Condensed Matter Physics Community Research Articles

Disentangling photoexcitation and photoluminescence processes in defective MgO

Oxygen vacancies are ubiquitous in oxides and strongly influence the material's electronic structure and catalytic and transport properties. Here we focus on a seemingly simple defective oxide, MgO, and on the electronic properties of oxygen vacancies, which remain controversial in spite of numerous studies. We present an ab initio investigation of the photoexcitation and photoionization process of these defects, using a newly developed embedding Bethe-Salpeter equation approach, implemented in the WEST code. We find absorption and emission energies in good agreement with experiments. Our results provide a detailed, microscopic understanding of the absorption and emission processes of the neutral and positively charged oxygen vacancy, reconciling different views present in the chemistry and condensed-matter physics communities.

Atomic-Resolution Cryogenic Scanning Transmission Electron Microscopy for Quantum Materials.

ConspectusThe rich physics permeating the phase diagrams of quantum materials have commanded the attention of the solid-state chemistry, materials science, and condensed-matter physics communities, sparking immense research into quantum phase transitions including superconducting, ferroic, and charge-order transitions. Many of these transitions occur at low temperatures and involve electronic, magnetic, or lattice order, which emerges on the atomic to mesoscopic scales. The complex interplay of these states and the heterogeneity that arises due to competition and intertwining of phases, however, is not fully understood and requires probes that capture ordering over multiple length scales down to the local atomic symmetries. Advances in scanning transmission electron microscopy (STEM) have enabled atomic-resolution imaging as well as mapping of functional picometer-scale atomic displacements inside materials. In this Account, we discuss our group's work to expand the reach of atomic-resolution STEM to cryogenic temperatures (cryo-STEM) to study quantum materials with focus on charge-ordered systems.Charge-ordered phases, in which electrons as well as the atomic lattice form periodic patterns that lift the translational symmetries of the crystal, are not only intertwined with superconductivity but also underlie other exotic electronic phenomena such as colossal magnetoresistance and metal-insulator transitions. The periodic lattice distortions (PLDs) modulate the positions of the crystal's nuclei, which can be readily probed by electron microscopy. In a set of examples, we demonstrate cryo-STEM as a powerful technique for probing local order, nanometer-scale heterogeneities, and topological defects in charge-ordered manganites and in transition metal dichalcogenide charge density wave (CDW) systems.With the nearly commensurate-to-commensurate CDW transition upon cooling in 1T-TaS2, we show that nanoscale lattice textures in CDW phases can be revealed through direct imaging. These early atomic-resolution results, however, also highlighted the need for improvements in cryo-STEM imaging, which led to a push to advance data collection and analysis for direct spatial mapping and quantification of PLDs. By introducing an image registration algorithm developed specifically to accommodate fast, low signal-to-noise image acquisitions of crystalline lattices, we address previous limitations due to sample drift in cryo-STEM experiments. This has enabled subangstrom cryo-STEM imaging with sufficient signal-to-noise to reveal the low temperature structure of 1T'-TaTe2. Furthermore, it allows mapping and quantification of PLD atomic displacements in the charge-ordered manganites Bi0.35Sr0.18Ca0.47MnO3 and Nd0.5Sr0.5MnO3 with picometer precision at ∼95 K to resolve not only distinct ordered phases (i.e., site- and bond-centered charge order) but also their nanoscale coexistence within the same sample.Atomic-resolution cryo-STEM opens new opportunities for understanding the microscopic underpinnings of quantum phases. In this Account, we focus on spatial mapping of lattice degrees of freedom in phases that are present at temperatures down to liquid nitrogen. Further advances in instrumentation are needed to expand the temperature range and to also enable atomic-resolution measurements that rely on weaker signals such as electron energy loss spectroscopy (EELS) for probing of electronic structure or 4D-STEM approaches to map electric and magnetic fields.

Novel synthesis approach for “stubborn” metals and metal oxides

Advances in physical vapor deposition techniques have led to a myriad of quantum materials and technological breakthroughs, affecting all areas of nanoscience and nanotechnology which rely on the innovation in synthesis. Despite this, one area that remains challenging is the synthesis of atomically precise complex metal oxide thin films and heterostructures containing "stubborn" elements that are not only nontrivial to evaporate/sublimate but also hard to oxidize. Here, we report a simple yet atomically controlled synthesis approach that bridges this gap. Using platinum and ruthenium as examples, we show that both the low vapor pressure and the difficulty in oxidizing a "stubborn" element can be addressed by using a solid metal-organic compound with significantly higher vapor pressure and with the added benefits of being in a preoxidized state along with excellent thermal and air stability. We demonstrate the synthesis of high-quality single crystalline, epitaxial Pt, and RuO2 films, resulting in a record high residual resistivity ratio (=27) in Pt films and low residual resistivity, ∼6 μΩ·cm, in RuO2 films. We further demonstrate, using SrRuO3 as an example, the viability of this approach for more complex materials with the same ease and control that has been largely responsible for the success of the molecular beam epitaxy of III-V semiconductors. Our approach is a major step forward in the synthesis science of "stubborn" materials, which have been of significant interest to the materials science and the condensed matter physics community.

On strong-scaling and open-source tools for analyzing atom probe tomography data

The development of strong-scaling computational tools for high-throughput methods with an open-source code and transparent metadata standards has successfully transformed many computational materials science communities. While such tools are mature already in the condensed-matter physics community, the situation is still very different for many experimentalists. Atom probe tomography (APT) is one example. This microscopy and microanalysis technique has matured into a versatile nano-analytical characterization tool with applications that range from materials science to geology and possibly beyond. Here, data science tools are required for extracting chemo-structural spatial correlations from the reconstructed point cloud. For APT and other high-end analysis techniques, post-processing is mostly executed with proprietary software tools, which are opaque in their execution and have often limited performance. Software development by members of the scientific community has improved the situation but compared to the sophistication in the field of computational materials science several gaps remain. This is particularly the case for open-source tools that support scientific computing hardware, tools which enable high-throughput workflows, and open well-documented metadata standards to align experimental research better with the fair data stewardship principles. To this end, we introduce paraprobe, an open-source tool for scientific computing and high-throughput studying of point cloud data, here exemplified with APT. We show how to quantify uncertainties while applying several computational geometry, spatial statistics, and clustering tasks for post-processing APT datasets as large as two billion ions. These tools work well in concert with Python and HDF5 to enable several orders of magnitude performance gain, automation, and reproducibility.

Coexistence of non-Fermi liquid behavior and biquadratic exchange coupling in La-substituted CeGe: Nonlinear susceptibility and DFT+DMFT study

Studies connected with the investigations of non-Fermi liquid (NFL) systems continue to attract interest in condensed matter physics community. Understanding the anomalous physical properties exhibited by such systems and its related electronic structures is one of the central research topics in this area. In this context, Ce-based and Ce-site diluted (with non-magnetic ions) compounds provide a fertile playground. Here, we present a detailed study of non-linear DC susceptibility and combined density functional theory plus dynamical mean field theory (DFT+DMFT) on Ce0.24La0.76Ge. Theoretical investigation of 4f partial density of states, local susceptibility and self-energy demonstrates the presence of NFL behavior which is associated with fluctuating local moments. Non-linear DC susceptibility studies on this compound reveal that the transition from NFL state to the new phase is due to development of the bi-quadratic exchange coupling and it obeys the non-linear susceptibility scaling. Under the application of magnetic fields, local moments interact spatially through conduction electrons resulting in magnetic fluctuations. Our studies point to the fact that the origin of the observed bi-quadratic exchange coupling is due to the spatial magnetic fluctuations.

Normal State of Nd1−xSrxNiO2 from Self-Consistent GW+EDMFT

The recent discovery of superconductivity in hole-doped NdNiO$_2$ thin films has captivated the condensed matter physics community. Such compounds with a formal Ni$^+$ valence have been theoretically proposed as possible analogues of the cuprates, and the exploration of their electronic structure and pairing mechanism may provide important insights into the phenomenon of unconventional superconductivity. At the modeling level, there are however fundamental issues that need to be resolved. While it is generally agreed that the low-energy properties of cuprates can to a large extent be captured by a single-band model, there has been a controversy in the recent literature about the importance of a multi-band description of the nickelates. The origin of this controversy is that studies based entirely on density functional theory (DFT) calculations miss important correlation and multi-orbital effects induced by Hund coupling, while model calculations or simulations based on the combination of DFT and (extended) dynamical mean field theory ((E)DMFT) involve ad-hoc parameters and double counting corrections that substantially affect the results. Here we use a multi-site extension of the recently developed $GW$+EDMFT method, which is free of adjustable parameters, to self-consistently compute the interaction parameters and electronic structure of hole-doped NdNiO$_2$. This full ab-initio simulation demonstrates the importance of a multi-orbital description, even for the undoped compound, and produces results for the resistivity and Hall conductance in qualitative agreement with experiment.

F90wrap: an automated tool for constructing deep Python interfaces to modern Fortran codes

f90wrap is a tool to automatically generate Python extension modules which interface to Fortran libraries that makes use of derived types. It builds on the capabilities of the popular f2py utility by generating a simpler Fortran 90 interface to the original Fortran code which is then suitable for wrapping with f2py, together with a higher-level Pythonic wrapper that makes the existance of an additional layer transparent to the final user. f90wrap has been used to wrap a number of large software packages of relevance to the condensed matter physics community, including the QUIP molecular dynamics code and the CASTEP density functional theory code.

Accelerating Time-Dependent Density Functional Theory and GW Calculations for Molecules and Nanoclusters with Symmetry Adapted Interpolative Separable Density Fitting

Computing integrals over orbital pairs is one of the most costly steps in many popular first-principles methods used by the quantum chemistry and condensed matter physics community. Here, we employ a recently proposed interpolative separable density fitting method (ISDF) to significantly reduce the cost of steps involving orbital pairs in linear response time-dependent density functional theory and GW calculations. In our implementation, we exploit the symmetry property of a system to effectively reduce the number of interpolation points and thus the computational cost. The performance of ISDF is illustrated with calculations on the GW100 set and silicon nanoclusters. We demonstrate the cost for constructing auxiliary basis and interpolation coefficients are negligible compared to the total cost. Compared to the conventional brute-force approach, the computation cost for evaluating all kernel matrix elements is reduced by nearly 3 orders of magnitude. The total cost for GW calculations can be reduced by four to eight times, depending on the system size.

Eigensolver performance comparison on Cray XC systems

SummaryHermitian (symmetric) eigenvalue solvers are the core constituents of electronic structure, quantum‐chemistry, and other HPC applications such as Quantum ESPRESSO, VASP, CP2K, and NWChem to name a few. Our understanding of the performance of symmetric eigenvalue algorithms on various hardware is clearly important to the quantum chemistry or condensed matter physics community but in fact goes beyond that community. For instance, big data analytics is increasingly utilizing eigenvalues solvers, in the study of randomized singular value decomposition (SVD) or principal component analysis (PCA). Noise, vibration, and harshness (NVH) is another field where fast and efficient eigenvalue solvers are required. Most eigenvalue solver packages feature numerous different parameters which can be tuned for performance, eg, the number of nodes, number of total ranks, the decomposition of the matrix, etc. In this paper, we investigate the performance of different packages as well as the influence of these knobs on the solver performance.

Inducing Strong Superconductivity in WTe2 by a Proximity Effect.

The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe2, in a van der Waals hybrid structure obtained by mechanically transferring NbSe2 onto various thicknesses of WTe2. When the WTe2 thickness ( tWTe2) reaches 21 nm, the superconducting transition occurs around the critical temperature ( Tc) of NbSe2 with a gap amplitude (Δp) of 0.38 meV and an unexpected ultralong proximity length ( lp) up to 7 μm. With the thicker 42 nm WTe2 layer, however, the proximity effect yields Tc ≈ 1.2 K, Δp = 0.07 meV, and a short lp of less than 1 μm. Our theoretical calculations, based on the Bogoliubov-de Gennes equations in the clean limit, predict that the induced superconducting gap is a sizable fraction of the NbSe2 superconducting one when tWTe2 is less than 30 nm and then decreases quickly as tWTe2 increases. This agrees qualitatively well with the experiments. Such observations form a basis in the search for superconducting phases in topological semimetals.

Non-commutative Chern numbers for generic aperiodic discrete systems

The search for strong topological phases in generic aperiodic materials and meta-materials is now vigorously pursued by the condensed matter physics community. In this work, we first introduce the concept of patterned resonators as a unifying theoretical framework for topological electronic, photonic, phononic etc (aperiodic) systems. We then discuss, in physical terms, the philosophy behind an operator theoretic analysis used to systematize such systems. A model calculation of the Hall conductance of a 2-dimensional amorphous lattice is given, where we present numerical evidence of its quantization in the mobility gap regime. Motivated by such facts, we then present the main result of our work, which is the extension of the Chern number formulas to Hamiltonians associated to lattices without a canonical labeling of the sites, together with index theorems that assure the quantization and stability of these Chern numbers in the mobility gap regime. Our results cover a broad range of applications, in particular, those involving quasi-crystalline, amorphous as well as synthetic (i.e. algorithmically generated) lattices.

Investigation of magnetic and transport properties of La0.8Sm0.2O0.5F0.5BiS2 superconductor under external hydrostatic pressure

The dc electrical resistivity and magnetization measurements as a function of temperature 1.8-300 K were formed at external hydrostatic pressures on normal and superconducting state of new polycrystalline BiS2 based layered La0.2Sm0.8BiS2O00.5F0.5 superconductor. In situ, electrical resistivity measurements, At ambient pressure the superconducting transition temperature (Tc) at ~3 K which are enhanced substantially under external pressure to 10.3 K at 1.74 GPa with positive pressure coefficient, dTc/dP= 4.19 K/GPa. Further increasing pressure Tc has been decreased upto 6.2 K at 1.96 GPa with negative pressure coefficient dTc/dP= -2.09 K/GPa. From the normal state resistivity exhibits the strong suppression of the semiconducting behavior in La/SmO0.5F0.5BiS2 compounds suggests that the Fermi surface is in the vicinity of some instability. We report here the enhancement of Tc from 2.83 to 5.86 K with small increase in pressure upto 0.8 GPa with similar pressure coefficient of dTc/dP= 3.78 K/GPa. However, it should be noted that either chemical substitution or physical pressure on these systems show a dome shaped Tc variation which means that the enhancement may not be achieved for higher doping concentration and (or) pressure s beyond an optimum level. In our results, exhibits which higher Tc may be achieved lies around 1.74 GPa. Above this pressure the Tc may decrease and exhibit a dome shaped behavior similar to other rare –earth systems of BiS2 and Fe-based superconductors. Substantial enhancement of superconductivity under moderate pressure in studied new BiS2 based superconductors call for the attention of condensed matter physics community.

Thermoelectric signals of state transition in polycrystalline SmB6

Topological Kondo insulator SmB6 has attracted quite a lot of attention from the condensed matter physics community. A number of unique electronic properties, including low-temperature resistivity anomaly, 1D electronic transport and 2D Fermi surfaces have been observed in SmB6. Here, we report on thermoelectric transport properties of polycrystalline SmB6 over a broad temperature from 300 to 2 K. An anomalous transition in the temperature-dependent Seebeck coefficient S from S(T) T−1 to S(T) T was observed around 12 K. Such a transition demonstrates a transition of conductivity from 3D metallic bulk states to 2D metallic surface states with insulating bulk states. Our results suggest that the thermotransport measurements could be used for the characterization of state transition in topological insulators.

A probabilistic model of the electron transport in films of nanocrystals arranged in a cubic lattice

The fabrication of nanocrystal (NC) films, starting from colloidal dispersion, is a very attractive topic in condensed matter physics community. NC films can be employed for transistors, light emitting diodes, lasers, and solar cells. For this reason the understanding of the film conductivity is of major importance. In this paper we describe a probabilistic model that allows the prediction of the conductivity of NC films, in this case of a cubic lattice of Lead Selenide or Cadmium Selenide NCs. The model is based on the hopping probability between NCs. The results are compared to experimental data reported in literature.

Effect of Hydrostatic Pressure on BiS2-Based Layered Superconductors: A Review

AbstractWe report impact of hydrostatic pressure on the newly discovered BiS2 based superconductors. In last couple of years the new BiS2 based superconductor attracted great attention of the condensed matter physics community. These new layered BiS2-based compounds are very sensitive to the concentration of carriers doping and pressure that cause profound changes in their physical properties with appearance of superconductivity in the vicinity of their insulating/semiconducting state. The BiS2-based new compounds are expected to provide us with the next stage of exploring new superconductors and to discuss their exotic superconductivity mechanisms. In current review, we present most of our findings related to impact of hydrostatic pressure on superconductivity of new BiS2 based superconductors at one place.

Hydrostatic pressure effect on new BiS2 based Bi4O4S3 and ReO/FBiS2 (Re = La, Pr, Nd, Sm) Superconductors

ABSTRACTDiscovery of superconductivity in BiS2 layers based systems has attracted tremendous interest of both experimentalists and theoreticians from condensed matter physics community. In this article, a review of our high pressure studies on BiS2 based superconductors is given. The pressure effects on magnetic, transport properties and superconducting transitions are discussed for different types of doped and undoped BiS2-based compounds such as Bi4O4S3 and ReO/FBiS2 (Re = rare-earth). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting Tc increases with low pressure for under doped BiS2 based compounds, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. Under pressure, the semiconducting behavior in the normal state is suppressed markedly and monotonically, whereas the evolution of Tc is nonlinear, the superconductivity in the BiS2 layer favors the Fermi surface at the boundary between the semiconducting and metallic behaviors. However, strong suppression of the semiconducting and induced metallic behavior without doping in ReO/FBiS2 suggests that the Fermi surface is located in the vicinity of some instability. Furthermore, notable properties under pressure in the BiS2 family are reported. The prospects for raising Tc in this family are proposed on the basis of experimental and theoretical studies.

New Insulating Antiferromagnetic Quaternary Iridates MLa10Ir4O24 (M = Sr, Ba).

Recently, oxides of Ir4+ have received renewed attention in the condensed matter physics community, as it has been reported that certain iridates have a strongly spin-orbital coupled (SOC) electronic state, Jeff = ½, that defines the electronic and magnetic properties. The canonical example is the Ruddlesden-Popper compound Sr2IrO4, which has been suggested as a potential route to a new class of high temperature superconductor due to the formal analogy between Jeff = ½ and the S = ½ state of the cuprate superconductors. The quest for other iridium oxides that present tests of the underlying SOC physics is underway. In this spirit, here we report the synthesis and physical properties of two new quaternary tetravalent iridates, MLa10Ir4O24 (M = Sr, Ba). The crystal structure of both compounds features isolated IrO6 octahedra in which the electronic configuration of Ir is d5. Both compounds order antiferromagnetically despite the lack of obvious superexchange pathways, and resistivity measurement shows that SrLa10Ir4O24 is an insulator.

Effect of hydrostatic pressures on the superconductivity of new BiS2 based REO0.5F0.5BiS2 (RE=La, Pr and Nd) superconductors

We study the effect of hydrostatic pressure on superconductivity of new BiS2 based layered REO0.5F0.5BiS2 (RE=La, Pr, and Nd) compounds through the measurements of dc electrical resistivity. The REO0.5F0.5BiS2 (RE=La, Pr and Nd) compounds synthesized by solid state reaction route via vacuum encapsulation are crystallized in the tetragonal P4/nmm space group. At ambient pressure the superconducting transition onset temperatures (Tconset) are 2.7K, 3.5K and 4.5K, which are enhanced substantially under external hydrostatic pressure to 10.5K, 7.8K and 7.5K for LaO0.5F0.5BiS2, PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2 respectively at 1.68GPa. The normal state electrical resistivity decreases with applied pressure for REO0.5F0.5BiS2 (RE=La, Pr and Nd). The electrical resistivity under magnetic field and applied pressure has been measured to estimate upper critical field Hc2(0), the values of which are 15.9Tesla, 8.8Tesla and 8.2Tesla for LaO0.5F0.5BiS2, PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2 compounds. Substantial enhancement of superconductivity under moderate pressures in studied new BiS2 based superconductors call for the attention of condensed matter physics community.

Superconductivity at 4.4 K in PdTe2 Chains of a Ta-Based Compound

Superconductivity in PdTe2 heavy transition metal-based compounds is a rapidly developing field in condensed matter physics community. Here, we report superconductivity in a nominal ternary telluride Ta2Pd0.97Te6 compound, which is synthesized in sealed evacuated quartz ampoule. The resultant compound is crystallized in layered monoclinic Ta4Pd3Te16 phase with space group C2/m, having lattice parameters a = 21.304(6)Å, b = 3.7365(7)Å and c = 17.7330Å with β = 120.65(1)∘. Both transport and magnetic measurements demonstrated bulk superconductivity at 4.4 K in studied polycrystalline sample. The metallic normal state conductivity can be well ascribed to Fermi liquid behaviour. The superconducting upper critical field of the compound is determined to be 5.5 T based on magnetoresistivity measurements.

Nanofragmentation of Ferroelectric Domains During Polarization Fatigue

The microscopic mechanism for polarization fatigue in ferroelectric oxides has remained an open issue for several decades in the condensed matter physics community. Even though numerous models are proposed, a consensus has yet to be reached. Since polarization reversal is realized through ferroelectric domains, their behavior during electric cycling is critical to elucidating the microstructural origin for the deteriorating performance. In this study, electric field in situ transmission electron microscopy is employed for the first time to reveal the domain dynamics at the nanoscale through more than 103 cycles of bipolar fields. A novel mechanism of domain fragmentation is directly visualized in polycrystalline [(Bi1/2Na1/2)0.95Ba0.05]0.98La0.02TiO3. Fragmented domains break the long‐range polar order and, together with domain wall pinning, contribute to the reduction of switchable polarization. Complimentary investigations into crystal structure and properties of this material corroborate our microscopic findings.