Anomalous Hall effect in Nd-doped Bi1.1Sb0.9STe2 topological insulator single crystals.

Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantum anomalous Hall effect, which has been observed in magnetic-topological-insulator-based devices. In this work, we report successful doping of rare-earth element Nd into Bi1.1Sb0.9STe2 bulk-insulating topological insulator single crystals, in which the Nd moments are ferromagnetically ordered at ∼100 K. Benefiting from the in-bulk-gap Fermi level, electronic transport behaviors dominated by the topological surface states are observed in the ferromagnetic region. At low temperatures, strong Shubnikov-de Haas oscillations with a nontrivial Berry phase are observed. The topological insulator with long range magnetic ordering in Nd-doped Bi1.1Sb0.9STe2 single crystals provides a good platform for quantum transport studies and spintronic applications.

High-mobility spin-polarized two-dimensional electron gas at the interface of EuTiO3/SrTiO3heterostructures.

Spin polarization of two-dimensional electron gas (2DEG) at the interface of EuTiO3/SrTiO3(STO) heterostructures has been theoretical predicted and experimentally observed via x-ray magnetic circular dichroism and polarized x-ray absorption spectroscopy, which, however, is lack of magnetotransport evidence. Here, we report the fabrication of high-quality EuTiO3/STO heterostructures by depositing antiferromagnetic insulating EuTiO3thin films onto STO substrates. Shubnikov-de Haas oscillation, Hall, and magnetoresistance (MR) measurements show that the interface is not only highly conducting, with electron mobility up to5.5×103cm2V-1s-1at 1.8 K, but also shows low-field hysteretic MR effects. MR of ∼9% is observed at 1.8 K and 20 Oe, which is one order of magnitude higher than those observed in other spin-polarized 2DEG oxide systems. Moreover, the heterostructures show ferromagnetic hysteresis loops. These results demonstrate that the high-mobility 2DEG is spin polarized, whose origin is attributed to the interfacial Ti3+-3dstates due to oxygen deficiency and the exchange interactions between interfacial Eu spins and itinerant Ti-3delectrons.

Coexistence of logarithmic and SdH quantum oscillations in ferromagnetic Cr-doped tellurium single crystals.

We report the synthesis of transition-metal-doped ferromagnetic elemental single-crystal semiconductors with quantum oscillations using the physical vapor transport method. The 7.7 atom% Cr-doped Te crystals (Cr:Te) show ferromagnetism, butterfly-like negative magnetoresistance in the low temperature (<3.8 K) and low field (<0.15 T) region, and high Hall mobility, e.g. 1320 cm2V-1s-1at 30 K and 350 cm2V-1s-1at 300 K, implying that Cr:Te crystals are ferromagnetic elemental semiconductors. WhenB// [001] // I, the maximum negative MR is ∼-27% atT= 20 K andB= 8 T. In the low temperature semiconducting region, Cr:Te crystals show strong discrete scale invariance dominated logarithmic quantum oscillations when the direction of the magnetic fieldBis parallel to the [100] crystallographic direction (B// [100]) and show Landau quantization dominated Shubnikov-de Haas oscillations forB// [210] direction, which suggests the broken rotation symmetry of the Fermi pockets in the Cr:Te crystals. The findings of coexistence of multiple quantum oscillations and ferromagnetism in such an elemental quantum material may inspire more study of narrow bandgap semiconductors with ferromagnetism and quantum phenomena.

Top-down patterning of topological surface and edge states using a focused ion beam.

The conducting boundary states of topological insulators appear at an interface where the characteristic invariant ℤ2 switches from 1 to 0. These states offer prospects for quantum electronics; however, a method is needed to spatially-control ℤ2 to pattern conducting channels. It is shown that modifying Sb2Te3 single-crystal surfaces with an ion beam switches the topological insulator into an amorphous state exhibiting negligible bulk and surface conductivity. This is attributed to a transition from ℤ2 = 1 → ℤ2 = 0 at a threshold disorder strength. This observation is supported by density functional theory and model Hamiltonian calculations. Here we show that this ion-beam treatment allows for inverse lithography to pattern arrays of topological surfaces, edges and corners which are the building blocks of topological electronics.

Spin Reorientation Transition and Negative Magnetoresistance in Ferromagnetic NdCrSb3 Single Crystals.

High-quality NdCrSb3 single crystals are grown using a Sn-flux method, for electronic transport and magnetic structure study. Ferromagnetic ordering of the Nd3+ and Cr3+ magnetic sublattices are observed at different temperatures and along different crystallographic axes. Due to the Dzyaloshinskii-Moriya interaction between the two magnetic sublattices, the Cr moments rotate from the b axis to the a axis upon cooling, resulting in a spin reorientation (SR) transition. The SR transition is reflected by the temperature-dependent magnetization curves, e.g., the Cr moments rotate from the b axis to the a axis with cooling from 20 to 9 K, leading to a decrease in the b-axis magnetization f and an increase in the a-axis magnetization. Our elastic neutron scattering along the a axis shows decreasing intensity of magnetic (300) peak upon cooling from 20 K, supporting the SR transition. Although the magnetization of two magnetic sublattices favours different crystallographic axes and shows significant anisotropy in magnetic and transport behaviours, their moments are all aligned to the field direction at sufficiently large fields (30 T). Moreover, the magnetic structure within the SR transition region is relatively fragile, which results in negative magnetoresistance by applying magnetic fields along either a or b axis. The metallic NdCrSb3 single crystal with two ferromagnetic sublattices is an ideal system to study the magnetic interactions, as well as their influences on the electronic transport properties.

Colossal Magnetoresistance in Ti Lightly Doped Cr2Se3 Single Crystals with a Layered Structure.

Stoichiometric Cr2Se3 single crystals are particular layer-structured antiferromagnets, which possess a noncollinear spin configuration, weak ferromagnetic moments, moderate magnetoresistance (MR ∼14.3%), and poor metallic conductivity below the antiferromagnetic phase transition. Here, we report an interesting >16 000% colossal magnetoresistance (CMR) effect in Ti (1.5 atomic percent) lightly doped Cr2Se3 single crystals. Such a CMR is approximately 1143 times larger than that of the stoichiometric Cr2Se3 crystals and is rarely observed in layered antiferromagnets and is attributed to the frustrated spin configuration. Moreover, the Ti doping not only dramatically changes the electronic conductivity of the Cr2Se3 crystal from a bad metal to a semiconductor with a gap of ∼15 meV but also induces a change in the magnetic anisotropy of the Cr2Se3 crystal from strong out-of-plane to weak in-plane. Further, magnetotransport measurements reveal that the low-field MR scales with the square of the reduced magnetization, which is a signature of CMR materials. The layered Ti:Cr2Se3 with the CMR effect could be used as two-dimensional (2D) heterostructure building blocks to provide colossal negative MR in spintronic devices.

Magnetic plasmon resonances in nanostructured topological insulators for strongly enhanced light-MoS2 interactions.

Magnetic resonances not only play crucial roles in artificial magnetic materials but also offer a promising way for light control and interaction with matter. Recently, magnetic resonance effects have attracted special attention in plasmonic systems for overcoming magnetic response saturation at high frequencies and realizing high-performance optical functionalities. As novel states of matter, topological insulators (TIs) present topologically protected conducting surfaces and insulating bulks in a broad optical range, providing new building blocks for plasmonics. However, until now, high-frequency (e.g. visible range) magnetic resonances and related applications have not been demonstrated in TI systems. Herein, we report for the first time, to our knowledge, a kind of visible range magnetic plasmon resonances (MPRs) in TI structures composed of nanofabricated Sb2Te3 nanogrooves. The experimental results show that the MPR response can be tailored by adjusting the nanogroove height, width, and pitch, which agrees well with the simulations and theoretical calculations. Moreover, we innovatively integrated monolayer MoS2 onto a TI nanostructure and observed strongly reinforced light-MoS2 interactions induced by a significant MPR-induced electric field enhancement, remarkable compared with TI-based electric plasmon resonances (EPRs). The MoS2 photoluminescence can be flexibly tuned by controlling the incident light polarization. These results enrich TI optical physics and applications in highly efficient optical functionalities as well as artificial magnetic materials at high frequencies.

Magnetotransport and magnetic properties of the layered noncollinear antiferromagnetic Cr2Se3 single crystals.

We report on the growth of high-quality stoichiometric layered Cr2Se3 single crystals with metallic and noncollinear antiferromagnetic ground state using the chemical vapor transport (CVT) method. The crystals show weak ferromagnetism in the in-plane and out-of-plane directions below the Neél temperature (T N), however, the field-cooled out-of-plane magnetization at 500 Oe and 10 K (∼0.24 µ B/f.u.) is approximately 15 times larger than that of the in-plane one, indicating strong c-axis easy uniaxial magnetic anisotropy, which is further supported by the in-plane and out-of-plane isothermal anisotropic magnetic hysteresis loops and the angular dependent magnetoresistance (MR). The latter also reveals a decrease of the coercive field of the crystal upon the tilting of the weak ferromagnetic easy axis away from the direction of the magnetic field. Further, the out-of-plane isothermal MR are negative below T N and show butterfly shapes for T < 10 K and couple with the magnetic hysteresis M(H) loop. These results may help researchers better understand the interplay between the weak ferromagnetism and the magnetotransport properties of 2D itinerant noncollinear antiferromagnetic systems.

Magnetotransport properties of square-net compounds of NbSiSb and NbGeSb single crystals.

We successfully grew single crystals of Si- and Ge-square-net compounds of NbSiSb and NbGeSb whose excellent crystalline quality are verified using single-crystal x-ray diffraction θ-2θ scans, rocking curves, scanning and transmission electron microscopies. Since these two compounds share major crystallographic similarity with the topological nodal-line semimetals of ZrSiS family, we employ density functional theory (DFT) calculations and magnetotransport measurements to demonstrate their band structures as well as the electron scattering mechanisms. DFT calculations show that the fermiology displays strong anisotropy from the crystallographic c-axis to the ab-plane and weak anisotropy within the ab plane, which is consistent with the strong anisotropic magnetotransport behaviors. Following the Kohler's scaling rule we prove that similar interband and intraband electron-phonon scattering mechanisms work in both the NbSiSb and NbGeSb compounds. The study of electronic transport mechanism in the presence of external magnetic field renders deep insight into topological behavior together with its Fermi surface, and the high similarity of crystallography and strong difference in band structures between the present single crystals and that of ZrSiS family provides the possibility to tune the band structure via element doping.

Graphene inclusion induced ultralow thermal conductivity and improved figure of merit in p-type SnSe.

The concept of composite material has been increasingly applied for the significant improvement in the thermoelectric performance because of the predictable effective medium properties and the unique interfacial correlated thermal and electrical transport mechanism. Herein, we report that the graphene inclusion can lead to a significant reduction in thermal conductivity and improve the overall thermoelectric figure-of-merit in SnSe. We demonstrate a systematic investigation on the microstructures and electrical and thermoelectric properties of the SnSe/graphene composite. HRTEM reveals the uniform distribution of graphene nanosheets in the SnSe matrix, forming a sharp interface with refined SnSe grain sizes and defects nearby the interfaces. Thermal conductivity decreases with graphene addition and can significantly reduce to as low as ∼0.18 W m-1 K-1, resulting in an enhanced figure of merit (ZT) of the SnSe/graphene composite by at least 50% compared with pristine SnSe. The significant reduction in thermal conductivity is attributed to the phonon scattering by densely distributed phase interfaces as well as defects and grain boundaries. The carbon element is also believed to potentially reduce long-range tin diffusion by acting as a confinement barrier to restrict heat and ion diffusion. Our work proves that the graphene secondary phase could enhance the ZT of the SnSe matrix, which might pave the way for achieving high-performance thermoelectric properties in carbon-induced composite materials.

Boosting Superconducting Properties of Fe(Se, Te) via Dual-Oscillation Phenomena Induced by Fluorine Doping.

Fluorine-doped Fe(Se, Te) has been successfully synthesized using the melting method. A dual-oscillation effect was found in the F-doped sample, which combined both microstructural oscillation and chemical compositional oscillation. The microstructural oscillation could be attributed to alternate growth of tetragonal ß-Fe(Se, Te) and hexagonal δ-Fe(Se, Te), which formed a pearlite-like structure and led to the enhancement of δ l flux pinning due to the alternating distributed nonsuperconducting δ-Fe(Se, Te) phase. The chemical compositional oscillations in ß-Fe(Se, Te) phase were because of the inhomogeneously distributed Se and Te, which changes the pinning mechanism from surface pinning in the undoped sample to Δκ pinning in the 5% F-doped one. As a result, the critical current, upper critical field, and thermally activated flux-flow activation energy of FeSe0.45Te0.5F0.05 were enhanced by 7, 2, and 3 times, respectively. Our work revealed the physical insights into F-doping resulting in high-performance Fe(Se, Te) superconductors and inspired a new approach to optimize superconductivities in iron-based superconductors through phase and element manipulations.

Nonvolatile and Reversible Ferroelectric Control of Electronic Properties of Bi2Te3 Topological Insulator Thin Films Grown on Pb(Mg1/3Nb2/3)O3-PbTiO3 Single Crystals.

Single-phase (00 l)-oriented Bi2Te3 topological insulator thin films have been deposited on (111)-oriented ferroelectric 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) single-crystal substrates. Taking advantage of the nonvolatile polarization charges induced by the polarization direction switching of PMN-PT substrates at room temperature, the carrier density, Fermi level, magnetoconductance, conductance channel, phase coherence length, and quantum corrections to the conductance can be in situ modulated in a reversible and nonvolatile manner. Specifically, upon the polarization switching from the positively poled Pr+ state (i.e., polarization direction points to the film) to the negatively poled Pr- (i.e., polarization direction points to the bottom electrode) state, both the electron carrier density and the Fermi wave vector decrease significantly, reflecting a shift of the Fermi level toward the Dirac point. The polarization switching from Pr+ to Pr- also results in significant increase of the conductance channel α from -0.15 to -0.3 and a decrease of the phase coherence length from 200 to 80 nm at T = 2 K as well as a reduction of the electron-electron interaction. All these results demonstrate that electric-voltage control of physical properties using PMN-PT as both substrates and gating materials provides a simple and a straightforward approach to realize reversible and nonvolatile tuning of electronic properties of topological thin films and may be further extended to study carrier density-related quantum transport properties of other quantum matter.

Tuning the Weak Ferromagnetic States in Dysprosium Orthoferrite.

RFeO3 orthoferrites, where R is a rare-earth ion of the lanthanide series, are attracting attention mostly because of their promising fast spin dynamics. The magnetic properties of these materials seem to crucially depend on whether the magnetizations of the R and Fe ions' weak ferromagnetic (WFM) components are parallel or antiparallel to each other. Here, we report an extensive investigation of a high-quality DyFeO3 single crystal in which the induced Dy3+ magnetization (FDy) has a natural tendency to be antiparallel to Fe3+ sublattice magnetization (FFe) within a large temperature window. Moreover, we find that specific variations of temperature and applied magnetic fields allow us to make FDy parallel to FFe, or force a spin-flip transition in FFe, among other effects. We found three different magnetic states that respond to temperature and magnetic fields, i.e. linear versus constant or, alternatively, presenting either behavior depending on the history of the sample. An original magnetic field-versus-temperature phase diagram is constructed to indicate the region of stability of the different magnetic phases, and to reveal the precise conditions yielding sudden spin switching and reversals. Knowledge of such a phase diagram is of potential importance to applications in spintronics and magnetic devices.

Semiconductor/Piezoelectrics Hybrid Heterostructures with Highly Effective Gate-Tunable Electrotransport and Magnetic Behaviors.

We report the epitaxial growth of oxygen deficient titanium dioxide thin films on 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) single crystals and realized highly effective in situ electrostatic manipulation of electrotransport and magnetism of TiO2-δ films via gate voltages. Upon the polarization switching in the PMN-PT, the carrier density of the TiO2-δ film could be reversibly modified, resulting in a large nonvolatile resistivity modulation by ∼51% at T = 300 K, approximately 4-12 times larger than that of other transition-metal oxide film/PMN-PT structures. By taking advantage of in situ manipulation of the carrier density via gate voltages, we found that competition between the trap of electrons by the Ti3+-VO pairs and that by the positive polarization charges at the interface results in a significant resistivity relaxation upon the polarization switching, and revealed that magnetization is inversely correlated with the carrier density of the TiO2-δ film. Such hybrid structures combining materials with dissimilar functionalities may have potential applications in multifunctional devices which can take advantage of the useful and unique properties of both materials.