Gapless Materials Research Articles

Proximity superconductivity in atom-by-atom crafted quantum dots

Gapless materials in electronic contact with superconductors acquire proximity-induced superconductivity in a region near the interface1,2. Numerous proposals build on this addition of electron pairing to originally non-superconducting systems and predict intriguing phases of matter, including topological3–7, odd-frequency8, nodal-point9 or Fulde–Ferrell–Larkin–Ovchinnikov10 superconductivity. Here we investigate the most miniature example of the proximity effect on only a single spin-degenerate quantum level of a surface state confined in a quantum corral11 on a superconducting substrate, built atom by atom by a scanning tunnelling microscope. Whenever an eigenmode of the corral is pitched close to the Fermi energy by adjusting the size of the corral, a pair of particle–hole symmetric states enters the gap of the superconductor. We identify these as spin-degenerate Andreev bound states theoretically predicted 50 years ago by Machida and Shibata12, which had—so far—eluded detection by tunnel spectroscopy but were recently shown to be relevant for transmon qubit devices13,14. We further find that the observed anticrossings of the in-gap states are a measure of proximity-induced pairing in the eigenmodes of the quantum corral. Our results have direct consequences on the interpretation of impurity-induced in-gap states in superconductors, corroborate concepts to induce superconductivity into surface states and further pave the way towards superconducting artificial lattices.

All-Optical Ultrafast Valley Switching in Two-Dimensional Materials

Electrons in two-dimensional materials possess an additional quantum attribute, the valley pseudospin, labeled as $\mathbf{K}$ and ${\mathbf{K}}^{\mathrm{\ensuremath{'}}}$---analogous to the spin up and spin down. The majority of research to achieve valley-selective excitations in valleytronics depends on resonant circularly polarized light with a given helicity. Not only acquiring valley-selective electron excitation but also switching the excitation from one valley to another is quintessential for bringing valleytronics-based technologies in reality. Present work introduces a coherent control protocol to initiate valley-selective excitation, de-excitation, and switch the excitation from one valley to another on the fly within tens of femtoseconds --- a timescale faster than any valley decoherence time. Our protocol is equally applicable to both gapped and gapless two-dimensional materials. Monolayer graphene and molybdenum disulfide are used to test the universality. Moreover, the protocol is robust as it is insensitive to significant parameters of the protocol, such as dephasing times, wavelengths, and time delays of the laser pulses. Present work goes beyond the existing paradigm of valleytronics, and opens an alternative realm of valley switch at petahertz rate.

Structural, magnetic, and transport properties of epitaxial thin films of equiatomic quaternary CoFeCrGa Heusler alloy

We present the first report on the structural, magnetic, and transport properties of epitaxial thin films of equiatomic quaternary CoFeCrGa, grown on a single crystal MgO(001) substrate. From the structure and surface morphology analyses, we have shown that the films grown are ordered, epitaxial, and homogeneous. Magnetic measurements confirm the soft ferromagnetic nature of the film along the in-plane direction with a coercivity of 4.6 Oe. The resistivity measurements indicate semiconducting nature with a very low temperature coefficient of resistivity (TCR) value, suggesting an almost temperature independent resistivity, similar to other reported spin-gapless semiconductors (SGSs) among the Heusler alloys. A non-saturating, quantum linear magnetoresistance is observed even in high fields of up to 12 kOe, another prerequisite for gapless materials. Hall measurements are also performed, and a detailed analysis has been carried out to estimate the ordinary and anomalous Hall contributions. The conductivity value (σxx) at 300 K is found to be 4280 S cm−1, which is comparable with the other reported SGS materials. We have also studied the effect of L21 and B2 type disorder on the electronic properties based on the first principle calculations and found that the SGS nature in CoFeCrGa is quite robust against the Co–Fe swap disorder (L21 disorder); however, with swap disorder (≥25%) between Cr and Ga sites, the system transits from SGS to half-metallic state. All these results indicate the possibility of spin-gapless semiconducting nature in the CoFeCrGa film, making it highly suitable for spin-based device applications.

Ultrashort pulse generation with MXene Ti3C2Tx embedded in PVA and deposited onto D-shaped fiber

A two-dimensional ternary metal carbide or/and nitride known as MXene, own countless merits in terms of electronic and optical properties. Owning to its fast optical-switching capability, strong saturable absorption, good optical damage tolerant, and gapless materials, it attracts a bunch of research efforts in an ultrafast photonics application. Here, the development of MXene-based SA devices for ultrafast laser in an erbium-doped fiber laser (EDFL) cavity was demonstrated. Two MXene Ti3C2Tx-based SAs were prepared, which were a thin film of MXene (MXene-film) and MXene deposited onto D-shaped fiber (MXene-DS), both revealed good nonlinear transmittance properties. The selective etching was used to provide a high-quality MXene powder, and the solution-casting method produced liquid MXene, which was dried to form a thin-film structure, and the liquid MXene was deposited onto the D-shaped fiber structure. The MXene-film and MXene-DS own a modulation depth of 1.22% and 3%, respectively. Later, both SA devices were incorporated into an EDFL cavity, and MXene-film initiated mode-locked (ML) at the low threshold pump of 44 mW, while MXene-DS produce ML at the pump power of 90 mW. The MXene-film generates a 3.64 ps ML with a pulse energy of 6.03 nJ, separately, MXene-DS initiates a 4.6 ps ML with a pulse energy of 7.69 nJ, highest ever reported among MXene-based SA. This demonstration provides a pathway toward high-energy mode-locking applications in the future.

Robust Spin-Gapless Behavior in the Newly Discovered Half Heusler Compound MnPK.

Spin gapless semiconductors have aroused high research interest since their discovery and a lot of effort has been exerted on their exploration, in terms of both theoretical calculation and experimental verification. Among different spin gapless materials, Heusler compounds stand out thanks to their high Curie temperature and highly diverse compositions. Especially, both theoretical and experimental studies have reported the presence of spin gapless properties in this kind of material. Recently, a new class of d0 − d Dirac half Heusler compound was introduced by Davatolhagh et al. and Dirac, and spin gapless semiconductivity has been successfully predicted in MnPK. To further expand the research in this direction, we conducted a systematical investigation on the spin gapless behavior of MnPK with both generalized gradient approximation (GGA) and GGA + Hubbard U methods under both uniform and tetragonal strain conditions by first principles calculation. Results show the spin gapless behavior in this material as revealed previously. Different Hubbard U values have been considered and they mainly affect the band structure in the spin-down channel while the spin gapless feature in the spin-up direction is maintained. The obtained lattice constant is very well consistent with a previous study. More importantly, it is found that the spin gapless property of MnPK shows good resistance for both uniform and tetragonal strains, and this robustness is very rare in the reported studies and can be extremely interesting and practical for the final end application. This study elaborates the electronic and magnetic properties of the half Heusler compound MnPK under uniform and tetragonal strain conditions, and the obtained results can give a very valuable reference for related research works, or even further motivate the experimental synthesis of the relative material.

Efficiency Limits of Solar Energy Harvesting via Internal Photoemission in Carbon Materials

We describe strategies to estimate the upper limits of the efficiency of photon energy harvesting via hot electron extraction from gapless absorbers. Gapless materials such as noble metals can be used for harvesting the whole solar spectrum, including visible and near-infrared light. The energy of photo-generated non-equilibrium or ‘hot’ charge carriers can be harvested before they thermalize with the crystal lattice via the process of their internal photo-emission (IPE) through the rectifying Schottky junction with a semiconductor. However, the low efficiency and the high cost of noble metals necessitates the search for cheaper abundant alternative materials, and we show here that carbon can serve as a promising IPE material candidate. We compare the upper limits of performance of IPE photon energy-harvesting platforms, which incorporate either gold or carbon as the photoactive material where hot electrons are generated. Through a combination of density functional theory, joint electron density of states calculations, and Schottky diode efficiency modeling, we show that the material electron band structure imposes a strict upper limit on the achievable efficiency of the IPE devices. Our calculations reveal that graphite is a good material candidate for the IPE absorber for harvesting visible and near-infrared photons. Graphite electron density of states yields a sizeable population of hot electrons with energies high enough to be collected across the potential barrier. We also discuss the mechanisms that prevent the IPE device efficiency from reaching the upper limits imposed by their material electron band structures. The proposed approach is general and allows for efficient pre-screening of materials for their potential use in IPE energy converters and photodetectors within application-specific spectral windows.

Quest of thermoelectricity in topological insulators: A density functional theory study

In this Paper, we have studied the structural, electronic and thermoelectric properties of LuAuPb and YAuPb compound respectively with frame work of density functional theory and Boltzmann equations. Our calculations show that both these compounds are gapless materials. Under strain, electronic and thermoelectric properties of these compounds are also studied. Under 5% strain, the maximum energy band gap opens up in LuAuPb compound. The thermoelectric properties of these compounds are studied between temperature range of 100–1000K. Thermoelectric properties of these compound as a function of temperature are studied for the first time in this work. Seebeck measurements indicate that both these compounds are n-type in nature. The calculated electrical conductivity of these compounds under 5% strain increases with increase in temperature. The trends of electrical conductivity confirms that these compounds are semiconductor in nature. As temperature increases the electronic thermal conductivity of these compounds also increase. The maximum figure of merit is achieved for LuAuPb compound which is equal to 0.4.

Universal Near-Infrared and Mid-Infrared Optical Modulation for Ultrafast Pulse Generation Enabled by Colloidal Plasmonic Semiconductor Nanocrystals.

Field effect relies on the nonlinear current-voltage relation in semiconductors; analogously, materials that respond nonlinearly to an optical field can be utilized for optical modulation. For instance, nonlinear optical (NLO) materials bearing a saturable absorption (SA) feature an on-off switching behavior at the critical pumping power, thus enabling ultrafast laser pulse generation with high peak power. SA has been observed in diverse materials preferably in its nanoscale form, including both gaped semiconductor nanostructures and gapless materials like graphene; while the presence of optical bandgap and small carrier density have limited the active spectral range and intensity. We show here that solution-processed plasmonic semiconductor nanocrystals exhibit superbroadband (over 400 THz) SA, meanwhile with large modulation depth (∼7 dB) and ultrafast recovery (∼315 fs). Optical modulators fabricated using these plasmonic nanocrystals enable mode-locking and Q-switching operation across the near-infrared and mid-infrared spectral region, as exemplified here by the pulsed lasers realized at 1.0, 1.5, and 2.8 μm bands with minimal pulse duration down to a few hundreds of femtoseconds. The facile accessibility and superbroadband optical nonlinearity offered by these nonconventional plasmonic nanocrystals may stimulate a growing interest in the exploiting of relevant NLO and photonic applications.

Phase diagrams of disordered Weyl semimetals

Weyl semimetals are gapless quasi-topological materials with a set of isolated nodal points forming their Fermi surface. They manifest their quasi-topological character in a series of topological electromagnetic responses including the anomalous Hall effect. Here we study the effect of disorder on Weyl semimetals while monitoring both their nodal/semi-metallic and topological properties through computations of the localization length and the Hall conductivity. We examine three different lattice tight-binding models which realize the Weyl semimetal in part of their phase diagram and look for universal features that are common to all of the models, and interesting distinguishing features of each model. We present detailed phase diagrams of these models for large system sizes and we find that weak disorder preserves the nodal points up to the diffusive limit, but does affect the Hall conductivity. We show that the trend of the Hall conductivity is consistent with an effective picture in which disorder causes the Weyl nodes move within the Brillouin zone along a specific direction that depends deterministically on the properties of the model and the neighboring phases to the Weyl semimetal phase. We also uncover an unusual (non-quantized) anomalous Hall insulator phase which can only exist in the presence of disorder.

Shockley-Ramo theorem and long-range photocurrent response in gapless materials

Scanning photocurrent maps of gapless materials, such as graphene, often exhibit complex patterns of hot spots positioned far from current-collecting contacts. We develop a general framework that helps to explain the unusual features of the observed patterns, such as the directional effect and the global character of photoresponse. We show that such a response is captured by a simple Shockley-Ramo-type approach. We examine specific examples and show that the photoresponse patterns can serve as a powerful tool to extract information about symmetry breaking, inhomogeneity, chirality, and other local characteristics of the system.

Anomalous magneto-transport properties in nearly gapless materials

We have measured magnetoresistance (MR) for semimetals Bi and graphite, nearly gapless materials Bi-Sb and Bi-Te alloys, and narrow gap semiconductor Ag2Se single crystals at 4.2 K and up to 4 T. We have observed a giant MR for Bi, a giant linear MR (LMR) for Bi-Sb alloy and Ag2Se single crystals. Anomalous completely linear weak-field LMR has been found for nearly gapless materials: this is transferred into the ordinal MR or other LMR in relatively strong field range. It suggests the linear band dispersion in nearly gapless materials as proposed by Abrikosov.

Giant transient Nernst–Ettingshausen effect in nearly gapless materials

We have measured a pulsed laser induced transient Nernst-Ettingshausen (TNE) effect for semimetal Bi and for nearly gapless materials Bi-Sb and Bi-Te alloy single crystals at 4.2 K and under magnetic fields up to 4 T. We have found the giant longitudinal-TNE (l-TNE) and transverse-TNE (t-TNE) signals for all materials. The l-TNE signals show a quite unusual antisymmetric magnetic field dependence, which suggests a mechanism of bipolar state caused by breakdown of space inversion symmetry.

Asymptotic forms of electrostatic potentials in zero-band-gap semiconductors: Bulk versus surface screening.

In this paper we calculate the asymptotic forms of the screened electrostatic potentials in both bulk and semi-infinite zero-band-gap semiconductors, which are described with the help of a random-phase approximation dielectric function. We consider the effective potential of a point charge as well as the screening of a uniform electric field. We show how, in comparison with the free-electron-gas case, the amplitudes of Friedel oscillations of the effective potentials are considerably reduced due to the specific band structure of the gapless materials. To analyze the surface screening, we use the specular electron reflection model. The long- and short-range forms of the potentials obtained in the framework of various asymptotic methods agree with the results of computer calculations.