Field Angle Dependence Research Articles

Theoretical characterization and its application of electromagnetic anisotropy in high-temperature superconducting bulks under rotated postures

The electromagnetic anisotropy of high-temperature superconducting (HTS) bulks limits their levitation ability in the applied magnetic fields from the permanent magnet guideway (PMG), thus impeding the enhancement of load-carrying capacity in HTS pinning maglev systems. Developing a suitable matching scheme between bulk orientation and magnetic field direction is a valuable way to relieve this restriction. In this paper, a method for characterizing bulk anisotropy in a rotating coordinate system is proposed to explore the best bulk orientation. The method is based on the concept of equivalent resistivity tensor and its eigenvectors, and includes an extended description of two types of anisotropy: conductivity anisotropy and magnetic field angle dependence. It provides a theoretical foundation for simulating anisotropic bulks under any rotated posture. Experimental investigations on the levitation force distribution of cylindrical bulks with different c-axis orientation were conducted, through which the accuracy of the characterization method and calculated results were validated. Analysis of current distribution reveals that aligning the c-axis parallel to the external magnetic field helps achieve the best match between the bulk and the PMG. Additionally, considering that the two types of anisotropy have opposite effects on levitation force distribution trends, prioritizing conductivity anisotropy when analyzing anisotropic bulk is recommended. This research not only offers a theoretical framework for simulating the anisotropy of rotated HTS bulks but also provides guidance for matching the optimal bulk orientation in applied magnetic fields.

Magnetic Field Angle Dependence of Critical Current in REBCO Tapes Produced by Different Techniques

Magnetic Field Angle Dependence of Critical Current in REBCO Tapes Produced by Different Techniques

Probing surface states: A study of UCF and WAL in Bi1.9Sb0.1Te2Se topological insulator

In the exploration of three-dimensional quaternary topological insulators, understanding surface states has become pivotal for unraveling the underlying physics and tapping into potential applications. Our study delves into the temperature and magnetic field-angle dependence of universal conductance fluctuations (UCF) and weak anti-localization (WAL) effects in a Bi1.9Sb0.1Te2Se topological insulator-based mesoscopic device. Conventionally, other low-temperature transport phenomena in probing surface states may inevitably face interference from three-dimensional bulk conductance. However, we experimentally demonstrate that, at low temperatures, UCF reflects the properties of two-dimensional topological surface states more accurately, thereby providing a more reliable and distinct way to confirm their existence. Moreover, we carefully analyze the temperature-dependent WAL using the Hikami–Larkin–Nagaoka model, proposing a crucial role for charge puddles associated with electrostatic fluctuations in the electron dephasing process. Our findings not only emphasize the key role of UCF in unveiling the underlying behavior of topological surface states but also offer a deeper understanding of phase-coherent transport in quaternary topological insulators.

Weakly coupled Majorana wire arrays under tilted magnetic fields

An array of Rashba-coupled semiconducting nanowires lying in proximity to an s-wave superconducting substrate, with weak inter-wire coupling, in the presence of an external magnetic field shows even–odd effect in the differential conductance over a chosen parameter space. Such an effect is a direct consequence of end Majoranas in each nanowire hybridizing into bonding and anti-bonding states. In the present work, we study in detail the impact of tilting of external magnetic field on the differential conductance of an array of both uncoupled and weakly coupled wires. The phase diagram evolution with various control parameters including the tilt angle of the magnetic field has also been presented. From detailed analysis of the field-angle dependence of the odd–even effect, and the evolution of the same over a large parameter space we summarize that the results can be used to exploit magnetic-field angle in an array of Rashba-coupled semiconducting nanowires on a superconducting substrate as an important tuning parameter to investigate zero-bias conductance peak arising from Majorana edge modes vis-a-vis that arising from a non-topological origin.

PO-1821 Neutron contribution in pediatric proton therapy and field angle dependence

PO-1821 Neutron contribution in pediatric proton therapy and field angle dependence

Tuning magnetic anisotropy in SrRuO3 thin film by Ru vacancies induced phase transition

Effective control of magnetic anisotropy, including perpendicular magnetic anisotropy (PMA) and lateral magnetic anisotropy, is important for the design of low-power and high-density spintronic devices. However, the rarity of oxide materials with PMA and stringent conditions required to control magnetic anisotropy have prevented its large-scale application. Here, we demonstrate that the magnetic anisotropy of SrRuO3 films can be specified on-demand by adjusting the content of Ru vacancies to control the structure of the films. With the increase in Ru vacancies, the structure of SrRuO3 changes from orthorhombic to tetragonal. The field angle dependence of the Hall resistance confirmed that the uniaxial magnetic easy axis of SrRuO3 thin films continuously rotates in the (100)pc crystallographic plane, which is identical to the continuous phase transition. Our results not only provide a way to continuously tune the physical properties of epitaxial oxide films by continuously changing the composition but also help to provide guidance for the on-demand design of spintronic devices.

Onset temperature of intrinsic pinning in a REBCO coated conductor from critical current anisotropy

• A detailed analysis is given of the evolution with temperature of the angle dependence of critical current in a REBCO coated conductor. • Particular attention is given to the development of the ubiquitous ab -plane peak for field parallel to the plane of the tape. We highlight that this peak appears as the result of two distinct mechanisms, stacking faults at higher temperatures and intrinsic pinning at lower temperatures. • This peak is unusually weak at 77K, due to process conditions favouring a low density of stacking fault defects. • The peak grows rapidly with decreasing temperature relative to the c-axis critical current starting from 60K, signalling the onset of intrinsic pinning by the layered structure of REBCO. • This growth is quantified through fitting with maximum entropy functions. • The angle dependence of n -values is also shown. With decreasing temperature, the ab -plane feature appears first as a dip at 65K, then a sharper peak starting from 45K. Both of these features broaden with decreasing temperature. The field-angle dependence of the critical current in a REBa 2 Cu 3 O 7 coated conductor has been measured over a fine range of temperatures from 15 K to 80 K. The particular sample is demonstrated to have a very low fraction of extrinsic planar defects by its near-isotropic critical current at 77 K. At a representative magnetic field of 3 T we are able to track the emergence of the ab-plane peak usually associated with intrinsic pinning and quantify its evolution with temperature by fitting to a maximum-entropy function. We are also able to observe the evolution of dip and peak features in the angle dependence of the power-law index n with decreasing temperature and show that a dip appearing at 65 K is supplanted by a sharper positive peak from 50 K. The combination of critical current and power-law index temperature dependences shows the onset of intrinsic pinning at 50 K.

Spin-Enhanced C–C Coupling in CO 2 Electroreduction with Oxide-Derived Copper

Spin-Enhanced C–C Coupling in CO ₂ Electroreduction with Oxide-Derived Copper

Field-orientation dependence of quantum phase transitions in the S=12 triangular-lattice antiferromagnet Ba3CoSb2O9

Ba$_3$CoSb$_2$O$_9$ approximates the two-dimensional spin-1/2 triangular-lattice Heisenberg antiferromagnet. This compound displays magnetic-field-induced quantum phase transitions, including the 1/3-magnetization-plateau, but its magnetization processes for the magnetic field $H$ parallel and perpendicular to the $c$ axis are different due to the weak easy-plane anisotropy and the weak interlayer antiferromagnetic exchange interaction. To elucidate how the quantum phase transitions change between these two field directions, we measured the field-angle dependence of the magnetization process in Ba$_3$CoSb$_2$O$_9$ using pulsed high magnetic fields. We compared obtained magnetic field-field angle phase diagram with those obtained by the large-size cluster mean-field method combined with a scaling scheme and the semiclassical theory. We also found a narrow 1/3-magnetization plateau and a high-field transition with a small magnetization jump for $H\,{\parallel}\,c$, not observed in the previous studies.

Nodal and Nematic Superconducting Phases in NbSe_{2} Monolayers from Competing Superconducting Channels.

Transition metal dichalcogenides like 2H-NbSe_{2} in their two-dimensional (2D) form exhibit Ising superconductivity with the quasiparticle spins being firmly pinned in the direction perpendicular to the basal plane. This enables them to withstand exceptionally high magnetic fields beyond the Pauli limit for superconductivity. Using field-angle-resolved magnetoresistance experiments for fields rotated in the basal plane we investigate the field-angle dependence of the upper critical field (H_{c2}), which directly reflects the symmetry of the superconducting order parameter. We observe a sixfold nodal symmetry superposed on a twofold symmetry. This agrees with theoretical predictions of a nodal topological superconducting phase near H_{c2}, together with a nematic superconducting state. We demonstrate that in NbSe_{2} such unconventional superconducting states can arise from the presence of several competing superconducting channels.

Temperature-linear resistivity in twisted double bilayer graphene

We report an experimental study of carrier density ($n$), displacement field, and twist angle (\ensuremath{\theta}) dependence of temperature ($T$)-linear resistivity in twisted double-bilayer graphene (TDBG). For a large twist angle ($\ensuremath{\theta}>1.{5}^{\ensuremath{\circ}}$) where correlated insulating states are absent, we observe a $T$-linear resistivity (with a slope on the order of $\ensuremath{\sim}10\phantom{\rule{0.28em}{0ex}}\mathrm{\ensuremath{\Omega}}/\mathrm{K}$) over a wide range of carrier densities, and its slope decreases with increasing $n$, which is in agreement with the acoustic phonon scattering model semiquantitatively. The slope of $T$-linear resistivity is nonmonotonically dependent on the displacement field with a single peak structure. For a device with $\ensuremath{\theta}\ensuremath{\sim}1.{23}^{\ensuremath{\circ}}$ at which correlated states emerge, the slope of $T$-linear resistivity is found to be a maximum ($\ensuremath{\sim}100\phantom{\rule{0.28em}{0ex}}\mathrm{\ensuremath{\Omega}}/\mathrm{K}$) at the boundary of the halo structure where phase transition occurs, with signatures of continuous phase transition, Planckian dissipation, and diverging effective mass. These observations are in line with quantum critical behaviors, which might be due to symmetry-breaking instability at the critical points. Our results shed new light on correlated physics in TDBG and other twisted moir\'e systems.

Isotropic and Anisotropic Flux Pinning Induced by Heavy-Ion Irradiation

Ion irradiation of REBCO films and coated conductors, in which the ions pass completely through the REBCO film, produces damage tracks which form near-ideal flux-pinning defects. The radius and aspect ratio of the tracks depends on the mass and energy of the incident ions. We have investigated the effect of Ag ion irradiation, at different incident energies and incidence angles, on REBCO production-quality coated conductors from American Superconductor Corp. Transmission electron microscopy and in-field transport critical current anisotropy analysis indicates that the ion-energy threshold for the formation of elongated tracks is around 50 MeV. At this energy tracks are not readily identifiable in low-resolution TEM, and enhancement of critical current is nearly isotropic. For a higher ion energy of 100 MeV, on the other hand, clear elongated (but still not fully continuous) tracks are visible in TEM, and the critical current is anisotropic with strong enhancement occurring when the applied field is parallel to the ion incidence angle. We particularly analyze the case of 60° inclined irradiation. This produces a clear peak in the field-angle dependence of critical current for 100 MeV irradiation, but only an incipient peak for 50 MeV irradiation. The incipient peak can be identified by curve fitting using a minimal number of maximum-entropy functional components.

Field-angle dependence of thermal transport in Kitaev-Γ model

We investigate the magnetic field effect on the Kitaev model with the Γ-type interaction, which plays an essential role in understanding the magnetic properties of real materials. We examine the mean-field phase diagram and thermal Hall conductivity using the linear spin-wave theory. We find that the Γ interaction substantially changes the field-angle dependence of the thermal Hall conductivity and suppresses its magnitude. The suppression is caused by the enhancement of the low-energy magnon gap while increasing the Γ interaction.

Identification of a Kitaev quantum spin liquid by magnetic field angle dependence

Quantum spin liquids realize massive entanglement and fractional quasiparticles from localized spins, proposed as an avenue for quantum science and technology. In particular, topological quantum computations are suggested in the non-abelian phase of Kitaev quantum spin liquid with Majorana fermions, and detection of Majorana fermions is one of the most outstanding problems in modern condensed matter physics. Here, we propose a concrete way to identify the non-abelian Kitaev quantum spin liquid by magnetic field angle dependence. Topologically protected critical lines exist on a plane of magnetic field angles, and their shapes are determined by microscopic spin interactions. A chirality operator plays a key role in demonstrating microscopic dependences of the critical lines. We also show that the chirality operator can be used to evaluate topological properties of the non-abelian Kitaev quantum spin liquid without relying on Majorana fermion descriptions. Experimental criteria for the non-abelian spin liquid state are provided for future experiments.

Twist engineering of the two-dimensional magnetism in double bilayer chromium triiodide homostructures

Twist engineering, or the alignment of two-dimensional (2D) crystalline layers with desired orientations, has led to tremendous success in modulating the charge degree of freedom in hetero- and homo-structures, in particular, in achieving novel correlated and topological electronic phases in moir\'e electronic crystals. However, although pioneering theoretical efforts have predicted nontrivial magnetism and magnons out of twisting 2D magnets, experimental realization of twist engineering spin degree of freedom remains elusive. Here, we leverage the archetypal 2D Ising magnet chromium triiodide (CrI3) to fabricate twisted double bilayer homostructures with tunable twist angles and demonstrate the successful twist engineering of 2D magnetism in them. Using linear and circular polarization-resolved Raman spectroscopy, we identify magneto-Raman signatures of a new magnetic ground state that is sharply distinct from those in natural bilayer (2L) and four-layer (4L) CrI3. With careful magnetic field and twist angle dependence, we reveal that, for a very small twist angle (~ 0.5 degree), this emergent magnetism can be well-approximated by a weighted linear superposition of those of 2L and 4L CI3 whereas, for a relatively large twist angle (~ 5 degree), it mostly resembles that of isolated 2L CrI3. Remarkably, at an intermediate twist angle (~ 1.1 degree), its magnetism cannot be simply inferred from the 2L and 4L cases, because it lacks sharp spin-flip transitions that are present in 2L and 4L CrI3 and features a dramatic Raman circular dichroism that is absent in natural 2L and 4L ones. Our results demonstrate the possibility of designing and controlling the spin degree of freedom in 2D magnets using twist engineering.

Coexistence of low-frequency spin-torque ferromagnetic resonance and unidirectional spin Hall magnetoresistance

We investigate the DC voltage under an applied microwave current for platinum(Pt)/cobalt(Co), tantalum(Ta)/Co, and tungsten(W)/Co bilayers, where the microwave-induced alternating magnetic field (${B}_{\mathrm{rf}}$) is parallel to an external static magnetic field (${B}_{\mathrm{ext}}$). Whereas spin torque ferromagnetic resonance (ST FMR) signals do not appear because of the parallel configuration of ${B}_{\mathrm{rf}}$ and the magnetization of the Co layer, we recently found a clear hysteresis signal around ${B}_{\mathrm{ext}}=0\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ only when the microwave frequency (${f}_{\mathrm{MW}}$) is low, typically less than 10 GHz. This low-frequency ST FMR (LFST FMR) signal enables us to detect magnetization switching induced by spin-orbit torque (SOT) with high sensitivity. In this study, we found an additional background (BG) signal superimposed on the LSFT FMR signal. The additional BG signal also has hysteresis behavior and appears even at high ${f}_{\mathrm{MW}}$, where the LSFT FMR signal completely disappears. The sign of the BG signal is changed by changing the nonmagnetic material from Pt to Ta or W. By measuring ${f}_{\mathrm{MW}}$, the microwave current, the temperature, and the magnetic field angle dependences, we conclude that the BG signal is induced by spin-dependent unidirectional spin Hall magnetoresistance (SD USMR), which is generated by a spin current induced by the spin Hall effect of nonmagnetic metals and spin-dependent electron mobility in ferromagnetic metals. The SD USMR signal appears in wide ranges of ${f}_{\mathrm{MW}}$ and ${B}_{\mathrm{ext}}$ because the SD USMR originates from the nonlinearity of current-dependent resistance and is not related to magnetization dynamics. From the magnitude of the BG signal, the spin Hall angles of Pt and Ta are calculated to be 0.026 \ifmmode\pm\else\textpm\fi{} 0.006 and \ensuremath{-}0.042 \ifmmode\pm\else\textpm\fi{} 0.006, respectively. In addition, we demonstrate SOT magnetization switching using the BG signal.

Role of asymmetric critical current on magnetization loss characteristics of (RE)Ba2Cu3O7−d coated conductors at various temperatures

Commercial high-Tc superconducting (HTS) coated conductors exhibit asymmetric Ic(B,θ) characteristics, where B presents a DC magnetic field and θ is defined as the angle between an applied magnetic field and the normal component of the superconductor plane. The asymmetric Ic(B,θ) characteristics have a non-trivial influence on the dominant loss component, magnetization loss, of various HTS applications where HTS conductors are exposed to an AC magnetic field. Here, we present measurements of Ic(B,θ) and magnetization loss in a 12 mm-wide (Rare Earth)Ba2Cu3O7−d (REBCO) commercial coated conductor at 77, 70, and 65 K. In the Ic(B,θ) measurement, θ was varied around a full 360° revolution and B was varied up to 0.2 T. In terms of the magnetization loss measurement, the applied AC magnetic field amplitude is up to 110 mT and the field angle varied from 0° to 180°. At the three given temperatures, we observed magnetization loss variations among the field-angle range, in particular, for θ and 180°-θ, which are in mirror symmetry relative to the superconducting plane. Furthermore, this asymmetric field-angle dependence of the magnetization loss becomes more apparent at higher applied field amplitudes and lower operating temperatures. A finite element method simulation using H-formulation was carried out by directly interpolating the measured Ic(B,θ) data, and the simulation results reproduce the trend of the experimental results. We also found that the magnetization loss is not equivalent within the positive and negative half field cycles due to the asymmetric Ic(B,θ) characteristics of the conductor. Numerical simulations revealed a clear correlation between the magnetization loss and the asymmetric Ic(B,θ) data for the whole 360° field-angle range in the REBCO conductor. The asymmetry in the Ic(B,θ) data about the ab-peak causes differences in magnetization loss values for the mirror-symmetric field angles. The asymmetry in the Ic(B,θ) data upon field reversal also results in differences between magnetization loss values for the positive and negative cycles.

Field-angle dependence of thermal Hall conductivity in a magnetically ordered Kitaev-Heisenberg system

We study magnetic excitations and thermal Hall effect on the Kitaev-Heisenberg model under magnetic fields. By employing the spin-wave theory for the magnetic orders realized in this model, we examine the topological nature of the spin-wave dispersions and calculate the thermal Hall conductivity. The comprehensive investigations on the field-angle dependence clarify that the thermal Hall conductivity is sensitive to the spin ordered pattern and excitation spectra of magnons; this quantity is enhanced by the noncoplanar spin configurations and small magnon gap in the excitation spectrum. On the other hand, we also find a common feature in the field-angle dependence of the thermal Hall conductivity. It vanishes when the magnetic field is on the planes spanned by the spin axes. We reveal that the behavior is intrinsic to the Kitaev -Heisenberg model in an applied field and demonstrate that the introduction of the off-diagonal spin interaction causes the disappearance of the feature in the thermal Hall conductivity.

Field-angle-dependent multi-frequency electron spin resonance spectroscopy in submillimeter wave range based on thermal detection.

We report the thermally detected electron spin resonance (ESR) spectroscopy in the frequency range of millimeter and submillimeter waves. Under high vacuum conditions, a cantilever-shaped device detects ESR absorption of a mounted sample as a temperature difference in its beam direction. Despite the simple experimental setup, the spin sensitivity of the order of 1012 spins/G was achieved at 10K. The developed sample stage is small enough to be used in a 10T split-pair superconducting magnet with a bore of 25mm, enabling precise field-angle-dependent ESR measurements at multi-frequencies above 500GHz. We demonstrate its usefulness by studying the field-angle dependence of the excitation energy of the dimer triplet state in the Shastry-Sutherland magnet SrCu2(BO3)2.

Characteristic singular behaviors of nodal-line materials emerging in orbital magnetic susceptibility and Hall conductivity

The bulk properties of nodal line materials have been an important research topic in recent years. In this paper, we study the orbital magnetic susceptibility and the Hall conductivity of nodal line materials using the formalism with thermal Green's functions and find characteristic singular behaviors of them. It is shown that, in the vicinity of the gapless nodal line, the orbital magnetic susceptibility shows a $\delta$-function singularity and the Hall conductivity shows a step function behavior in their chemical potential dependences. Furthermore, these singular behaviors are found to show strong field angle dependences corresponding to the orientation of the nodal line in the momentum space. These singular behaviors and strong angle dependences will give clear evidence for the presence of the nodal line and its orientation and can be used to experimentally detect nodal line materials.