Xuekui Xi

Periodic corrugation on dynamic fracture surface in brittle bulk metallic glass

Dynamic crack propagation in a model brittle bulk metallic glass (BMG) is studied. Contrary to other brittle glassy materials, the authors find nanometer scale out-of-plane periodic corrugations along the crack surface of the BMG. The nanoscale periodicity remains nearly constant at different loading rates. An interpretation is presented to explain the evolution and the periodic coalescence of the nanometer scale cavities along the crack surface. The observation sheds light on the origin of dynamic fracture surface roughening in brittle materials and could be generally applicable to brittle materials.

Formation and properties of new heavy rare-earth-based bulk metallic glasses

Abstract A series of new families of heavy rare-earth (RE) based bulk metallic glasses (BMGs) with excellent glass-forming ability and high thermal stability have been obtained by a copper mold casting method. Compared with the light RE-based BMGs, the heavy RE-based BMGs have much higher glass transition and crystallization temperatures and higher elastic moduli. It is found that the thermal stability is closely correlated with the elastic constants in the RE-based BMGs. The correlations found are useful for guiding BMG-forming alloy design to enhance stability.

Unprecedentedly Wide Curie-Temperature Windows as Phase-Transition Design Platform for Tunable Magneto-Multifunctional Materials

A series of unprecedentedly wide Curie-temperature windows (CTWs) between 40 and 450 K are realized by employing the isostructural alloying principle for the strongly coupled magnetostructural phase transitions in a single host system. The CTWs provide a design platform for magneto-multifunctional multiferroic alloys that can be manipulated in a quite large temperature space in various scales and patterns, as well as by multiple physical fields.

On the correlation of Young's modulus and the fracture strength of metallic glasses

It is generally believed that there is no simple relationship between ultimate fracture strength and stiffness for an elastically isotropic material. By taking bulk metallic glasses (BMGs) as model materials, the relation between ultimate fracture strength and elastic moduli was analyzed within Griffith theory framework. The observed correlation between elastic moduli and fracture strength in BMGs reveals the effects of BMG defects and plastic zone size on the crack resistance of these model materials.

Large anisotropic thermal transport properties observed in bulk single crystal black phosphorus

The anisotropy of thermal transport properties for bulk black phosphorus (BP) single crystal, which might be of particular interest in the fabrication of thermoelectric/optoelectronic devices, was investigated by using angular dependent thermal conductivity and Seebeck coefficient measurements at various temperatures. We found that the maximum thermal conductivities in x (zigzag), y (armchair), and z (perpendicular to the puckered layers) directions are 34, 17, and 5 W m−1 K−1, respectively, exhibiting large anisotropy. At temperature around 200 K, a large Seebeck coefficient up to +487 ± 10 μV/K has been obtained in x direction, which is 1.5 times higher than that in z direction. The large anisotropy of thermal transport properties can be understood from the crystal structure and bonding characters of BP. In addition, the energy gap has been obtained from nuclear spin lattice relaxation measurements, which is consistent with the value derived from temperature-dependent Seebeck coefficient measurements.

Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases

Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.

Understanding of nanoscale periodic stripes on fracture surface of metallic glasses

We report the observation of nanoscale striped periodic pattern with similar distinctive characteristics independent of loading conditions on the fracture surface of various bulk metallic glasses. We demonstrate that the periodic stripes are formed by the orderly assembly of nanoscale regular dimples. The similarities between our observed striped pattern and various unequilibrium systems such as oscillating granular and colloidal suspensions systems are found. By drawing an analogy between glassy and granular materials, we propose a model that can capture and simulate the characteristics of the observed corrugations. Our results would provide insight into the origin of fracture surface roughening in brittle materials.

NMR Evidence for the Topologically Nontrivial Nature in a Family of Half-Heusler Compounds

Spin-orbit coupling (SOC) is expected to partly determine the topologically nontrivial electronic structure of heavy half-Heusler ternary compounds. However, to date, attempts to experimentally observe either the strength of SOC or how it modifies the bulk band structure have been unsuccessful. By using bulk-sensitive nuclear magnetic resonance (NMR) spectroscopy combined with first-principles calculations, we reveal that 209Bi NMR isotropic shifts scale with relativity in terms of the strength of SOC and average atomic numbers, indicating strong relativistic effects on NMR parameters. According to first-principles calculations, we further claim that nuclear magnetic shieldings from relativistic p1/2 states and paramagnetic contributions from low-lying unoccupied p3/2 states are both sensitive to the details of band structures tuned by relativity, which explains why the hidden relativistic effects on band structure can be revealed by 209Bi NMR isotropic shifts in topologically nontrivial half-Heusler compounds. Used in complement to surface-sensitive methods, such as angle resolved photon electron spectroscopy and scanning tunneling spectroscopy, NMR can provide valuable information on bulk electronic states.

Degradation of black phosphorus: a real-time 31P NMR study

In this work, degradation behaviors and mechanisms of black phosphorus (BP) crystals in air under ambient conditions were investigated by nuclear magnetic resonance spectroscopy. It has been found that the 31P NMR line intensity for BP decreases exponentially during aging even at the very first several hours, suggesting the origin of the degradation of transport properties. In addition to phosphoric acid, new phosphorous acid was also well resolved in the final aging products. Moreover, BP has been found to be stable in water without the presence of oxygen molecules. These findings are relevant for better understanding of degradation behaviors of BP upon aging and should be helpful for overcoming a barrier that might hamper progress toward applications of BP as a 2D material.

High electron mobility and large magnetoresistance in the half-Heusler semimetal LuPtBi

We report on the discovery of an electron-hole-compensated half-Heusler semimetal LuPtBi that exhibits an extremely high electron mobility up to 79,000 cm2/Vs with a non-saturating positive MR as large as 3200% at 2 K. Remarkably, the mobility at 300 K is found to exceed 10,500 cm2/Vs, which is among the highest values reported in three-dimensional bulk materials thus far. From the observed Shubnikov-de Haas quantum oscillations together with the first-principles calculations, we confirm the bulk three-dimensional Fermi surface of LuPtBi having signature of Dirac-like behavior with a very small effective carrier mass, which gives rise to the extremely high electron mobility. Our work may provide a new recipe for searching high-mobility and large MR materials by designing an appropriate Fermi surface topology starting from the simple electron-hole-compensated semimetals.