Family Of Semimetals Research Articles

Symmetry of Identical Particles, Modern Achievements in the Pauli Exclusion Principle, in Superconductivity and in Some Other Phenomena

In this review, the modern achievements in studies of the Pauli exclusion principle (PEP) and the properties of the identical particle systems when PEP is not fulfilled are discussed. The validity of conception of the spin in the framework of density functional theory (DFT) approaches is analyzed. The modern state of the recently discovered Fe-based superconductors is discussed in detail. These materials belong to the paramagnetic semimetal family and become superconductors upon doping. Recently, in 2020, room-temperature superconductivity was realized. However, from the following discussion in the SC community, it was not evident that the results of room-temperature superconductivity have been repeated by other laboratories. Thus, the question “is room temperature really achieved?” is still open. In the concluding remarks, we present the explanation of why the PEP limitations on the symmetry of identical particles system exist in nature, and following from it, some important consequences.

125Te spin-lattice relaxation in a candidate to Weyl semimetals WTe2

The tungsten ditelluride WTe2 was suggested to belong to the Weyl semimetal family. We studied 125Te spin-lattice relaxation and NMR spectra in a WTe2 single crystal within a large range from 28 K up to room temperature. Measurements were carried out on a Bruker Avance 500 NMR pulse spectrometer for the crystalline c axis directed in parallel and perpendicular to the magnetic field. Relaxation proved to be single-exponential. The relaxation time varied depending on the sample position in magnetic field and frequency offset. The relaxation rate increased about linearly with temperature below 70 K, however the dependence became nearly quadratic at higher temperatures. The relaxation rate within the total temperature range was fitted using a theoretical model developed in Ref. [41] for Weyl semimetals and assuming the decrease of the chemical potential with increasing temperature. The results obtained for 125Te spin-lattice relaxation evidence in favor of the topological nontriviality of the WTe2 semimetal. The 125Te NMR spectra agreed with the occurrence of nonequivalent tellurium sites and varied insignificantly with temperature.

Topological properties after light ion irradiation on Weyl semimetal niobium phosphide from first principles

A member of the nonmagnetic Weyl semimetal family, niobium phosphide (NbP), is a new low-energy dissipative material due to the properties of its nontrivial energy band structure and topologically protected nodes. This paper mainly focus on the changes of the topological properties of the crystal NbP after H and He ions irradiation, a novel research view. In order to demonstrate the formation process of doping defects, we use TD-DFT to simulate the whole path of H ion passing the bulk NbP, while both the total energy change for system and the radial drag force as a function of the projectile position are investigated. The energy band structures and density of states for the equilibrium point defect have been analyzed by means of the DFT method based on the CASTEP package. A strikingly phenomenon discovered is that diverse kinds of defects are able to modify the symmetrically protected Weyl points to some extent. Specifically, when H or He as an interstitial particle in the crystal NbP, the degenerate bands are uncoupled; meanwhile, the Fermi levels are shifted upward 0.12 eV and 0.18 eV, respectively. In contrast to the defect structures formed by P, the defects caused by Nb present the greater destructive power to the Weyl points. By contrast, the calculations of formation energy have demonstrated that H substitute for Nb is the most easily formed defect structure, and the defect with interstitial H atom is most stable structure within the scope of this research.

Multiple metamagnetism, extreme magnetoresistance and nontrivial topological electronic structures in the magnetic semimetal candidate holmium monobismuthide

Inconceivably large changes (up to 106%) of the resistivity induced by external magnetic field—a phenomenon known as the extreme magnetoresistance effect has been reported in a great number of exotic semimetals. The very recent and exciting discoveries mainly pay attention to the compounds without magnetic ground states, which appears to limit the potential growth of semimetal family. For fundamental scientific interests, introduction of spin degree of freedom would provide an almost ideal platform for investigating the correlation effect between magnetism, crystallographic structure and electric resistivity in materials. Here, we report the experimental observation of metamagnetic behaviors and transport properties of HoBi single crystals. Being a magnetic member of the rare earth monopnictide family, the magnetoresistance of HoBi is significantly modulated by the magnetic orders at low temperature, which shows a nonmonotonic increment across the successive magnetic phases and reaches 104% (9 T and 2 K) in the ferromagnetic state. Kohler’s rule predicts that more than one type of carriers dominates the transport properties. Well fitted magnetoresistance and Hall resistivity curves by the semiclassical two-band model suggest that the densities of electron and hole carriers are nearly compensated and the carrier mobilities in this compound are ultrahigh. Besides, the inverted band structures and nonzero Z2 topological invariant indicate that possible nontrivial electronic states could generate in the ferromagnetic phase of HoBi. Combining the experimental and theoretical results, it is found that the cooperative action of carrier compensation effect and ultrahigh mobility might contribute to the extreme magnetoresistance observed in the titled compound. These findings suggest a paradigm for obtaining the extreme magnetoresistance in magnetic compounds and are relevant to understand the rare-earth-based correlated topological materials.

Anomalous quantum oscillations and evidence for a non-trivial Berry phase in SmSb

Topologically non-trivial electronic structures can give rise to a range of unusual physical phenomena, and the interplay of band topology with other effects such as electronic correlations and magnetism requires further exploration. The rare earth monopnictides X(Sb,Bi) (X = lanthanide) are a large family of semimetals where these different effects may be tuned by the substitution of rare-earth elements. Here we observe anomalous behavior in the quantum oscillations of one member of this family, antiferromagnetic SmSb. The analysis of Shubnikov-de Haas (SdH) oscillations provides evidence for a non-zero Berry phase, indicating a non-trivial topology of the α-band. Furthermore, striking differences are found between the temperature dependence of the amplitudes of de Haas-van Alphen effect oscillations, which are well fitted by the Lifshitz-Kosevich (LK) formula across the measured temperature range, and those from SdH measurements which show a significant disagreement with LK behavior at low temperatures. Our findings of unusual quantum oscillations in an antiferromagnetic, mixed valence semimetal with a possible non-trivial band topology can provide an opportunity for studying the interplay between topology, electronic correlations and magnetism.

Nodal-line semimetal states in the positive-electrode material of a lead-acid battery: Lead dioxide family and its derivatives

Based on first-principles calculations and symmetry analysis, we report that the three-dimensional (3D) nodal line (NL) semimetal phases can be realized in the lead dioxide family (\emph{$\alpha$}-PbO$_2$ and \emph{$\beta$}-PbO$_2$) and its derivatives. The \emph{$\beta$}-PbO$_2$ features two orthogonal nodal rings around the Fermi level, protected by the mirror reflection symmetry. The effective model is developed and the related parameters are given by fitting with the HSE06 band structures. The NLs mainly come from the $p$ orbitals of the light element O and are rather robust against such tiny spin-orbit coupling. The NL phase of the \emph{$\alpha$}-PbO$_2$ can be effectively tailored by strain, making a topological phase transition between a semiconductor phase and a NL phase. In addition, the exploration of \emph{$\beta$}-PbO$_2$ derivatives (i.e. \emph{$\beta$}-PbS$_2$ and \emph{$\beta$}-PbSe$_2$) and confirmation of their topological semimetallicity greatly enrich the NL semimetal family. These findings pave a route for designing topological NL semimetals and spintronic devices based on realistic PbO$_2$ family.

Superconductivity in the 2-Dimensional Homologous Series AMm Bi3 Q5+m (m=1, 2) (A=Rb, Cs; M=Pb, Sn; Q=Se, Te).

Superconductivity in the two-dimensional AMm Bi3 Q5+m family of semimetals is reported. The AMBi3 Te6 (m=1) and AM2 Bi3 Te7 (m=2) members of the homologous series with A=Rb, Cs and M=Pb, Sn undergo a bulk superconducting transition ranging from 2.7 to 1.4 K depending on the composition. The estimated superconducting volume fraction is about 90 %. Superconducting phase diagrams as a function of chemical pressure are constructed for the solid solution products of each member of the homologous series, AMBi3-x Sbx Te6-y Sey and AM2 Bi3-x Sbx Te7-y Sey (0≤x≤3 or 0≤y≤2). The structural flexibility of the ternary AMm M'3 Te5+m semiconducting homology to form isostructural analogues with a variety of metals, M=Pb, Sn; M'=Bi, Sb, gives access to a large number of electronic configurations and superconductivity due to chemical pressure effects.

Magneto-transport properties of proposed triply degenerate topological semimetal Pd3Bi2S2

We report transport properties of single-crystalline Pd3Bi2S2, which has been predicted to host an unconventional electronic phase of matter beyond three-dimensional Dirac and Weyl semimetals. Similar to several topological systems, the resistivity shows field-induced metal to semiconductor-like crossover at low temperature. Large, anisotropic, and non-saturating magnetoresistance has been observed in the transverse experimental configuration. At 2 K and 9 T, the MR value reaches as high as ∼1.1 × 103%. Hall resistivity reveals the presence of two types of charge carriers and has been analyzed using the two-band model. In spite of the large density (>1021 cm−3), the mobility of charge carriers is found to be quite high (∼ 0.75 × 104 cm2 V−1 s−1 for the hole and ∼ 0.3 × 104 cm2 V−1 s−1 for the electron). The observed magneto-electrical properties indicate that Pd3Bi2S2 may be a new member of the topological semimetal family, which can have a significant impact in technological applications.

Extreme magnetoresistance in magnetic rare-earth monopnictides

The acute sensitivity of the electrical resistance of certain systems to magnetic fields known as extreme magnetoresistance (XMR) has recently been explored in a new materials context with topological semimetals. Exemplified by WTe$_{2}$ and rare earth monopnictide La(Sb,Bi), these systems tend to be non-magnetic, nearly compensated semimetals and represent a platform for large magnetoresistance driven by intrinsic electronic structure. Here we explore electronic transport in magnetic members of the latter family of semimetals and find that XMR is strongly modulated by magnetic order. In particular, CeSb exhibits XMR in excess of $1.6 \times 10^{6}$ % at fields of 9 T while the magnetoresistance itself is non-monotonic across the various magnetic phases and shows a transition from negative magnetoresistance to XMR with field above magnetic ordering temperature $T_{N}$. The magnitude of the XMR is larger than in other rare earth monopnictides including the non-magnetic members and follows an non-saturating power law to fields above 30 T. We show that the overall response can be understood as the modulation of conductivity by the Ce orbital state and for intermediate temperatures can be characterized by an effective medium model. Comparison to the orbitally quenched compound GdBi supports the correlation of XMR with the onset of magnetic ordering and compensation and highlights the unique combination of orbital inversion and type-I magnetic ordering in CeSb in determining its large response. These findings suggest a paradigm for magneto-orbital control of XMR and are relevant to the understanding of rare earth-based correlated topological materials.

Observation of topological surface states and strong electron/hole imbalance in extreme magnetoresistance compound LaBi

The recent discovery of the extreme magnetoresistance (XMR) in the nonmagnetic rare-earth monopnictides LaX (X = P, As, Sb, Bi,), a recently proposed new topological semimetal family, has inspired intensive research effort in the exploration of the correlation between the XMR and their electronic structures. In this work, using angle-resolved photoemission spectroscopy to investigate the three-dimensional band structure of LaBi, we unraveled its topologically nontrivial nature with the observation of multiple topological surface Dirac fermions, as supported by our ab initio calculations. Furthermore, we observed substantial imbalance between the volume of electron and hole pockets, which rules out the electron-hole compensation as the primary cause of the XMR in LaBi.

Dirac line nodes and effect of spin-orbit coupling in the nonsymmorphic critical semimetals MSiS(M=Hf,Zr)

Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk (conduction and valance) bands at discrete points in the momentum space (i.e. 3D Dirac points), such as in Na3Bi and Cd3As2. More recently, new types of Dirac semimetals with robust Dirac line-nodes (with non-trivial topology or near the critical point between topological phase transitions) have been proposed that extends the bulk linear touching from discrete points to 1D lines. In this work, using angle-resolved photoemission spectroscopy (ARPES), we explored the electronic structure of the non-symmorphic crystals MSiS (M=Hf, Zr). Remarkably, by mapping out the band structure in the full 3D Brillouin Zone (BZ), we observed two sets of Dirac line-nodes in parallel with the kz-axis and their dispersions. Interestingly, along directions other than the line-nodes in the 3D BZ, the bulk degeneracy is lifted by spin-orbit coupling (SOC) in both compounds with larger magnitude in HfSiS. Our work not only experimentally confirms a new Dirac line-node semimetal family protected by non-symmorphic symmetry, but also helps understanding and further exploring the exotic properties as well as practical applications of the MSiS family of compounds.

Fermi surface interconnectivity and topology in Weyl fermion semimetals TaAs, TaP, NbAs, and NbP

The family of binary compounds including TaAs, TaP, NbAs, and NbP was recently discovered as the first realization of Weyl semimetals. In order to develop a comprehensive description of the charge carriers in these Weyl semimetals, we performed systematic electronic structure calculations which reveal the nature of Fermi surfaces and their complex interconnectivity in TaAs, TaP, NbAs, and NbP. Our work report the first comparative and comprehensive study of Fermi surface topology and band structure details of all known members of the Weyl semimetal family and provide the necessary building blocks for advancing our understanding of their unique topologically protected low-energy Weyl fermion physics.