Yuheng Zhang

Pressure-induced superconductivity in a three-dimensional topological material ZrTe5

Significance Three-dimensional (3D) Dirac semimetals have attracted a lot of advanced research recently on many exotic properties and their association with crystalline and electronic structures under extreme conditions. As one of the fundamental state parameters, high pressure is an effective, clean way to tune lattice as well as electronic states, especially in quantum states, thus their electronic and magnetic properties. In this paper, by combining multiple experimental probes (synchrotron X-ray diffraction, low-temperature transport under magnetic field) and theoretical investigations, we discover the pressure-induced 3D Dirac semimetal to superconductor transition in ZrTe5. As a new type of topological materials, ZrTe5 shows many exotic properties under extreme conditions. Using resistance and ac magnetic susceptibility measurements under high pressure, while the resistance anomaly near 128 K is completely suppressed at 6.2 GPa, a fully superconducting transition emerges. The superconducting transition temperature Tc increases with applied pressure, and reaches a maximum of 4.0 K at 14.6 GPa, followed by a slight drop but remaining almost constant value up to 68.5 GPa. At pressures above 21.2 GPa, a second superconducting phase with the maximum Tc of about 6.0 K appears and coexists with the original one to the maximum pressure studied in this work. In situ high-pressure synchrotron X-ray diffraction and Raman spectroscopy combined with theoretical calculations indicate the observed two-stage superconducting behavior is correlated to the structural phase transition from ambient Cmcm phase to high-pressure C2/m phase around 6 GPa, and to a mixture of two high-pressure phases of C2/m and P-1 above 20 GPa. The combination of structure, transport measurement, and theoretical calculations enable a complete understanding of the emerging exotic properties in 3D topological materials under extreme environments.

Pressure-Induced New Topological Weyl Semimetal Phase in TaAs

TaAs as one of the experimentally discovered topological Weyl semimetal has attracted intense interests recently. The ambient TaAs has two types of Weyl nodes which are not on the same energy level. As an effective way to tune lattice parameters and electronic interactions, high pressure is becoming a significant tool to explore new materials as well as their exotic states. Therefore, it is highly interesting to investigate the behaviors of topological Weyl fermions and possible structural phase transitions in TaAs under pressure. Here, with a combination of ab initio calculations and crystal structure prediction techniques, a new hexagonal P-6m2 phase is predicted in TaAs at pressure around 14 GPa. Surprisingly, this new phase is a topological semimetal with only single set of Weyl nodes exactly on the same energy level. The phase transition pressure from the experimental measurements, including electrical transport measurements and Raman spectroscopy, agrees with our theoretical prediction reasonably. Moreover, the P-6m2 phase seems to be quenched recoverable to ambient pressure, which increases the possibilities of further study on the exotic behaviors of single set of Weyl fermions, such as the interplay between surface states and other properties.

Direct imaging of magnetic field-driven transitions of skyrmion cluster states in FeGe nanodisks

Significance The rapid growth of data volume demands faster and denser storage devices. The noncoplanar swirling spin texture, known as magnetic skyrmion, has potential application in future memory devices. To realize such applications, it is essential to understand the properties of individual skyrmion in patterned nanoelements. While quite a number of theoretical efforts have been made in this field, direct experimental demonstration in such a real modeling system is a challenge. Here, we report the direct visualization of skyrmion cluster states in FeGe nanodisks. We determine the common relationship among the temperature, magnetic field, and disk size. These results have an immediate implication for designing future skyrmion-based devices. Magnetic skyrmion is a nanosized magnetic whirl with nontrivial topology, which is highly relevant for applications on future memory devices. To enable the applications, theoretical efforts have been made to understand the dynamics of individual skyrmions in magnetic nanostructures. However, directly imaging the evolution of highly geometrically confined individual skyrmions is challenging. Here, we report the magnetic field-driven dynamics of individual skyrmions in FeGe nanodisks with diameters on the order of several skyrmion sizes by using Lorentz transmission electron microscopy. In contrast to the conventional skyrmion lattice in bulk, a series of skyrmion cluster states with different geometrical configurations and the field-driven cascading phase transitions are identified at temperatures far below the magnetic transition temperature. Furthermore, a dynamics, namely the intermittent jumps between the neighboring skyrmion cluster states, is found at elevated temperatures, at which the thermal energy competes with the energy barrier between the skyrmion cluster states.

Experimental observation of chiral magnetic bobbers in B20-type FeGe

Chiral magnetic skyrmions1,2 are nanoscale vortex-like spin textures that form in the presence of an applied magnetic field in ferromagnets that support the Dzyaloshinskii–Moriya interaction (DMI) because of strong spin–orbit coupling and broken inversion symmetry of the crystal3,4. In sharp contrast to other systems5,6 that allow for the formation of a variety of two-dimensional (2D) skyrmions, in chiral magnets the presence of the DMI commonly prevents the stability and coexistence of topological excitations of different types7. Recently, a new type of localized particle-like object—the chiral bobber (ChB)—was predicted theoretically in such materials8. However, its existence has not yet been verified experimentally. Here, we report the direct observation of ChBs in thin films of B20-type FeGe by means of quantitative off-axis electron holography (EH). We identify the part of the temperature–magnetic field phase diagram in which ChBs exist and distinguish two mechanisms for their nucleation. Furthermore, we show that ChBs are able to coexist with skyrmions over a wide range of parameters, which suggests their possible practical applications in novel magnetic solid-state memory devices, in which a stream of binary data bits can be encoded by a sequence of skyrmions and bobbers.Electron holography enables direct experimental verification of the existence of chiral bobbers in thin films of chiral magnets.The use of chiral skyrmions, which are nanoscale vortex-like spin textures, as movable data bit carriers forms the basis of a recently proposed concept for magnetic solid-state memory. In this concept, skyrmions are considered to be unique localized spin textures, which are used to encode data through the quantization of different distances between identical skyrmions on a guiding nanostripe. However, the conservation of distances between highly mobile and interacting skyrmions is difficult to implement in practice. Here, we report the direct observation of another type of theoretically-predicted localized magnetic state, which is referred to as a chiral bobber (ChB), using quantitative off-axis electron holography. We show that ChBs can coexist together with skyrmions. Our results suggest a novel approach for data encoding, whereby a stream of binary data representing a sequence of ones and zeros can be encoded via a sequence of skyrmions and bobbers. The need to maintain defined distances between data bit carriers is then not required. The proposed concept of data encoding promises to expedite the realization of a new generation of magnetic solid-state memory.

Concurrence of superconductivity and structure transition in Weyl semimetal TaP under pressure

Weyl semimetal defines a material with three dimensional Dirac cones which appear in pair due to the breaking of spatial inversion or time reversal symmetry. Superconductivity is the state of quantum condensation of paired electrons. Turning a Weyl semimetal into superconducting state is very important in having some unprecedented discoveries. In this work, by doing resistive measurements on a recently recognized Weyl semimetal TaP under pressure up to about 100 GPa, we observe superconductivity at about 70 GPa. The superconductivity retains when the pressure is released. The systematic evolutions of resistivity and magnetoresistance with pressure are well interpreted by the relative shift between the chemical potential and paired Weyl points. Calculations based on the density functional theory also illustrate the structure transition at about 70GPa, the phase at higher pressure may host superconductivity. Our discovery of superconductivity in TaP by pressure will stimulate further study on superconductivity in Weyl semimetals.

Scotch tape induced strains for enhancing superconductivity of FeSe0.5Te0.5 single crystals

We investigated the superconducting transition temperature Tc of FeSe0.5Te0.5 single crystals, which can be enhanced up to 14% by attaching onto a commercial Scotch tape. The Scotch tape exhibits a large cooling shrinkage at low temperatures, which is considerably more pronounced than that of the metallic FeSe0.5Te0.5 single crystal, thus providing a compressive strain of 2.4u2009×u200910−3 at 15u2009K. For such strain, we calculated that the lattice parameter of c/a can be increased to ∼0.31%, which corresponds to the enhancement of the superconductivity. The present finding provides a rapid and simple method to examine the microstructure sensitive physical properties of the layered-structure materials by using the Scotch tape as strain generator.

Enhanced electrical conductivity and diluted Ir4+ spin orders in electron doped iridates Sr2–xGaxIrO4

Sr2IrO4 represents a fascinating system to study comparable electronic correlations and spin-orbit couplings, and recently attracts considerable attention in high-temperature superconductivity. Here, we report on the transport and magnetic properties in gallium-doped Sr2IrO4. A metallic state is discovered when doping x is over 0.1, which could be understood in terms of the quickly decreased energy gap and increased carrier concentration. In addition to the high-temperature magnetic transition (TCu2009>u2009200u2009K), a low-temperature one ( TC′) is also observed for the xu2009=u20090.05–0.10 samples. Both of the magnetic states are found to be canted antiferromagnetism. The low-temperature phase is strongly depressed by doping and vanishes when doping is further increased, which is probably stabilized by the long-way exchange interactions of diluted Ir4+ spins via Ir3+ ions. Our studies provide an insight into the electrical and magnetic states tuned by chemical doping in Sr2IrO4, thereby facilitating the seeking of superc...

Size effect on the magnetic phase in Sr4Ru3O10

High quality Sr4Ru3O10 nanoflakes are obtained by the scotch tape-based micro-mechanical exfoliation method. The metamagnetic transition temperature is found to decrease in line with the decrease of thickness, while the ferromagnetic (FM) phase, the ordinary, and anomalous Hall effects (OHE and AHE) are independent on the thickness of the flake. Analysis of the data demonstrates that the AHE reflects the FM nature of Sr4Ru3O10, and the decrease of thickness favors the Ru moments aligned in the ab-plane, which induces a decrease of the metamagnetic transition temperature compared with the bulk.

Proposal for a topological spin Chern pump

The quantum Hall (QH) effect discovered in 1980 (1) is the first example of topological state in the field of condensed matter physics. Since then, there has been continuously strong interest in topological phenomena of condensed matter systems. Laughlin (2) interpreted the integer QH effec as a quantum charge pump. In- creasing the magnetic flux by a single flux quantum that threads a looped QH ribbon constitutes a cycle of the pump due to gauge invariance, transferring an integer- quantized amount of charge from one edge of the rib- bon to the other. Thouless, Kohmoto, Nightingale, and Nijs (3) showed that the QH state can be classified by a topological invariant, the Chern number. Thouless and Niu (4, 5) also established a general relation between the Chern number and the charge pumped during a period of slow variation of potential in the Schrodinger equation. Recently, an important discovery was the topological insulator, (6-9) a new quantum state of matter existing in nature. Different from the QH systems, the topolog- ical insulators preserve the time-reversal (TR) symme- try. Two-dimensional topological insulators, also called the quantum spin Hall (QSH) systems, have a bulk band gap and a pair of gapless helical edge states traversing the bulk gap. When electron spin is conserved, the topo- logical properties of the QSH systems can be easily un- derstood, as a QSH system can be viewed as two inde- pendent QH systems without Landau levels. (10) When the spin conservation is destroyed, unconventional topo- logical invariants are needed to classify the QSH systems. The Z2 index (11) and the spin Chern numbers (12-14) have been proposed to describe the QSH systems. While the two different invariants are found to be equivalent to each other for TR-invariant systems, (13, 14) they lead to controversial predictions when the TR symmetry is bro- ken. The definition of the Z2 index explicitly relies on the presence of TR symmetry, suggesting that the QSH state turns into a trivial insulator once the TR symme- try is broken. However, calculations (15) based upon the spin Chern numbers showed that the nontrivial topolog- ical properties of the QSH systems remain intact when the TR symmetry is broken, as long as the band gap and spin spectrum gap stay open. The nonzero spin Chern numbers guarantee that the edge states must appear on the sample boundary, (16) which could be either gaped or gapless, depending on symmetries or spatial distributions of the edge states. (17) This prediction was supported by the recent experimental observation of the QSH effect in InAs/GaSb bilayers under broken TR symmetry. (18) Spin pumps promise broad applications in spintronics,

In-plane magnetic anisotropy of the Sr4Ru3O10 nanosheet probed by planar Hall effect

The planar Hall effect (PHE) in a ruthenate Sr4Ru3O10 nanosheet as a function of the magnetic field direction and temperature has been investigated. From the magnetic reversal induced PHE signal, we find that the [ 1 ¯ 10] direction is the in-plane ferromagnetic easy-axis and the [110] direction is a metastable magnetic axis of the Sr4Ru3O10 nanosheet. This in-plane magnetic anisotropy can lead to a large, measurable, and field direction sensitive resistance switching when an in-plane magnetic field is swept, suggesting that Sr4Ru3O10 may have potential applications in spintronic and magnetic sensor devices.