Mn Te Research Articles

Magnetically tunable supercurrent in dilute magnetic topological insulator-based Josephson junctions

A superconductor, when exposed to a spin-exchange field, can exhibit spatial modulation of its order parameter, commonly referred to as the Fulde–Ferrell–Larkin–Ovchinnikov state. Such a state can be induced by controlling the spin-splitting field in Josephson junction devices, allowing access to a wide range of the phase diagram. Here we demonstrate that a Fulde–Ferrell–Larkin–Ovchinnikov state can be induced in Josephson junctions based on the two-dimensional dilute magnetic topological insulator (Hg,Mn)Te. We do this by observing the dependence of the critical current on the magnetic field and temperature. The substitution of Mn dopants induces an enhanced Zeeman effect, which can be controlled with high precision by using a small external magnetic field. We observe multiple re-entrant behaviours of the critical current as a response to an in-plane magnetic field, which we assign to transitions between ground states with a phase shifted by π. This will enable the study of the Fulde–Ferrell–Larkin–Ovchinnikov state in much more accessible experimental conditions.

Spectral Asymmetry Induces a Re-Entrant Quantum Hall Effect in a Topological Insulator.

The band inversion of topological materials in three spatial dimensions is intimately connected to the parity anomaly of 2D massless Dirac fermions, known from quantum field theory. At finite magnetic fields, the parity anomaly reveals itself as a non-zero spectral asymmetry, i.e., an imbalance between the number of conduction and valence band Landau levels, due to the unpaired zero Landau level. This work reports the realization of this 2D Dirac physics at a single surface of the 3D topological insulator (Hg,Mn)Te. An unconventional re-entrant sequence of quantized Hall plateaus in the measured Hall resistance can be directly related to the occurrence of spectral asymmetry in a single topological surface state. The effect should be observable in any topological insulator where the transport is dominated by a single Dirac surfacestate.

Polarized Raman scattering and magnetic ordering in Mn lightly doped van der Waals Cr1−xMnxGeTe3 crystals

A layered van der Waals ferromagnet CrGeTe3 has attracted considerable interest due to the great potential for exploring low-power functional spintronic devices. However, research on the incorporation of chemical substitution in CrGeTe3 to tune the lattice vibrations and magnetic interactions is still lacking. In this work, an understanding of the phonon property and magnetic orders in Mn lightly doped Cr1−xMnxGeTe3 single crystals has been reported. It is found that the phonon symmetries are preserved over the studied Mn substitution content, as confirmed by angle-resolved polarized Raman spectroscopy and first-principles calculations. Furthermore, clear signatures of improved ferromagnetic Curie–Weiss temperature and magnetic moment are observed by detailed magnetic measurements. The higher magnetic moment of Mn ions, the variation of Mn–Te–Cr angle, Mn-induced hole doping, and the concomitant strain effect may all contribute to the enhanced ferromagnetism. Overall, our results demonstrate that chemical substitution is an effective pathway to engineering the lattice and magnetism of CrGeTe3.

Bridgman-Grown (Cd,Mn)Te and (Cd,Mn)(Te,Se): A Comparison of Suitability for X and Gamma Detectors.

This study explores the suitability of (Cd,Mn)Te and (Cd,Mn)(Te,Se) as room-temperature X-ray and gamma-ray detector materials, grown using the Bridgman method. The investigation compares their crystal structure, mechanical and optical properties, and radiation detection capabilities. Both crystals can yield large-area single crystal samples measuring approximately 30 × 30 mm2. In low-temperature photoluminescence analysis, both materials showed defect states, and annealing in cadmium vapors effectively eliminated donor-acceptor pair luminescence in (Cd,Mn)Te but not in (Cd,Mn)(Te,Se). Moreover, harder (Cd,Mn)(Te,Se) exhibited a higher etch pit density compared to softer (Cd,Mn)Te. X-ray diffraction examination revealed uniform lattice constant distribution in both compounds, with variations at a part per million level. (Cd,Mn)Te crystals demonstrated excellent single crystal properties with narrower omega scan widths, while (Cd,Mn)(Te,Se) exhibited a high contribution of block-like structures with significantly larger misorientation angles. Spectroscopic evaluations revealed better performance of a pixelated (Cd,Mn)Te detector, in comparison to (Cd,Mn)(Te,Se), achieving a mean full width at half maximum of 14% for the 122 keV gamma peak of Co-57. The reduced performance of the (Cd,Mn)(Te,Se) detector may be attributed to deep trap-related luminescence or block-like structures with larger misorientation angles. In conclusion, Bridgman-grown (Cd,Mn)Te emerges as a more promising material for X-ray and gamma-ray detectors when compared to (Cd,Mn)(Te,Se).

Double resonant Raman scattering and photoluminescence mediated by heavy and light hole excitons in a (Cd, Mn)Te quantum well in external magnetic field

Double resonant Raman scattering and photoluminescence mediated by heavy and light hole excitons in a (Cd, Mn)Te quantum well in external magnetic field

The Forms and Evolution of Free‐Cutting Phase in Bi–Te–S Free‐Cutting Steel

The free‐cutting phase (FCP) plays a crucial role in the machinability of steel materials. This article investigates the existence forms of the Bi–Te–S FCP in steel according to 2D and 3D morphology and observes its evolution process through a high‐temperature confocal laser scanning microscope combined with thermodynamic calculation and crystallographic analysis. The forms of Bi–Te–S composite FCP are classified comprehensively. Six types of Bi–sulfide exist in this work, including separate (SP), cavity bridge (CB), acute angle coated (AAC), obtuse angle coated (OAC), and semicoated types (SC). Five types of Te–sulfide existence forms are found, including CB, AAC, OAC, SC, and complete‐coated types (CC). Regarding the existence forms of Bi–Te–sulfide, five types are identified, including SP, CB, OAC, SC, and CC. The evolution of Bi–Te–S composite FCP is closely related to the proportion of Bi, Te, and S. When the Te/S ratio is below 0.07, Bi–Mn(Te, S)‐bridged two sulfides form, breaking the chain‐like FCP and avoiding the formation of chain‐like FCP. Te mainly dissolves in the sulfide, promoting spherical sulfide formation. When the Te/S ratio is beyond 0.07, MnTe precipitates easily on MnS with more nucleation sites and grows to larger‐sized FCP; subsequently, Bi precipitates around Mn(Te, S), forming the large‐sized spherical MnS–MnTe–Bi FCP.

Interplay between Magnetism and Topology in MnBi2Te4

The dependence of the topology of the fermion excitation spectrum on the magnetic state of the system is analyzed taking into account the structure of the Te–Mn–Te trilayer in the Te–Bi–Te–Mn–Te–Bi–Te layer sequence of the MnBi2Te4 van der Waals single crystal, crystal field effects, spin–orbit coupling, and the covalent mixing of electronic states of Mn2+ ions with electronic states of Te2– ions in the strong electron correlation regime. The Chern number in the ferromagnetic phase, which is due to the kinematic interaction between Hubbard fermions, is equal to 1; i.e., the topology of the band structure of the Te–Mn–Te trilayer is nontrivial. The Chern number in the paramagnetic phase is zero; i.e., the topology is trivial. The magnetic moments of Mn2+ ions for the constructed spin orbitals are perpendicular to the layers. The magnetic moments of Mn2+ ions in the nearest layers are antiferromagnetically ordered via the Anderson mechanism.

Role of Magnetic Defects in Tuning Ground States of Magnetic Topological Insulators (Adv. Mater. 21/2023)

Magnetic Topological Insulators Inelastic neutron scattering measurements, as reported by Farhan Islam and co-workers in article number 2209951, reveal the formation of linear Mn–Te–Mn antiferromagnetic dimers in Mn-doped Sb2Te3 magnetic topological insulators, following the Goodenough–Kanamori superexchange rule. The experiments also identify ferromagnetic correlations within quintuple blocks and across the van der Waals gap, which mediate long-range ferromagnetic order. These competing interactions explain the role of defects in tuning ground states in magnetic topological insulators.

Quantum confinement in optically active ultrathin (Cd,Mn)Te/(Cd,Mg)Te core/shell nanowires

The fabrication of ultrathin (Cd,Mn)Te nanowires with diameters reaching values below 10 nm is reported. The thinning procedure relies on a thermally induced reverse reaction process applied to (Cd,Mn)Te nanowires grown using the vapor-liquid-solid method. The impact of quantum confinement is manifested by a distinct blueshift of the excitonic emission from individual nanowires. Cathodoluminescence mapping reveals that the optical emission originates from relatively short nanowire segments, which is explained in terms of exciton localization on irregularities of nanowire core diameter. Magneto-optical study performed on several individual nanowires reveals a change from light hole to heavy hole character of the excitonic emission depending on the emission energy, i.e., on the size of the emitting object. The results are interpreted in terms of the influence of axial and radial quantum confinement on excitons in ultrathin nanowires.

Quantitative theory of backscattering in topological HgTe and (Hg,Mn)Te quantum wells: Acceptor states, Kondo effect, precessional dephasing, and bound magnetic polaron

We present the theory and numerical evaluations of the backscattering rate determined by acceptor holes or Mn spins in HgTe and (Hg,Mn)Te quantum wells in the quantum spin Hall regime. The role of anisotropic s-p and sp-d exchange interactions, Kondo coupling, Luttinger liquid effects, precessional dephasing, and bound magnetic polarons is quantified. The determined magnitude and temperature dependence of conductance are in accord with experimental results for HgTe and (Hg,Mn)Te quantum wells.

Effects of Charge Dopants in Quantum Spin Hall Materials.

Semiconductors' sensitivity to electrostatic gating and doping accounts for their widespread use in information communication and new energy technologies. It is demonstrated quantitatively and with no adjustable parameters that the presence of paramagnetic acceptor dopants elucidates a variety of hitherto puzzling properties of two-dimensional topological semiconductors at the topological phase transition and in the regime of the quantum spin Hall effect. The concepts of resonant states, charge correlation, Coulomb gap, exchange interaction between conducting electrons and holes localized on acceptors, strong coupling limit of the Kondo effect, and bound magnetic polaron explain a short topological protection length, high hole mobilities compared with electron mobilities, and different temperature dependence of the spin Hall resistance in HgTe and (Hg,Mn)Te quantum wells.

Effect of doping and annealing on resistivity, mobility-lifetime product, and detector response of (Cd,Mn)Te

We present the effect of doping with In, V, and Cl on the values of resistivity (ρ) and mobility-lifetime (μτ) in (Cd,Mn)Te crystals grown by the Bridgman method and with diameters of 2 or 3 in. This material is investigated to use in room-temperature X-ray and gamma-ray detectors. We carried out post-growth annealing processes in cadmium vapors at 500 °C to obtain more homogeneous crystal properties. The EU-ρ-μτ-SCAN machine, which uses the principle of time-dependent charge measurement, allowed us to map the values of resistivity and mobility-lifetime in crystal plates. High values of those two quantities are crucial in nuclear detector applications. Crystals doped with In show inhomogeneous properties, which makes the fabrication of competitive large-area crystals more difficult. Low-temperature post-growth annealing in Cd vapors is insufficient for (Cd,Mn)Te:In crystals, where the decrease of ρ and μτ values is observed. Such annealing works for (Cd,Mn)Te:V crystals, where the mean values of ρ and μτ do not change significantly, but their distributions are more uniform. Both dopants, vanadium and indium, used together with an excess of Te, effectively increased ρ and μτ. Using the 57Co source, the spectroscopic performance of each pixel of the detector co-doped with In and V was shown.

Energy and spin relaxation of excitons in CdMnTe/CdMgTe quantum wells

The energy and spin relaxation of excitons and trions is studied in the (Cd, Mn)Te/(Cd,Mg)Te quantum wells containing hole magnetic polarons by means of polarized photoluminescence technique. It was found that under circularly polarized photoexcitation with an energy higher than the ground state energy of the exciton, photoluminescence is also polarized. This fact points to the presence of the spin memory of the excitons in these systems. It is suggested that the long spin memory is due to the magnetic polaron formation stabilizing the spin of the exciton. Under quasiresonant excitation of the trion states an inverted sign of circular polarization degree is observed in the radiation spectrum. Such inversion of the polarization degree requires the special mechanism of the spin relaxation, associated with the spin flip of an electron in the trion magnetic polaron state. Application of the external magnetic field in the Voigt geometry leads to the depolarization of the excitonic radiation. The cause of this depolarization is the destabilization of the magnetic polaron and the shortening of the spin relaxation time of the exciton.

Magnetic ion relaxation time distribution within a quantum well

Time-resolved optically detected magnetic resonance (ODMR) is a valuable technique to study the local deformation of the crystal lattice around magnetic ion as well as the ion spin relaxation time. Here we utilize selective Mn-doping to additionally enhance the inherent locality of the ODMR technique. We present the time-resolved ODMR studies of single {(Cd,Mg)Te/(Cd,Mn)Te} quantum wells (QWs) with manganese ions located at different positions along the growth axis -- in the center or on the sides of the quantum well. We observe that spin-lattice relaxation of Mn$^{2+}$ significantly depends on the ion-carrier wavefunction overlap at low-magnetic fields. Interestingly, the effect is clearly observed in spite of very low carrier density, which suggests the potential for control of the Mn$^{2+}$ ion relaxation rate by means of the electric field in future experiments.

Nonreciprocal electrical transport in the multiferroic semiconductor (Ge,Mn)Te

We have investigated the nonreciprocal electrical transport, that is a nonlinear resistance effect depending on the current direction, in multiferroic Rashba semiconductor (Ge,Mn)Te. Due to coexistence of ferromagnetic and ferroelectric orders, (Ge,Mn)Te provides a unique platform for exploring the nonreciprocal electrical transport in a bulk form. (Ge,Mn)Te thin films shows a large nonreciprocal resistance compared to GeTe, the nonmagnetic counterpart with the same crystal structure. The magnetic-field-angle dependence of the nonreciprocal resistance is maximized when magnetic field is orthogonal to both current and electric polarization, in accord with the symmetry argument. From the analysis of temperature and magnetic field dependence, we deduce that inelastic scatterings of electrons mediated by magnons dominantly contribute to the observed nonreciprocal response. Furthermore, the nonreciprocal resistance is significantly enhanced by lowering hole density. The Fermi level dependence is attributed to the deformation of the Rashba band in which the spin-momentum locked single Fermi surface appears by exchange field from the in-plane magnetization. The present study provides a key insight to the mechanisms of novel transport phenomena caused by the interplay of ferroelectric and ferromagnetic orders in a semiconductor.

Ultraefficient resistance switching between charge ordered phases in 1T-TaS2 with a single picosecond electrical pulse

Progress in high-performance computing demands significant advances in memory technology. Among novel memory technologies that promise efficient device operation on a sub-ns timescale, resistance switching between charge ordered phases of 1T-TaS2 has shown to be potentially useful for development of high-speed, energy efficient nonvolatile memory devices. Measurement of the electrical operation of such devices in the picosecond regime is technically challenging and hitherto still largely unexplored. Here, we use an optoelectronic “laboratory-on-a-chip” experiment for measurement of ultrafast memory switching, enabling accurate measurement of electrical switching parameters with 100 fs temporal resolution. Photoexcitation and electro-optic sampling on a (Cd,Mn)Te substrate are used to generate and, subsequently, measure electrical pulse propagation with intra-band excitation and sub-gap probing, respectively. We demonstrate high contrast nonvolatile resistance switching from high to low resistance states of a 1T-TaS2 device using single sub-2 ps electrical pulses. Using detailed modeling, we find that the switching energy density per unit area is exceptionally small, EA= 9.4 fJ/μm2. The speed and energy efficiency of an electronic “write” process place the 1T-TaS2 devices into a category of their own among new generation nonvolatile memory devices.

Polishing of CdTe, Cd(Zn)Te, Cd(Mn)Te Single Crystals by Iodine in Dimethylformamide

The process of chemical-mechanical polishing of CdTe, Cd0,9Zn0,1Te and Cd0.95Mn0.05Te surface by etchants, based on solutions of the I2-dimethylformamide system, has been studied. Ethylene glycol and lactic acid were used as modifying components. The dependences of chemical-mechanical polishing rates on the content of ethylene glycol and lactic acid in the composition of the basic solution have been studied. The technological stability of the developed etchants has been determined. Qualitative characteristics of the surface are established by atomic force microscopy and non-contact profilographic analysis. Based on the obtained results, the compositions of etching solutions and technological modes of chemical-mechanical treatment of CdTe, Cd0,9Zn0,1Te and Cd0.95Mn0.05Te single crystals were optimized. Etchants modified with an organic component are promising for use in the semiconductor materials technology in the case when the main goal is to obtain high-quality, clean from contaminants and impurities, structurally perfect surface.

Lattice Dynamics of Bi2Те3 and Vibrational Modes in Raman Scattering of Topological Insulators MnBi2Te4·n(Bi2Te3)

This work is devoted to the experimental study and symmetry analysis of the Raman-active vibration modes in MnBi2Te4·n(Bi2Te3) van der Waals topological insulators, where n is the number of Te–Bi–Te–Bi–Te quintuple layers between two neighboring Te–Bi–Te–Mn–Te–Bi–Te septuple layers. Confocal Raman spectroscopy is applied to study Raman spectra of crystal structures with n = 0,1,2,3,4,5,6, and ∞. The experimental frequencies of vibration modes of the same symmetry in the structures with different n are compared. The lattice dynamics of free-standing one, three, and four quintuple layers, as well as of bulk Bi2Те3(n = infty ) and MnBi2Te4(n = 0), is considered theoretically. Vibrational modes of the last two systems have the same symmetry, but different displacement fields. These fields in the case of a Raman-active mode do not contain displacements of manganese atoms for any finite n. It is shown that two vibrational modes in the low-frequency region of the spectrum (35–70 cm–1) of structures with n = 1,;2,;3,;4,;5, and 6 practically correspond to the lattice dynamics of n free quintuple Bi2Те3 layers. For this reason, the remaining two vibration modes, which are observed in the high-frequency region of the spectrum (100–140 cm–1) and are experimentally indistinguishable in the sense of belonging to quintuple or septuple layer or to both layers simultaneously, should also be assigned to vibrations in quintuple layers under immobile atoms of septuple layers.

Finite Field Transport Response of a Dilute Magnetic Topological Insulator-Based Josephson Junction.

Hybrid samples combining superconductors with magnetic topological insulators are a promising platform for exploring exotic new transport physics. We examine a Josephson junction of such a system based on the dilute magnetic topological insulator (Hg,Mn)Te and the type II superconductor MoRe. In the zero and very low field limits, to the best of our knowledge, the device shows, for the first time, an induced supercurrent through a magnetically doped semiconductor, in this case, a topological insulator. At higher fields, a rich and hysteretic magnetoresistance is revealed. Careful analysis shows that the explanation of this behavior can be found in magnetic flux focusing stemming from the Meissner effect in the superconductor, without invoking any role of proximity-induced superconductivity. The phenomena is important because it will ubiquitously coexist with any exotic new physics that may be present in this class of devices.

Surface Recombination and Space-Charge-Limited Photocurrent-Voltage (PC-V) Measurements in (Cd,Mn)Te Samples-Kinetics of Photocurrent (PC).

Photocurrent-voltage characteristic (PC-V) is a method of determining the critical parameter in X-ray and gamma-ray detector plates, i.e., the carrier mobility-lifetime product, μτ. We show for the (Cd,Mn)Te samples that the measurement results depend strongly on the surface treatment and the space charge distribution. The PC-V characteristics obtained for ħω > Eg and ħω ~ Eg indicated that etching with 20% HCl caused an appearance of a significant concentration of very shallow surface traps at the (Cd,Mn)Te sample surface. These traps seriously changed the results of measurements of PC-V characteristics and PC kinetics. We also noticed a small contribution of holes to photoconductivity in the PC kinetics. The measurements of PC-V characteristics for ħω > Eg may test the detector plate surface quality.