the quantum dot in the $N=9$ charge state. By changing the voltages applied to the quantum dot barrier gates, we find a zero-field splitting of the Kondo resonance and a zero-bias differential conductance, which shows a nonmonotonic in-plane magnetic field and temperature dependence. Using a two-site Hubbard model, we show that the main observed features can be explained in terms of Kondo correlation effects resulting from the exchange interaction between two localized spins.

Temperature, electron density and in-plane magnetic field dependence of cyclotron relaxation time in the two-dimensional metallic phase

We have performed cyclotron resonance measurements on two-dimensional electrons in the metallic phase. The temperature, electron density and in-plane magnetic field dependence of cyclotron and transport scattering time are reported for a high-mobility Si/SiGe quantum well.

In-plane magnetic field dependence of cyclotron relaxation time in a Si two-dimensional electron system

Cyclotron resonance of two-dimensional electrons is studied for a high-mobility Si/SiGe quantum well in the presence of an in-plane magnetic field, which induces spin polarization. The relaxation time ${\ensuremath{\tau}}_{\mathrm{CR}}$ shows a negative in-plane magnetic-field dependence, which is similar to that of the transport scattering time ${\ensuremath{\tau}}_{t}$ obtained from dc resistivity. The resonance magnetic field shows an unexpected negative shift with increasing in-plane magnetic field.

In-plane magnetic anisotropy and coercive field dependence upon thickness of CoFeB

The structural and magnetic properties of as-grown 5–50nm thin ion-beam sputter deposited transition metal–metalloid Co20Fe60B20 (CFB) films are reported in this communication. A broad peak observed at 2θ∼45° in the glancing angle X-ray diffraction pattern revealed the formation of very fine nano-sized grains embedded in majority amorphous CFB matrix. Although no magnetic field is applied during deposition, the longitudinal magneto-optic Kerr effect measurements performed at 300K in these as-grown films clearly established the presence of in-plane uniaxial magnetic anisotropy (Ku). It is argued that this observed anisotropy is strain-induced. This is supported by the observed dependence of direction of Ku on the angle between applied magnetic field and crystallographic orientation of the underlying Si(100) substrate, and increase in the coercivity with the increase of the film thickness.

Non-planar spin texture driven dissipation in a ferromagnet–superconductor bilayer

We report the observation of a pronounced dip in the in-plane magnetic field ( H ∥) dependence of the critical current density J c ( H) and a peak in resistance R( H) of a NbN–HoNi 5 bilayer at temperatures below the magnetic ordering temperature ( T Curie ≈ 3.5 K) of HoNi 5, which is lower than the onset temperature (≈9 K) of superconductivity in the NbN layer. The extrema in J c ( H) and R( H) appear at fields much below the upper critical field of NbN. We attribute these features to a coupling between localized out-of-plane moments present in the magnetic film and Pearl vortices of the superconducting layer. A spin re-orientation transition of the localized moments by H ∥ breaks this coupling, leading to the observed excess dissipation.

Local investigation of the energy gap within the incompressible strip in the quantum Hall regime

We experimentally study the energy gap within the incompressible strip at local filling factor ν c = 1 at the quantum Hall edge for samples of very different mobilities. The obtained results indicate strong enhancement of the energy gap in comparison to the single-particle Zeeman splitting. We identify the measured gap as a mobility gap, so a pronounced experimental in-plane magnetic field dependence can both be attributed to the spin effects as well as to the change in the energy levels broadening.

Scanning Kerr Microscopy of the Spin Hall Effect in n-Doped GaAs with Various Doping Concentration

We investigated the doping concentration (ND) dependence of the extrinsic spin Hall effect (SHE) in n-doped GaAs with ND raging from 3×1016 cm−3 to 5×1017 cm−3. By using scanning Kerr microscopy (SKM) measurements, we observed the Kerr rotation signal due to the spin accumulation near the channel edges in all the samples with different ND. Moreover, the position and in-plane magnetic field dependence of the Kerr rotation signal are found to vary with ND. We analyzed the ND dependence of the spin Hall conductivity by taking account of the ND-dependent spin lifetime based on the typical drift-diffusion model.

Stoner ferromagnetic phase of graphene in the presence of an in-plane magnetic field

We study the effects of an in-plane magnetic field on the ground-state properties of both gapless and gapped graphene sheets within random-phase approximation. The critical magnetic field which leads to a fully spin polarized phase increases by decreasing the carrier density at zero gap indicating that no spontaneous magnetic phase transition occurs. However, at large energy-gap values it decreases by decreasing the density. We find a continuous quantum magnetic phase transition (Stoner phase) for Dirac fermions in a doped graphene sheet. Novel in-plane magnetic field dependence of the charge and spin susceptibilities are uncovered.

Suppression of vortex channeling in meandered YBa2Cu3O7−δ grain boundaries

We report on the in-plane magnetic field (H) dependence of the critical current density (Jc) in meandered and planar single grain boundaries (GBs) isolated in YBa2Cu3O7−δ (YBCO) coated conductors. The meandered boundaries resulted from metal-organic deposition, and the planar boundaries resulted from physical vapor deposition. The Jc(H) properties of the planar GB are consistent with those previously seen in single GBs of YBCO films grown on SrTiO3 bicrystals. In the straight boundary, a characteristic flux channeling regime when H is oriented near the GB plane, associated with a reduced Jc, is seen. The meandered GB does not show vortex channeling since it is not possible for a sufficient length of vortex line to lie within it.

Hall coefficient and magnetoresistance of two-dimensional spin-polarized electron systems

Recent measurements of the 2D Hall resistance show that the Hall coefficient is independent of the applied in-plane magnetic field, i.e., the spin-polarization of the system. We calculate the weak-field Hall coefficient and the magnetoresistance of a spin polarized 2D system using the semi-classical transport approach based on the screening theory. We solve the coupled kinetic equations of the two carrier system including electron-electron interaction. We find that the in-plane magnetic field dependence of the Hall coefficient is suppressed by the weakening of screening and the electron-electron interaction. However, the in-plane magnetoresistance is mostly determined by the change of the screening of the system, and can therefore be strongly field dependent.

In-plane magnetic field dependence of cyclotron resonance in two-dimensional electron system

The in-plane magnetic field dependence of cyclotron resonance is measured both in single and double layer 2DES. In particular, we observe that the in-plane dependence of cyclotron mass in single-layer 2DES changes with carrier density and perpendicular magnetic field in the low electron density sample, which cannot be explained by the semiclassical calculation.

Effect of in-plane magnetic field on cyclotron resonance in various types of 2DES

The in-plane magnetic field ( B ||) dependence of cyclotron resonance is investigated in both single and double layer two-dimensional electron systems by Fourier spectrometry in tilted magnetic fields. The observed B || dependence of the cyclotron mass ( m C) is complicated and varies with perpendicular magnetic field in the double layer system. The splitting of m C by B || around filling factor 2 is observed in the single layer system.