Mingda Li

Proximity-Driven Enhanced Magnetic Order at Ferromagnetic-Insulator-Magnetic-Topological-Insulator Interface

Magnetic exchange driven proximity effect at a magnetic-insulator-topological-insulator (MI-TI) interface provides a rich playground for novel phenomena as well as a way to realize low energy dissipation quantum devices. Here we report a dramatic enhancement of proximity exchange coupling in the MI/magnetic-TI EuS/Sb(2-x)V(x)Te3 hybrid heterostructure, where V doping is used to drive the TI (Sb2Te3) magnetic. We observe an artificial antiferromagneticlike structure near the MI-TI interface, which may account for the enhanced proximity coupling. The interplay between the proximity effect and doping in a hybrid heterostructure provides insights into the engineering of magnetic ordering.

Cellulose nanofiber backboned Prussian blue nanoparticles as powerful adsorbents for the selective elimination of radioactive cesium

On 11 March 2011, the day of the unforgettable disaster of the 9 magnitude Tohoku earthquake and quickly followed by the devastating Tsunami, a damageable amount of radionuclides had dispersed from the Fukushima Daiichi’s damaged nuclear reactors. Decontamination of the dispersed radionuclides from seawater and soil, due to the huge amounts of coexisting ions with competitive functionalities, has been the topmost difficulty. Ferric hexacyanoferrate, also known as Prussian blue (PB), has been the most powerful material for selectively trapping the radioactive cesium ions; its high tendency to form stable colloids in water, however, has made PB to be impossible for the open-field radioactive cesium decontamination applications. A nano/nano combinatorial approach, as is described in this study, has provided an ultimate solution to this intrinsic colloid formation difficulty of PB. Cellulose nanofibers (CNF) were used to immobilize PB via the creation of CNF-backboned PB. The CNF-backboned PB (CNF/PB) was found to be highly tolerant to water and moreover, it gave a 139 mg/g capability and a million (106) order of magnitude distribution coefficient (Kd) for absorbing of the radioactive cesium ion. Field studies on soil and seawater decontaminations in Fukushima gave satisfactory results, demonstrating high capabilities of CNF/PB for practical applications.

Experimental verification of the van Vleck nature of long-range ferromagnetic order in the vanadium-doped three-dimensional topological insulator Sb(2)Te(3).

We demonstrate by high resolution low temperature electron energy loss spectroscopy (EELS) measurements that the long range ferromagnetic (FM) order in the vanadium- (V-)doped topological insulator Sb_{2}Te_{3} has the nature of van Vleck-type ferromagnetism. The positions and the relative amplitudes of two core-level peaks (L_{3} and L_{2}) of the V EELS spectrum show unambiguous change when the sample is cooled from room temperature to T=10  K. Magnetotransport and comparison of the measured and simulated EELS spectra confirm that these changes originate from the onset of FM order. Crystal field analysis indicates that in V-doped Sb_{2}Te_{3}, partially filled core states contribute to the FM order. Since van Vleck magnetism is a result of summing over all states, this magnetization of core level verifies the van Vleck-type ferromagnetism in a direct manner.

Quantum anomalous Hall effect in time-reversal-symmetry breaking topological insulators.

The quantum anomalous Hall effect (QAHE), the last member of Hall family, was predicted to exhibit quantized Hall conductivity σ(yx) = e2/h without any external magnetic field. The QAHE shares a similar physical phenomenon with the integer quantum Hall effect (QHE), whereas its physical origin relies on the intrinsic topological inverted band structure and ferromagnetism. Since the QAHE does not require external energy input in the form of magnetic field, it is believed that this effect has unique potential for applications in future electronic devices with low-power consumption. More recently, the QAHE has been experimentally observed in thin films of the time-reversal symmetry breaking ferromagnetic (FM) topological insulators (TI), Cr- and V- doped (Bi,Sb)2Te3. In this topical review, we review the history of TI based QAHE, the route to the experimental observation of the QAHE in the above two systems, the current status of the research of the QAHE, and finally the prospects for future studies.

Magnetic proximity effect and interlayer exchange coupling of ferromagnetic/topological insulator/ferromagnetic trilayer

Magnetic proximity effect between topological insulator (TI) and ferromagnetic insulator (FMI) is considered to have great potential in spintronics. However, a complete determination of interfacial magnetic structure has been highly challenging. We theoretically investigate the interlayer exchange coupling of two FMIs separated by a TI thin film, and show that the particular electronic states of the TI contributing to the proximity effect can be directly identified through the coupling behavior between two FMIs, together with a tunability of coupling constant. Such FMI/TI/FMI structure not only serves as a platform to clarify the magnetic structure of FMI/TI interface, but also provides insights into designing the magnetic storage devices with ultrafast response.

Scalable synthesis of a sulfur nanosponge cathode for a lithium–sulfur battery with improved cyclability

Although lithium–sulfur batteries exhibit a high initial capacity, production costs and lack of cyclability are major limitations. Here we report a liquid-based, low-cost and reliable synthesis method of a lithium–sulfur composite cathode with improved cyclability. An open network of Conductive Carbon Black nanoparticles (Cnet) is infused with a sulfur network (Snet) to form sponge-like networks (Cnet + Snet). Initially, Snet is open to the outside, allowing liquid electrolyte to infiltrate and impart Snet Li+ conductivity. During lithiation, Cnet could accommodate the volume expansion of Snet largely without losing electrical contact. During delithiation, the carbon nanoparticles would preferably flocculate on the outer surface due to polysulfide dissolution and depletion of sulfur, to form a passivation layer that still allows Li+ exchange, but prevents more polysulfides from escaping, thus slowing the leaching of polysulfides into the bulk electrolyte liquid. The plausibility of a carbonaceous passivation layer was checked using an extra carbon deposition layer to achieve an improved performance of ∼400 mA h g−1 after 250 cycles under a high rate 2.0 C. A 763 mA h g−1 discharge specific capacity of this sulfur nanosponge cathode (abbreviated as “SULFUN”) was obtained after 100 cycles under a rate of 0.2 C. Discharge capacities of 520 mA h g−1 and 290 mA h g−1 were attained after 300 and 500 cycles, respectively, making this cathode material attractive for rechargeable battery applications.

Boson Peak in Deeply Cooled Confined Water: A Possible Way to Explore the Existence of the Liquid-to-Liquid Transition in Water

In their Letter, Wang et al. [1] report on an inelastic neutron scattering (INS) experiment where they describe the pressure evolution of a low energy (E ∼ 6 meV) excitation, emerging in confined protonated water only below 230 K at an exchanged momentum Q 1⁄4 2.0 Å−1. Water confinement was used to overcome the unavoidable crystallization occurring below ∼250 K in bulk water. The authors report that a similar finding was also obtained in both bulk (numerical simulations [2]) and confined water at ambient pressure. They refer to this low temperature excitation as a boson peak (BP) [3], and relate its occurrence to the Widom line, concluding that the observed pressure behavior of the BP reveals the signature of the high-density liquid (HDL) to the low-density liquid (LDL) transition proposed [4], though severely questioned [5], for bulk water. We believe these claims to be unconvincing for the following reasons. Comparison with corresponding findings in liquid water.—The authors not only overlook commenting on the actual density of confined liquid water [6–9], but they also neglect to establish any physical relationship with the well known excitations of coherent or incoherent origin occurring at similar energies in bulk liquid water. Since the seminal Raman scattering room temperature studies by Bolla [10], a mode in the ∼5–7 meV range has indeed been regularly observed with optical [11–14], numerical [15], and inelastic x-ray scattering [16–18] and INS techniques [19,20] over a wide thermodynamic range (0–2 kbar, 250–450 K) in both H2O and D2O (see Fig. 4 in Ref. [21]). The microscopic nature of such a mode, underdamped and still well defined at Q 1⁄4 2.0 Å−1, is the subject of controversial single-particle [12,22,23] or collective [21,24] interpretations. Irrespective of its incoherent or coherent nature, this evidence is unquestionable and cannot be ignored. This mode is not easily detectable in high temperature neutron spectra from H2O because of the overwhelming quasielastic contribution. However, its presence always emerges in calculating the hydrogen vibrational density of states, as was done in Ref. [22] at T 1⁄4 256 K, and in bulk or confined H2O from 300 K down to 242 K [25], but not mentioned in Ref. [1]. This mode, but not the BP, was also observed when investigating the vibrational dynamics in amorphous ices [26,27]. Moreover, a bulklike excitation not dependent on temperature was observed down to 205 K in an INS measurement on slightly salty liquid water [28]. Data analysis and treatment.—(i) INS probes at the same time the coherent and incoherent properties of matter with a weight given by their respective neutron cross section and dynamic structure factor. H2O is considered as an incoherent scatterer by reason of the high σinc=σcoh ratio. Yet, this approximation cannot be uncritically adopted as was done in Ref. [1] and a proper estimation of the related ratio SincðQ;ωÞ=ScohðQ;ωÞ at the thermodynamic (P, T) and kinetic (Q;ω) investigated point should be addressed. (ii) An arbitrary interpolating metric is adopted to determine the locus of the BP appearance: the TB parameter is a clumsy, large-error quantity inherent in the slowing down of the thermal diffusion. The peak associated with the low energy—and virtually temperature independent (see Fig. 4 of Ref. [1])—excitation is enhanced by the narrowing of the quasielastic signal upon lowering the temperature. (iii) The exact internal pressure existing in such tiny pores is not directly related to the He applied pressure and is therefore unknown [29]. As a consequence, the confined water phase diagram and properties cannot be unconditionally assigned to those of bulk water. (iv) In order to support the authors’ claims at a less speculative level, the correct BP shape should be determined by calculating the vibrational density of states in excess of that of the corresponding crystalline phase. In conclusion, the whole large body of numeric and experimental investigations on the single particle and collective properties of liquid water report the presence of a weakly dispersing excitation in the 5–7 meV range. We believe that, in order to use the BP as a marker of the HDL or LDL bulk water phases, the authors should perform a more complete data treatment and establish a relation, if any, between the supposed BP peak they observe in a confined environment and the well established bulk mode present in a wide portion of the phase diagram at the same energy.

Phonon Hydrodynamic Heat Conduction and Knudsen Minimum in Graphite

In the hydrodynamic regime, phonons drift with a nonzero collective velocity under a temperature gradient, reminiscent of viscous gas and fluid flow. The study of hydrodynamic phonon transport has spanned over half a century but has been mostly limited to cryogenic temperatures (∼1 K) and more recently to low-dimensional materials. Here, we identify graphite as a three-dimensional material that supports phonon hydrodynamics at significantly higher temperatures (∼100 K) based on first-principles calculations. In particular, by solving the Boltzmann equation for phonon transport in graphite ribbons, we predict that phonon Poiseuille flow and Knudsen minimum can be experimentally observed above liquid nitrogen temperature. Further, we reveal the microscopic origin of these intriguing phenomena in terms of the dependence of the effective boundary scattering rate on momentum-conserving phonon-phonon scattering processes and the collective motion of phonons. The significant hydrodynamic nature of phonon transport in graphite is attributed to its strong intralayer sp2 hybrid bonding and weak van der Waals interlayer interactions. More intriguingly, the reflection symmetry associated with a single graphene layer is broken in graphite, which opens up more momentum-conserving phonon-phonon scattering channels and results in stronger hydrodynamic features in graphite than graphene. As a boundary-sensitive transport regime, phonon hydrodynamics opens up new possibilities for thermal management and energy conversion.

Topological effect of surface plasmon excitation in gapped isotropic topological insulator nanowires

We present a theoretical investigation of the surface plasmon (SP) at the interface between a topologically nontrivial cylindrical core and a topologically trivial surrounding material, from the ax...

Phonon-like excitation in secondary and tertiary structure of hydrated protein powders

Existence of sub-thermal collective excitations in proteins is of great interest due to its possible close coupling with the onset of their biological functions. We use high-energy resolution inelastic X-ray scattering to directly measure phonon dispersion relations and their damping in two hydrated proteins, α-chymotrypsinogen A and casein, differing in their secondary and tertiary structures. We observe that specific phonons in the Q range 28–30 nm−1 are markedly softened only above TD = 220 K, the observed protein dynamic transition temperature. This might indicate that only phonon modes within the wavelengths in the length scale comparable to the secondary structure dimension could be linked to the onset of protein biological activity. We also infer that the presence of tertiary structure contributes little to the population of phonons, while the α-helix seems to be the major contributor to phonons propagation.