Semimetal Research Articles

Superfluid stiffness and Josephson quantum capacitance: Adiabatic approach and topological effects

We bring forward a unified framework for the study of the superfluid stiffness and the quantum capacitance of superconducting platforms exhibiting conventional spin-singlet pairing. We focus on systems which in their normal phase contain topological band touching points or crossings, while in their superconducting regime feature a fully gapped energy spectrum. Our unified description relies on viewing these two types of physical quantities as the charge current and density response coefficients obtained for slow spatiotemporal variations of the superconducting phase. Within our adiabatic formalism, the two coefficients are given in terms of Berry curvatures defined in synthetic spaces. Our paper lays the foundation for the systematic description of topological diagonal superfluid responses induced by singularities dictating the synthetic Berry curvatures. We exemplify our approach for concrete one- and two-dimensional models of superconducting topological (semi)metals. We discuss topological phenomena which arise in the superfluid stiffness of bulk systems and the quantum capacitance of Josephson junctions. We show that both coefficients become proportional to a topological invariant which counts the number of topological touchings or crossings of the normal phase band structure. These topological effects can be equivalently viewed as manifestations of chiral anomaly. Our predictions appear experimentally testable in topological semimetals with proximity-induced pairing, such as in graphene-superconductor hybrids at charge neutrality. Published by the American Physical Society 2024

Metal and non–metal dynamics and distribution in soil profiles across selected pans in the Ramsar declared subtropical within a national protected area

ABSTRACT The study investigated the spatial distributions of selected metals, semi–metals and non–metals within a floodplain pan ecosystem in the Ramsar declared Makuleke Wetlands within the Makuleke Contractual National Park, in the northern Kruger National Park (South Africa), along varying soil depths (0–120 cm) at 20 cm intervals. The study identified significant differences in metal concentrations (i.e. Ca, Mn, Fe) and non–metals (i.e. C, S) across sediment depths. Metal and non–metal concentrations in surface sediments (0–40 cm) were generally high. Compared with the sediment quality guidelines, all measured metals were within the ‘no effect’ level across different sites and depths, except for one site (i.e. Mambvumbvanyi pan). In contrast, enrichment factors showed that K, Ca and Mg were enriched in sediments across all the floodplain pans and depths. Principal component and cluster analyses indicated that various metals originated from different sources. Although a high concentration of metals was found in the topsoil, no potential detrimental effects on the aquatic systems could be observed. Based on the findings, this study provides a baseline overview of sediment metal pollution that can inform effective management of these floodplain wetlands systems.

Arsenite Impairs BRCA1-Dependent DNA Double-Strand Break Repair, a Mechanism Potentially Contributing to Genomic Instability.

BRCA1 is a key player in maintaining genomic integrity with multiple functions in DNA damage response (DDR) mechanisms. Due to its thiol-rich zinc-complexing domain, the protein may also be a potential target for redox-active and/or thiol-reactive (semi)metal compounds. The latter includes trivalent inorganic arsenic, which is indirectly genotoxic via induction of oxidative stress and inhibition of DNA repair pathways. In the present study, we investigated the effect of NaAsO2 on the transcriptional and functional DDR. Particular attention was paid to the potential impairment of BRCA1-mediated DDR mechanisms by arsenite by comparing BRCA1-deficient and -proficient cells. At the transcriptional level, arsenite itself activated several DDR mechanisms, including a pronounced oxidative stress and DNA damage response, mostly independent of BRCA1 status. However, at the functional level, a clear BRCA1 dependency was observed in both cell cycle regulation and cell death mechanisms after arsenite exposure. Furthermore, in the absence of arsenite, the lack of functional BRCA1 impaired the largely error-free homologous recombination (HR), leading to a shift towards the error-prone non-homologous end-joining (NHEJ). Arsenic treatment also induced this shift in BRCA1-proficient cells, indicating BRCA1 inactivation. Although BRCA1 bound to DNA DSBs induced via ionizing radiation, its dissociation was impaired, similarly to the downstream proteins RAD51 and RAD54. A shift from HR to NHEJ by arsenite was further supported by corresponding reporter gene assays. Taken together, arsenite appears to negatively affect HR via functional inactivation of BRCA1, possibly by interacting with its RING finger structure, which may compromise genomic stability.

Su Kalitesi Durum Değerlendirmesi- Salda Gölü, Türkiye

Inland water bodies, either flowing or non-flowing all around the world, are under the adverse effect of both the climate change and anthropogenic activities. The water surface areas of lakes and wetlands diminish over time and thus, this global reality brings together the deterioration of water quality especially if such water bodies confront with a variety of human activities exerting pollutants. An example of such a vulnerable ecosystem is the Lake Salda located in the southwestern part of Turkey. This lake has gained the attention of international public due to its similarities with Mars. Over a long period of time, many researches have been conducted in this lake by different disciplines and scientists to better understand its unique features. In this study, water samples from the three selected stations on the lake at three different depths were collected and analyzed on physico-chemical parameters, organic matter content, nutrients, metals and semi metals according to national legislation and Water Framework Directive (WFD) of EU. The results are tabulated and analyzed in detail considering the potential pollution sources arising from the lake’s basin. The pollutants are linked with the experimental measurements, and those parameters that exceeded the Environmental Quality Standards (EQS) are underlined. It is for sure that the lake necessitates utmost care and attention according to the results achieved.

High refractive index sensitivity adjustable six band absorber based on dual regulation of embedded Dirac semi metal and active graphene

High refractive index sensitivity adjustable six band absorber based on dual regulation of embedded Dirac semi metal and active graphene

First-principles prediction of topological Dirac semimetallic phase in NaHgX (X = As and Bi)

The topological semimetals (TSMs) are the electronic gap-less phases with topological band crossing near the Fermi-level and have attracted a lot of attention from researchers in recent years. In this work, we have identified topological Dirac semimetallic phase in hexagonal NaHgAs and NaHgBi via first-principles DFT calculations. We have studied structural, electronic, elastic, mechanical, thermodynamic and topological properties of NaHgX (X = As, Sb and Bi) without and with application of external uniaxial strain. Our findings reveal that NaHgAs is a Dirac semi metal (DSM) in its ground state as well as in strained state, while NaHgBi is DSM only in its strained state. The elastic constants and mechanical properties of NaHgX (X = As, Sb and Bi) were also calculated and it was found that only NaHgAs and NaHgBi are mechanically and thermodynamically stable.

Ohmic Behavior in Metal Contacts to n/p-Type Transition-Metal Dichalcogenides: Schottky versus Tunneling Barrier Trade-off

High contact resistance (RC) between 3D metallic conductors and single-layer 2D semiconductors poses major challenges toward their integration in nanoscale electronic devices. While in experiments the large RC values can be partly due to defects, ab initio simulations suggest that, even in defect-free structures, the interaction between metal and semiconductor orbitals can induce gap states that pin the Fermi level in the semiconductor band gap, increase the Schottky barrier height (SBH), and thus degrade the contact resistance. In this paper, we investigate, by using an in-house-developed ab initio transport methodology that combines density functional theory and nonequilibrium Green’s function (NEGF) transport calculations, the physical properties and electrical resistance of several options for n-type top metal contacts to monolayer MoS2, even in the presence of buffer layers, and for p-type contacts to monolayer WSe2. The delicate interplay between the SBH and tunneling barrier thickness is quantitatively analyzed, confirming the excellent properties of the Bi–MoS2 system as an n-type ohmic contact. Moreover, simulation results supported by literature experiments suggest that the Au–WSe2 system is a promising candidate for p-type ohmic contacts. Finally, our analysis also reveals that a small modulation of a few angstroms of the distance between the (semi)metal and the transition-metal dichalcogenide (TMD) leads to large variations of RC. This could help to explain the scattering of RC values experimentally reported in the literature because different metal deposition techniques can result in small changes of the metal-to-TMD distance besides affecting the density of possible defects.

Delocalization of topological surface states by diagonal disorder in nodal loop semimetals

The effect of Anderson diagonal disorder on the topological surface (``drumhead'') states of a Weyl nodal loop semimetal is addressed. Since diagonal disorder breaks chiral symmetry, a winding number cannot be defined. Seen as a perturbation, the weak random potential mixes the clean exponentially localized drumhead states of the semimetal, thereby producing two effects: (i) the algebraic decay of the surface states into the bulk; (ii) a broadening of the low energy density of surface states of the open system due to degeneracy lifting. This behavior persists with increasing disorder, up to the bulk semimetal-to-metal transition at the critical disorder $W_{c}$. Above $W_{c}$, the surface states hybridize with bulk states and become extended into the bulk.

Reactive Solubilization of Heterometallic Clusters by Treatment of (TrBi3)2− Anions (Tr=Ga, In, Tl) with [Mn{N(SiMe3)2}2

AbstractLowering the charge of Zintl anions by (element‐)organic substituents allows their use as sources of (semi)metal nanostructures in common organic solvents, as realized for group 15 anions or Ge94− and Sn94−. We developed a new strategy for other anions, using low‐coordinate 3d metal complexes as electrophiles. [K(crypt‐222)]+ salts of (TrBi3)2− anions dissolved in situ in Et2O and/or THF when reacted with [Mn(hmds)2]. Work‐up afforded soluble [K(crypt‐222)]+ salts of [{(hmds)2Mn}2(TlBi3)]2− (in 1), [{(hmds)2Mn}2(Bi2)]2− (in 2), and [{(hmds)Mn}4(Bi2)2]2− (in 3) (crypt‐222=4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane; Tr=Ga, In, Tl; hmds=N(SiMe3)2), representing rare cases of Zintl clusters with open‐shell metal atoms. 1 comprises the first coordination compound of the (TlBi3)2− anion, 2 features a diamond‐shaped {Pn2M2} unit, and 3 is a mixed‐valent MnI/MnII compound. The uncommon electronic structures in 1–3 and magnetic coupling were studied by comprehensive DFT calculations.

Reactive Solubilization of Heterometallic Clusters by Treatment of (TrBi3 )2- Anions (Tr=Ga, In, Tl) with [Mn{N(SiMe3 )2 }2

Lowering the charge of Zintl anions by (element-)organic substituents allows their use as sources of (semi)metal nanostructures in common organic solvents, as realized for group 15 anions or Ge9 4- and Sn9 4- . We developed a new strategy for other anions, using low-coordinate 3d metal complexes as electrophiles. [K(crypt-222)]+ salts of (TrBi3 )2- anions dissolved in situ in Et2 O and/or THF when reacted with [Mn(hmds)2 ]. Work-up afforded soluble [K(crypt-222)]+ salts of [{(hmds)2 Mn}2 (TlBi3 )]2- (in 1), [{(hmds)2 Mn}2 (Bi2 )]2- (in 2), and [{(hmds)Mn}4 (Bi2 )2 ]2- (in 3) (crypt-222=4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane; Tr=Ga, In, Tl; hmds=N(SiMe3 )2 ), representing rare cases of Zintl clusters with open-shell metal atoms. 1 comprises the first coordination compound of the (TlBi3 )2- anion, 2 features a diamond-shaped {Pn2 M2 } unit, and 3 is a mixed-valent MnI /MnII compound. The uncommon electronic structures in 1-3 and magnetic coupling were studied by comprehensive DFT calculations.

Analysis of the Interphase Formation of Thiophosphate Solid Electrolytes and the Lithium Metal Anode in Solid-State Batteries

Due to their high ionic conductivities and better safety, thiophosphate solid electrolytes (SEs) such as Li6PS5Cl or Li7SiPS8 are of interest for applications in solid-state batteries (SSBs). However, drawbacks such as their inherent reduction instability, which impede their application with a lithium metal anode (LMA) [1] as well as low gravimetric and volumetric energy densities of the resulting full cells [2] still have to be overcome.If the decomposition products of SEs formed upon contact with lithium metal are sufficiently electronically conductive, the decomposition layer forms a so-called mixed conducting interphase (MCI). A (semi)metal dispersed in the MCI should decrease the internal resistance and lead to short circuits [3]. However, this is not the case for all metal ion containing SEs [4]. For example, the resistance of a Li|Li7SiPS8|Li symmetric cell constantly increases. Thus, knowledge about the composition of the interphase between lithium metal and SEs as well as the resistance evolution of a cell containing the respective SE is necessary.We report our latest findings of the stability of several thiophosphate SEs in contact with lithium metal. We show the kinetics of the heterogeneous solid-state reaction and the chemical composition of the resulting interphase layer. Impedance spectroscopy measurements were conducted to investigate the temporal evolution of the interphase resistance between the lithium metal anode and the SE. In situ-sputter-deposition of lithium metal onto the surface of the SE pellets and subsequent X-ray photoelectron spectroscopy (XPS) measurements are performed to study the chemical composition upon contact. Multiple repetitions of these steps allow a detailed characterization and identification of the decomposition products within the forming interphase. The combined results of our studies allow us to distinguish between the possible interphases of metal ion containing electrolytes and promotes the understanding of the SE stability towards LMAs. Reference s : [1] X. Li, J. Liang, X. Yang, K. Adair, C. Wang, F. Zhao, X. Sun, Energy Environ. Sci. (2020) 10.1039/C9EE03828K.[2] P. Braun, C. Uhlmann, M. Weiss, A. Weber, E. Ivers-Tiffée, J. Power Sources 393 (2018) 119-127.[3] S. Wenzel, T. Leichtweiss, D. Krüger, J. Sann, J. Janek, Solid State Ion. 278 (2015) 98-105.[4] L. Riegger, R. Schlem, J. Sann, W. Zier, J. Janek, Angew. Chem. (2020) 10.1002/ange.202015238.

Excitonic insulators and Gross-Neveu models

We introduce a generalized Gross-Neveu (GN) model to describe the excitonic instabilities in two different systems: a small overlap semi-metal (SM) and a small gap semi-conductor (SMC), both in two (2d) and three-dimensions (3d). We identify the excitonic order parameter (EOP) and obtain the effective potential within the Large $N$ limit approach where the GN model can be exactly solved. We obtain the excitonic insulator (EI) phase diagrams as a function of temperature, chemical potential, overlap between bands and gaps of the system. We show that the EI may undergo first- or second-order thermal transitions depending on the regime whereupon this phase is approached. We also investigate the expected thermodynamic signatures for the specific heat above the fine-tuned excitonic quantum critical point (EQCP), in both 2d and 3d, in the SMC regime. We show that the EQCP is a different kind of critical point since although the EOP vanishes at the EQCP, there is always a finite gap in the SMC regime. We find that for high temperatures, the specific heat might exhibit a scaling behavior in the form $C_V/T \propto T^{(d-z)/z}$, where $d$ is the dimension of the system and $z$ is the dynamical critical exponent. The very low temperature behavior has a dominant exponential thermally activated term due to the presence of a gap that does not vanish at the excitonic transition.

How electrons Coulomb repulsion changes graphene band structure

Base on effective medium theory we introduce a multi sites method for calculation of realistic energy bands of strongly correlated systems. We found due to approximated self energy, the density of states that obtained directly by calculated local Green function does not reflects system energy bands truly. By using this method we investigated how electrons repulsion renormalizes graphene bands. Graphene realistic bands calculated in both the dynamical mean field theory (DMFT) and four sites beyond super cell approximation for different repulsions. Our calculated interacting graphene bands illustrate a semi metal to a Mott insulator anti ferromagnetic phase transition at repulsions u = 2.2 t and u = 0.6 t for DMFT and four sites beyond super cell approximation respectively. These values are much less than finite size quantum Monte Carlo calculation prediction. We showed that the graphene bands are very sensitive to electrons repulsions and this phase transition happens at low repulsions in comparison to graphene band width.

RKKY interaction of magnetic impurities in nodal-line semimetals

Motivated by recent upsurge in research of three-dimensional topological semimetals (SMs), we theoretically study the RKKY interaction between magnetic impurities in nodal-line SMs with and without the chirality and obtain the analytical expressions. We find that unique toroidal Fermi surface (FS) in nodal-line SMs, distinctly different from the spheroid FS in the SMs with the point nodes, has significant influence on the RKKY interaction, which leads to strong anisotropic oscillation and unique decay features. In the direction perpendicular to nodal-line plane, as usual, there is only one oscillation period related to the Fermi energy. In contrast, in the nodal-line plane, the RKKY interaction form a beating pattern and oscillates more rapidly with two distinct periods: one is coming from the Fermi energy and the other is from the radius of nodal-line. More importantly, inside nodal-line SMs bulk, the decay rate of RKKY interaction manifests a typical two-dimensional feature for impurities aligned along the direction perpendicular to nodal-line plane. Furthermore, the magnetic interactions in nodal-line SMs with linear and quadratic dispersions in the nodal-line plane are compared. We also discuss the possible application of the present theory on realistic nodal-line SM ZrSiSe. Our results shed a light for application of magnetically doped nodal-line SMs in spintronics.

Half antiperovskites VII – DFT modelling of shandites that are isoelectronic to Co3Sn2S2

AbstractOur discovery of half metal ferromagnetic (HFM) properties on shandite type Co3Sn2S2=Sn2Co3S2 about 20 years ago by DFT calculations opened the gate for fascinating discoveries like giant anomalous Hall effect and Weyl semimetal characteristics. Thereby, interest arose on electronic and magnetic structure effects upon substitution of M=Co and A=Sn sites in and between Co Kagomé layers. Non isoelectronic substitution to A=In or M=Ni causes a decay of magnetic properties to semiconducting diamagnetic InSnCo3S2 or paramagnetic semi metal Ni3Sn2S2. The present study addresses simultanious substitutions on both A and M sites to novel isoelectronic compounds of Co3Sn2S2. Therefore, DFT calculations were performed on model structures MM'2AA'X2 (M=Fe, Co, Ni; A=In, Sn, Sb; X=S). By the given approach, target compositions are identified that are interesting for future investigations and discoveries.

Magnetic, charge, and transport properties of graphene nanoflakes

We investigate magnetic, charge and transport properties of hexagonal graphene nanoflakes (GNFs) connected to two metallic leads by using the functional renormalization group (fRG) method. The interplay between the on-site and long-range interactions leads to a competition of semimetal (SM), spin density wave (SDW), and charge-density-wave (CDW) phases. The ground-state phase diagrams are presented for the GNF systems with screened realistic long-range electron interaction [T. O. Wehling, et. al., Phys. Rev. Lett. 106, 236805 (2011)], as well as uniformly screened long-range Coulomb potential $\propto 1/r$. We demonstrate that the realistic screening of Coulomb interaction by $\sigma$ bands causes moderate (strong) enhancement of critical long-range interaction strength, needed for the SDW (CDW) instability, compared to the results for the uniformly screened Coulomb potential. This enhancement gives rise to a wide region of stability of the SM phase for realistic interaction, such that freely suspended GNFs are far from both SM-SDW and SM-CDW phase-transition boundaries and correspond to the SM phase. Close relation between the linear conductance and the magnetic or charge states of the systems is discussed. A comparison of the results with those of other studies on GNFs systems and infinite graphene sheet is presented.

A study on the optical properties of the PMMA polymer doped with some semi metal dioxides nanoparticles

This work on a compound polymer PMMA doped with some semi-metal oxides nanoparticles (SiO2, TiO2) NPs, intended to be utilized in corrective Contact Lenses. For this purpose the optical properties of the PMMA compound (PMMA+2%SiO2, PMMA+2%TiO2, and PMMA+1%SiO2+1%TiO2) NPs were measured and calculated to get the proper refractive indices and dispersion properties (Abbe number) for these compounds for optical design optimization.

Topological phase transition in Sb-doped Mg3Bi2 monocrystalline thin films

${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ has been proved to be a semimetal with topological surface states (referred to as a topological semimetal, TSM) in the presence of spin-orbit coupling (SOC), and predicted to be a type-II nodal line semimetal (NSM) in the absence of SOC. It is possible to tune the effective SOC in ${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ by substituting the heavy Bi atom with lighter atoms, such as Sb or As, which results in topological phase transitions. Here in our work we investigate the evolution of the transport properties and band structures of ${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ by doping different concentrations of Sb through molecular beam epitaxy. Our results demonstrate the existence of phase transitions of TSM to trivial semimetal (SM) and SM to trivial insulator, with Sb concentration $x$ at ${x}_{1}\ensuremath{\approx}0.44--0.64$ and ${x}_{2}\ensuremath{\approx}1.39$, respectively. Further theoretical calculations give values of ${x}_{1}\ensuremath{\approx}0.523$ and ${x}_{2}\ensuremath{\approx}1.10$, which agrees well with our experimental results. And all these results are concluded into a ${\text{Mg}}_{3}{\text{Bi}}_{2\ensuremath{-}x}{\text{Sb}}_{x}$ phase diagram. Our work will stimulate more explorations to the possibilities of the realization of type-II NSM in ${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ by various elements substitution.

Emergence of −s, −p–d band inversion in zincblende gold iodide topological insulator and its thermoelectric properties

We employ first-principles calculations to investigate the topological states (TS) and thermoelectric (TE) transport properties of three dimensional (3D) gold iodide (AuI) which belongs to the zincblende family. We explore, semi-metal (SM) to topological conductor (TC) and topological insulator (TI) phase transitions. Under pristine conditions, AuI exhibits Dirac SM nature but, under the influence of mild isotropic compressive pressure the system undergoes electronic quantum phase transition driving it into non-trivial topological state. This state exhibits Dresselhaus like band spin splitting leading to a TC state. In order to realize TI state from the SM state, we break the cubic symmetry of the system by introducing a compressive pressure along (001) crystal direction. The non-trivial TI nature of the system is characterized by the emergence of robust surface states and the invariant ν 0 = 1 which indicates a strong TI nature. A novel facet of the phase transition discussed here is, the −s and −p, −d orbital band inversion mechanism which is unconventional as compared to previously explored TI families. This mechanism unravels new path by which TI materials can be predicted. Also, we investigated the lattice and electronic contributions to the TE transport properties. We characterize the TE performance by calculating the figure of merit (zT) and find that, at room temperature (300 K) and for a fixed doping concentration (i.e., n = 1 × 1019 cm−3) the zT is 0.55 and 0.53 for electrons and holes respectively. This is quite remarkable since, higher values of zT are generally predicted at higher temperature scales whereas, zT values as in the present case are desired at room temperatures for various energy applications. The manifestation of non-trivial TS governed by the unconventional band inversion mechanism and the TE properties of AuI make it a unique multi-functional candidate with probable thermoelectric and spintronic applications.

Publisher Correction: A charge-density-wave topological semimetal

A Correction to this paper has been published: https://doi.org/10.1038/s41567-021-01169-4 .