Metallic Ground State Research Articles

Semiconducting B13C2 system: structure search and DFT-based analysis

DFT calculation on Boron Carbide in B13C2 stoichiometry using a 15-atom unit cell necessarily results in metallic ground state regardless of the crystal structure. This is because such a unit cell consists of odd number of electrons, and hence complete filling of the top most band(s) of nonzero occupancy is impossible. This is in contrast to the observed semiconducting nature. If the crystal structure of B13C2 is made of a 30-atom unit cell which cannot be reduced to a 15 atom cell, there is a possibility of obtaining either a metallic or a semiconducting state as such a cell consists of an even number of electrons. In this work the evolutionary algorithm based structure search using 30-atom unit cells has yielded a previously unreported semiconducting system of B13C2 with unique bonding pattern. The mechanical and dynamical stability of the system have been properly established through the computation of elastic constants and phonon spectra. Its bond lengths, elastic moduli, hardness and infrared spectrum are in good agreement with experimental data.

Transport and Thermodynamic Properties of CeRu<sub>2</sub>Al<sub>10</sub> Controlled by Pressure at around Critical Pressure

We present transport and thermodynamic properties of CeRu2Al10 controlled by pressure in a vicinity of a critical pressure PC ~ 4GPa, where antiferromagnetic ordering disappears. The resistivity under pressure was measured with DC four terminal method and the AC specific heat under pressure was measured by Joule heating type technique. The pressure was applied by cubic-anvil-apparatus and palm-cubic-anvil-apparatus. The results of AC specific heat indicate TN holds at high temperature up to 3.9 GPa but suddenly disappears above this pressure. We confirmed TN from thermodynamic properties. Although CeRu2Al10 is in a Kondo semiconducting ground state at 4 GPa, temperature dependences of electrical resistivity at 4.6 GPa and 5.9 GPa indicate metallic ground state in these pressures. CeRu2Al10 does not show superconductivity down to 0.7 K at 4.6 GPa and 5.9 GPa.

Giant magnetoresistance control and nontrivial metallic state manipulation in a transition-metal dichalcogenide spin-valve using a gate voltage

Here, we have theoretically studied the valley- and spin-resolved transport in a monolayer transition metal dichalcogenides based spin valve device, where both the Rashba spin orbit interaction and a gate voltage coexist in the central lead. In contrast to conventional semiconductor, nontrivial metallic states, such as, normal Rashba metal state (NRMS), anomalous Rashba metal state (ARMS), and Rashba ring metal state (RRMS), can be generated and manipulated by Rashba spin orbit interaction without the magnetic effect. For a nonferromagnetic double junction, it was found that the valley- and spin-resolved tunneling conductance can be effectively tuned by the incident energy, the junction length, the Rashba spin orbit interaction strength, and the gate voltage. Due to the spin texture and the Fermi wavevectors in the central lead, both the tunneling coefficient and the tunneling conductance all exhibit the remarkable characteristic features which enable us to diagnose the special states. For a ferromagnetic spin valve device, the resulting nontrivial metallic groundstates in the central lead also demonstrate directly in the giant magnetoresistance with notable unique features. We have further revealed that a perfect valley and spin giant magnetoresistance stems from the spin splitting and the spin-valley coupling. These valley- and spin-resolved phenomena are interesting for both fundamental research and applications.

Carrier density and disorder tuned superconductor-metal transition in a two-dimensional electron system

Quantum ground states that arise at atomically controlled oxide interfaces provide an opportunity to address key questions in condensed matter physics, including the nature of two-dimensional metallic behaviour often observed adjacent to superconductivity. At the superconducting LaAlO3/SrTiO3 interface, a metallic ground state emerges upon the collapse of superconductivity with field-effect gating and is accompanied with a pseudogap. Here we utilize independent control of carrier density and disorder of the interfacial superconductor using dual electrostatic gates, which enables the comprehensive examination of the electronic phase diagram approaching zero temperature. We find that the pseudogap corresponds to precursor pairing, and the onset of long-range phase coherence forms a two-dimensional superconducting dome as a function of the dual-gate voltages. The gate-tuned superconductor–metal transitions are driven by macroscopic phase fluctuations of Josephson coupled superconducting puddles.

Investigation of correlation effects on the electronic structure of 3d1 perovskites

We have investigated the electronic structure of the 3d1 perovskites SrVO3, CaVO3, and YTiO3 by performing ab initio full-potential linearized augmented plane wave band-structure calculations within the density functional theory framework. The electronic band structure with the correct ground state for 3d1 perovskites could not be realized with the generalized gradient approximation (GGA). Although the GGA can explain the metallic ground state of SrVO3 and CaVO3, it fails to find the correct ground state of YTiO3, which is a Mott-Hubbard insulator with a band gap of approximately 0.7eV. Our GGA + U approach with systematic variation of both the electron (U) and the exchange (J) correlation energies has helped in understanding the correct electronic structure of these perovskites. We conducted a series of calculations with different Ueff (U − J) in the GGA + U approach to understand the effect of electron correlation on the electronic structure. Our procedure ultimately succeeded in describing the insulating electronic structure and correct ferromagnetic ground state of YTiO3.

Magnetic and magnetocaloric properties of the coloured Heusler phases GdAg2Mg andREAgAuMg (RE=Gd, Tb, Dy)

AbstractThe magnetocaloric effect (MCE) of the ferromagnetic compound GdAg2Mg [TC=98.3(5) K] was investigated along with its electrical resistivity and the specific heat capacity. The magnetic entropy changes (–ΔSM) as well as the changes in adiabatic temperature (ΔTad) have been calculated from these data. Furthermore, the magnetic susceptibility of the pseudo-quaternary Heusler phases GdAgAuMg, TbAgAuMg and DyAgAuMg [i.e.RE(Ag0.5Au0.5)2Mg] were measured and compared to the data for the pure silver and gold compoundsREAg2Mg andREAu2Mg. The substitution of the transition metal at the crystallographic Wyckoff site 8cinfluences the magnetic ground state of the trivalent rare earth metals and therefore drastically alters the Curie temperatures. The structure of GdAgAuMg was refined from single crystal X-ray diffraction data, revealing a small deviation from the equiatomic composition leading to the refined formula GdAg0.92(6)Au1.08(6)Mg [space groupFm3̅m,Z=4,a=695.03(10) pm,wR2=0.0883, 55F2values, six parameters]. The intermetallic compounds were synthesised in sealed niobium ampoules under high temperature conditions. They have reddish to brassy colour.

Valence and spin fluctuations in the Mn-doped ferroelectric BaTiO3

We study Mn substitution for Ti in BaTiO3 with and without compensating oxygen vacancies using density functional theory (DFT) in combination with dynamical mean field theory (DMFT). We find strong charge and spin fluctuations. Without compensating oxygen vacancies, the ground state is found to be a quantum superposition of two distinct atomic valences, 3d4 and 3d5. Introducing a compensating oxygen vacancy at a neighboring site reduces both charge and spin fluctuations due to the reduction of electron hopping from Mn to its ligands. As a consequence, valence fluctuations are reduced, and is closely fixed to the high spin 3d5 state. Here we show that inclusion of charge and spin fluctuations is necessary to obtain an accurate ground state of transition metal doped ferroelectrics.

Role of oxygen vacancy in the spin-state change and magnetic ordering in SrCoO3−δ

We present the first-principles investigation of the structural, electronic, and magnetic properties of SrCoO$_{3-\delta}$ ($\delta=0, 0.25, 0.5$) to understand the multivalent nature of Co ions in SrCoO$_{3-\delta}$ along the line of topotactic transition between perovskite SrCoO$_{3}$ and brownmillerite SrCoO$_{2.5}$. From the on-site Coulomb interaction $U$-dependent ground state of stoichiometric SrCoO$_{3}$, we show the proximity of its metallic ferromagnetic ground state to other antiferromagnetic states. The structural and magnetic properties of SrCoO$_{3-\delta}$ depending on their oxygen-content provide an interesting insight into the relationship between the Co-Co distances and the magnetic couplings so that the spin-state transition of Co spins can understood by the change of $pd$-hybridization depending on the Co-Co distances. The \emph{strong} suppression of the $dp\sigma$-hybridization between Co $d$ and O $p$ orbitals in brownmillerite SrCoO$_{2.5}$ brings on the high-spin state of Co$^{3+}$ $d^{6}$ and is responsible for the antiferromagnetically ordered insulating ground state. The increase of effective Co-Co distances driven by the presence of oxygen vacancies in SrCoO$_{3-\delta}$ is consistent with the reduction of the effective $pd$-hybridization between Co $d$ and O $p$ orbitals. We conclude that the configuration of neighboring Co spins is shown to be crucial to their local electronic structure near the metal-to-insulator transition along the line of the topotactic transition in SrCoO$_{3-\delta}$. Incidentally, we also find that the \textit{I2mb} symmetry of SrCoO$_{2.5}$ is energetically stable and exhibits ferroelectricity via the ordering of CoO$_{4}$ tetrahedra, where this polar lattice can be stabilized by the presence of a large activation barrier.

Static and dynamic signatures of anisotropic electronic phase separation in La2/3Ca1/3MnO3 thin films under anisotropic strain

The electronic phase separation (EPS) of optimally doped ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) thin films under various degrees of anisotropic strain is investigated by static magnetotransport and dynamic relaxation measurements. Three LCMO films were grown simultaneously on (001) $\mathrm{NdGa}{\mathrm{O}}_{3}$ substrates by pulsed laser deposition, and then postgrowth annealed at $780{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ in ${\mathrm{O}}_{2}$ for different durations of time. With increasing annealing time, the films developed significant strains of opposite signs along the two orthogonal in-plane directions. The static temperature-dependent resistivity, $\ensuremath{\rho}(T)$, was measured simultaneously along the two orthogonal directions. With increasing annealing time, both zero-field-cooled and field-cooled $\ensuremath{\rho}(T)$ show significant increases, suggesting strain-triggered EPS and appearance of antiferromagnetic insulating (AFI) phases in a ferromagnetic metallic (FMM) ground state. Meanwhile, $\ensuremath{\rho}(T)$ along the tensile-strained [010] direction becomes progressively larger than that along the compressive-strained [100]. The enhanced resistivity anisotropy indicates that the EPS is characterized by phase-separated FMM entities with a preferred orientation along [100], possibly due to the cooperative deformation and rotation/tilting of the $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra under the enhanced anisotropic strain. The anisotropic EPS can also be tuned by an external magnetic field. During a field cycle at several fixed temperatures, the AFI phases are melted at high fields and recovered at low fields, resulting in sharp resistance changes of ratio as high as ${10}^{4}$. Furthermore, the resistivity was found to exhibit glasslike behavior, relaxing logarithmically in the phase-separated states. Fitting the data to a phenomenological model, the resulting resistive viscosity and characteristic relaxation time are found to evolve with temperature, showing a close correlation with the static measurements in the EPS states.

Single-atom vacancy in monolayer phosphorene: A comprehensive study of stability and magnetism under applied strain

Using first-principles calculations based on density-functional theory we systematically investigate the effect of applied strain on the stability and on the electronic and magnetic properties of monolayer phosphorene with single-atom vacancy. We consider two types of single vacancies: the symmetric SV-55|66, which has a metallic and non-magnetic ground state, and the asymmetric SV-5|9, which is energetically more favorable and exhibits a semiconducting and magnetic character. Our results show that compressive strain up to 10%, both biaxial and uniaxial along the zigzag direction, reduces the formation energy of both single-atom vacancies with respect to the pristine configuration and can stabilize these defects in phosphorene. We found that the magnetic moment of the SV-5|9 system is robust under uniaxial strain in the range of −10 to +10%, and it is only destroyed under biaxial compressive strain larger than 8%, when the system also suffers a semiconductor-to-metal transition. Additionally, we found that magnetism can be induced in the SV-55|66 system under uniaxial compressive strain larger than 4% along the zigzag direction and under biaxial tensile strain larger than 6%. Our findings of small formation energies and non-zero magnetic moments for both SV-5|9 and SV-55|66 systems under zigzag uniaxial compressive strain larger than 4% strongly suggest that a magnetic configuration in monolayer phosphorene can be easily realized by single-vacancy formation under uniaxial compressive strain.

Dirac Nodes and Magnetic Order in M2X2 Transition‐Metal Chalcogenides

In this study, we perform a computational analysis of the M2X2 transition‐metal chalcogenides (TMCs). Using density functional theory with a spin‐polarized generalized gradient approximation, we examine the magnetic and electronic properties for the antiferromagnetic and ferromagnetic states with M = Cr, Mn, and Fe and X = S and Se. After optimizing the geometric structure for stability, we examine the spin‐polarized electronic structure, density of states, and Mulliken population. It is discovered that these materials are quasi‐two‐dimensional honeycomb lattices with metallic antiferromagnetic ground states. The structures consist of a distorted tetrahedral crystal‐field symmetry that has a distinct magnetic moment. An analysis of the electronic structure shows the presence of nodal points that resemble Dirac nodes for all cases, which leads to the possibility of the realization of magnetic Dirac materials.

Development of half metallicity within mixed magnetic phase of Cu1−xCoxMnSb alloy

Cubic half-Heusler Cu1−xCoxMnSb () compounds have been investigated both experimentally and theoretically for their magnetic, transport and electronic properties in search of possible half metallic antiferromagnetism. The systems (Cu,Co)MnSb are of particular interest as the end member alloys CuMnSb and CoMnSb are semi metallic (SM) antiferromagnetic (AFM) and half metallic (HM) ferromagnetic (FM), respectively. Clearly, Co-doping at the Cu-site of CuMnSb introduces changes in the carrier concentration at the Fermi level that may lead to half metallic ground state but there remains a persistent controversy whether the AFM to FM transition occurs simultaneously. Our experimental results reveal that the AFM to FM magnetic transition occurs through a percolation mechanism where Co-substitution gradually suppresses the AFM phase and forces FM polarization around every dopant cobalt. As a result a mixed magnetic phase is realized within this composition range while a nearly HM band structure is developed already at the 10% Co-doping. Absence of T2 dependence in the resistivity variation at low T-region serves as an indirect proof of opening up an energy gap at the Fermi surface in one of the spin channels. This is further corroborated by the ab initio electronic structure calculations that suggests that a nearly ferromagnetic half-metallic ground state is stabilized by Sb-p holes produced upon Co doping.

Electronic and optical responses of quasi-one-dimensional phosphorene nanoribbons to strain and electric field

Electronic and optical responses of zigzag- and armchair-edge quasi-one-dimensional phosphorene nanoribbons (Q1D-PNRs) to strain and external field are comparatively studied based on the tight-binding calculations. The results show that: (i) Zigzag-edge Q1D-PNR has the metallic ground state; applying global strains can not open the gap at the Fermi level but applying the electric field can achieve it; the direct/indirect character of the field-induced gap is determined by the electron-hole symmetry; an electric-field-enhanced optical absorption of low-energy photons is also predicted. (ii) Armchair-edge Q1D-PNR turns out an insulator with the large direct band gap; the inter-plane strain modulates this gap non monotonically while the in-plane one modulates it monotonically; in addition, the gap responses to electric fields also show strong direction dependence, i. e., increasing the inter-plane electric field will monotonically enlarge the gap but the electric field along the width direction modulates the gap non monotonically with three characteristic response regions.

X-ray absorption spectroscopy study of annealing process on Sr1–xLaxCuO2 electron-doped cuprate thin films

The superconducting properties of Sr1–xLaxCuO2 thin films are strongly affected by sample preparation procedures, including the annealing step, which are not always well controlled. We have studied the evolution of Cu L2,3 and O K edge x-ray absorption spectra (XAS) of Sr1–xLaxCuO2 thin films as a function of reducing annealing, both qualitatively and quantitatively. By using linearly polarized radiation, we are able to identify the signatures of the presence of apical oxygen in the as-grown sample and its gradual removal as a function of duration of 350 °C Ar annealing performed on the same sample. Even though the as-grown sample appears to be hole doped, we cannot identify the signature of the Zhang-Rice singlet in the O K XAS, and it is extremely unlikely that the interstitial excess oxygen can give rise to a superconducting or even a metallic ground state. XAS and x-ray linear dichroism analyses are, therefore, shown to be valuable tools to improving the control over the annealing process of electron doped superconductors.

Pauli metallic ground state in Hubbard clusters with Rashba spin-orbit coupling

We study the ``phase diagram'' of a Hubbard plaquette with Rashba spin-orbit coupling. We show that the peculiar way in which Rashba coupling breaks the spin-rotational symmetry of the Hubbard model allows a mixing of singlet and triplet components in the ground state that slows down and changes the nature of the Mott transition and of the Mott insulating phases.

Experimental and Theoretical Studies of the Metallic Conductivity in Cubic PbVO3 under High Pressure

The physical properties of the high-pressure (HP) cubic phase of PbVO3 were investigated on experimental and theoretical bases. Above 3 GPa, PbVO3 exhibits a structural transition from the tetragonal phase to the cubic phase. Electrical resistivity measurement on a twinned single-crystalline sample under HP showed the metallic conductivity of the HP phase, which was absent in the polycrystalline sample. The observed metallic ground state is in accordance with our theoretical calculations, which also suggest the presence of ferromagnetic ordering in the HP phase. However, the magnetic measurement under HP above the transition pressure did not reveal ferromagnetic behavior. Theoretical calculations produced the magnetic moments of the V ions in the HP phase; thus, this difference can be attributed to a lack of ferromagnetic ordering of magnetic moments.

Magnetic ground state of orthorhombically distorted CaCoO3 perovskite oxide: GGA and GGA+U calculations

Cobalt-based perovskite oxides have been attracted considerable attention due to their fascinating properties, especially entangled magnetic spin structures. Here, we comprehensively study the various magnetic ordering in recently reported orthorhombically distorted CaCoO3 perovskite oxide by employing the density functional theory calculations. A strong ferromagnetic coupling due to double exchange interactions between neighbouring Co atoms within the CoO2 bilayers is found than the anti-ferromagnetic coupling, which results in ferromagnetic spin ordering as a magnetic ground state with metallic electronic ground state (even Co is in its +4 state) for both exchange-correlation potentials (generalized gradient approximation (GGA) and GGA+U). The estimated magnetic transition temperature TC = 116/139 K for GGA/GGA+U, which is close to the experimentally observed value of 95 K for helimagnetic ground state.

Novel boron channel-based structure of boron carbide at high pressures

Boron carbide (B4C) is one of the hardest materials known to date. The extreme hardness of B4C arises from architecturally efficient B12 or B11C icosahedrons and strong inter-icosahedral B–C bonding. As an excellent material for use in ballistic armor, the mechanic limit of B4C and possible phase transitions under extreme stress conditions are of great interest. Here we systematically explored the post-icosahedral solid structures of B4C under high pressure, using an unbiased structure search method. A new structure composed of extended framework of B and zigzag chains of C is predicted to be stable above 96 GPa. The new structure was predicted to have a high Vickers hardness of 55 GPa and simultaneously to retain a metallic ground state. The exceptional mechanical properties found in this structure are attributed to strong sp3 covalent network formed under extreme pressure conditions. The predicted structure represents a new type of superhard boron carbides that form under high pressure without the presence of boron icosahedrons, which encourages experimental exploration in this direction.

Stability, electronic and phononic properties of β and 1T structures of SiTex (x = 1, 2) and their vertical heterostructures

By performing first-principles calculations, we predict a novel, stable single layer phase of silicon ditelluride, 1T-, and its possible vertical heterostructures with single layer β-SiTe. Structural optimization and phonon calculations reveal that 1T- structure has a dynamically stable ground state. Further analysis of the vibrational spectrum at the point shows that single layer 1T- has characteristic phonon modes at 80, 149, 191 and 294 . Electronic-band structure demonstrates that 1T- phase exhibits a nonmagnetic metallic ground state with a negligible intrinsic spin–orbit splitting. Moreover, it is shown that similar structural parameters of 1T- and existing β-SiTe phases allows construction of 1T-β heterostructures with a negligible lattice mismatch. In this regard, it is found that two energetically favorable stacking orders, namely AA and B, have distinctive shear and layer breathing phonon modes. It is important to note that the combination of semiconducting β-SiTe and metallic 1T- building blocks forms ultra-thin Schottky barriers that can be used in nanoscale optoelectronic device technologies.

Metallic phases from disordered (2+1)-dimensional quantum electrodynamics

Metallic phases have been observed in several disordered two dimensional (2d) systems, including thin films near superconductor-insulator transitions and quantum Hall systems near plateau transitions. The existence of 2d metallic phases at zero temperature generally requires an interplay of disorder and interaction effects. Consequently, experimental observations of 2d metallic behavior have largely defied explanation. We formulate a general stability criterion for strongly interacting, massless Dirac fermions against disorder, which describe metallic ground states with vanishing density of states. We show that (2+1)-dimensional quantum electrodynamics (QED$_3$) with a large, even number of fermion flavors remains metallic in the presence of weak scalar potential disorder due to the dynamic screening of disorder by gauge fluctuations. We also show that QED$_3$ with weak mass disorder exhibits a stable, dirty metallic phase in which both interactions and disorder play important roles.