Dirac semimetals have attracted extensive attentions in recent years. It has been theoretically suggested that many-body interactions may drive exotic phase transitions, spontaneously generating a Dirac mass for the nominally massless Dirac electrons. So far, signature of interaction-driven transition has been lacking. In this work, we report high-magnetic-field transport measurements of the Dirac semimetal candidate ZrTe5. Owing to the large g factor in ZrTe5, the Zeeman splitting can be observed at magnetic field as low as 3 T. Most prominently, high pulsed magnetic field up to 60 T drives the system into the ultra-quantum limit, where we observe abrupt changes in the magnetoresistance, indicating field-induced phase transitions. This is interpreted as an interaction-induced spontaneous mass generation of the Dirac fermions, which bears resemblance to the dynamical mass generation of nucleons in high-energy physics. Our work establishes Dirac semimetals as ideal platforms for investigating emerging correlation effects in topological matters.

We demonstrate by magneto-transport measurements that a Curie temperature as high as 200 K can be obtained in nanostructures of (Ga,Mn)As. Heavily Mn-doped (Ga,Mn)As films were patterned into nanowires and then subject to low-temperature annealing. Resistance and Hall effect measurements demonstrated a consistent increase of T(C) with decreasing wire width down to about 300 nm. This observation is attributed primarily to the increase of the free surface in the narrower wires, which allows the Mn interstitials to diffuse out at the sidewalls, thus enhancing the efficiency of annealing. These results may provide useful information on optimal structures for (Ga,Mn)As-based nanospintronic devices operational at relatively high temperatures.

Three-dimensional topological Dirac semimetals (TDSs) are a new kind of Dirac materials that exhibit linear energy dispersion in the bulk and can be viewed as three-dimensional graphene. It has been proposed that TDSs can be driven to other exotic phases like Weyl semimetals, topological insulators and topological superconductors by breaking certain symmetries. Here we report the first transport experiment on Landau level splitting in TDS Cd3As2 single crystals under high magnetic fields, suggesting the removal of spin degeneracy by breaking time reversal symmetry. The detected Berry phase develops an evident angular dependence and possesses a crossover from non-trivial to trivial state under high magnetic fields, a strong hint for a fierce competition between the orbit-coupled field strength and the field-generated mass term. Our results unveil the important role of symmetry breaking in TDSs and further demonstrate a feasible path to generate a Weyl semimetal phase by breaking time reversal symmetry.

In 1933, Meissner and Ochsenfeld reported the expulsion of magnetic flux-the diamagnetic Meissner effect-from the interior of superconducting lead. This discovery was crucial in formulating the BardeenCooper-Schrieffer (BCS) theory of superconductivity. In exotic superconducting systems BCS theory does not strictly apply. A classical example is a superconductor-magnet hybrid system where magnetic ordering breaks time-reversal symmetry of the superconducting condensate and results in the stabilization of an odd-frequency superconducting state. It has been predicted that under appropriate conditions, odd-frequency superconductivity should manifest in the Meissner state as fluctuations in the sign of the magnetic susceptibility, meaning that the superconductivity can either repel (diamagnetic) or attract (paramagnetic) external magnetic flux. Here, we report local probe measurements of faint magnetic fields in a Au=Ho=Nb trilayer system using low-energy muons, where antiferromagnetic Ho (4.5 nm) breaks time-reversal symmetry of the proximity-induced pair correlations in Au. From depth-resolved measurements below the superconducting transition of Nb, we observe a local enhancement of the magnetic field in Au that exceeds the externally applied field, thus proving the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state.

Thin films of CrSb grown by solid-source molecular-beam epitaxy on GaAs, (Al, Ga)Sb, and GaSb are found to exhibit ferromagnetism. Reflection high-energy electron diffraction and high-resolution cross sectional transmission electron microscopy both indicate that the structure is zincblende. Temperature dependence of remanent magnetization shows that the ferromagnetic transition temperature is beyond 400 K.

Self-catalyzed growth of GaAs nanowires are widely ascribed to the vapor-liquid-solid (VLS) mechanism due to the presence of Ga particles at the nanowire tips. Here we report synthesis of self-catalyzed GaAs nanowires by molecular-beam epitaxy covering a large growth parameter space. By carefully controlling the Ga flux and its ratio with the As flux, GaAs nanowires without Ga particles and exhibiting a flat growth front are produced. Using scanning electron microscopy and high-resolution transmission electron microscopy, we compare the growth rate and structure, especially near the growth front, of the nanowires with and without Ga droplets. We find that regardless of whether Ga droplets are present on top, the nanowires have a short wurtzite section following the zinc-blende bulk structure. The nanowires without Ga droplets are terminated by a thin zinc-blende cap, while the nanowires with Ga droplets do not have such a cap. The bulk zinc-blende phase is attributed to the Ga droplet wetting the sidewall during growth, pinning the triple phase line on the sidewall. The zinc-blend/wurtzite/(zinc-blende) phase transitions at the end of growth are fully consistent with the triple phase line shifting up to the growth front due to the progressive consumption of the Ga in the droplet by crystallization with As. The results imply an identical VLS growth mechanism for both types of GaAs NWs, and their intricate structures provide detailed comparison with and specific experimental verification of the recently proposed growth mechanism for self-catalyzed III-V semiconductor nanowires ( Phy. Rev. Lett. 2011 , 106 , 125505 ). Using this mechanism as a guideline, we successfully demonstrated controllable fabrication of two distinct types of axial superlattice GaAs NWs consisting of zinc-blende/defect-section and wurtzite/defect-section units.

Here we report on the Ga self-catalyzed growth of near full-composition-range energy-gap-tunable GaAsSb nanowires by molecular-beam epitaxy. GaAsSb nanowires with different Sb content are systematically grown by tuning the Sb and As fluxes, and the As background. We find that GaAsSb nanowires with low Sb content can be grown directly on Si(111) substrates (0 ≤ x ≤ 0.60) and GaAs nanowire stems (0 ≤ x ≤ 0.50) by tuning the Sb and As fluxes. To obtain GaAsSb nanowires with x ranging from 0.60 to 0.93, we grow the GaAsSb nanowires on GaAs nanowire stems by tuning the As background. Photoluminescence measurements confirm that the emission wavelength of the GaAsSb nanowires is tunable from 844 nm (GaAs) to 1760 nm (GaAsSb). High-resolution transmission electron microscopy images show that the grown GaAsSb nanowires have pure zinc-blende crystal structure. Room-temperature Raman spectra reveal a redshift of the optical phonons in the GaAsSb nanowires with x increasing from 0 to 0.93. Field-effect transistors based on individual GaAsSb nanowires are fabricated, and rectifying behavior is observed in devices with low Sb content, which disappears in devices with high Sb content. The successful growth of high-quality GaAsSb nanowires with near full-range bandgap tuning may speed up the development of high-performance nanowire devices based on such ternaries.

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