337 research outputs found
Tension-compression asymmetry in superelasticity of SrNi2P2 single crystals and the influence of low temperatures
Get PDFThCr2Si2-type intermetallic compounds are known to exhibit superelasticity associated with structural transitions through lattice collapse and expansion. These transitions occur via the formation and breaking of Si-type bonds, respectively, under uniaxial loading along the [0 0 1] direction. Unlike most ThCr2Si2-type intermetallic compounds, which have either an uncollapsed tetragonal structure or a collapsed tetragonal structure, SrNi2P2 possesses a third type of collapsed structured: a one-third orthorhombic structure, for which one expects the occurrence of unique structural transitions and superelastic behavior. In this study, uniaxial compression and tension tests were conducted on micron-sized SrNi2P2 single crystalline columns at room temperature, 200K, and 100K, to investigate the influence of loading direction and temperature on the superelasticity of SrNi2P2. Experimental data and density functional theory calculations revealed the presence of tension-compression asymmetry in the structural transitions and superelasticity, as well as an asymmetry in their temperature dependence, due to the opposite superelastic process associated with compression (forming P-P bonds) and tension (breaking P-P bonds). Additionally, following thermodynamics, the observations suggest that this asymmetric superelasticity could lead to an opposite elastocaloric effect between compression and tension, which could be beneficial potentially in obtaining large temperature changes compared to conventional superelastic solids that show the same elastocaloric effect regardless of loading direction. These results provide an important fundamental insight into the structural transitions, superelasticity processes, and potential elastocaloric effects in SrNi2P2.This is a preprint from Xiao, Shuyang, Adrian Valadkhani, Sarshad Rommel, Paul C. Canfield, Mark Aindow, Roser Valentí, and Seok-Woo Lee. "Tension-compression asymmetry in superelasticity of SrNi2P2 single crystals and the influence of low temperatures." arXiv preprint arXiv:2405.08294 (2024). doi: https://doi.org/10.48550/arXiv.2405.08294. Published as Xiao, Shuyang, Adrian Valadkhani, Sarshad Rommel, Paul C. Canfield, Mark Aindow, Roser Valentí, and Seok-Woo Lee. "Tension-compression asymmetry in superelasticity of SrNi2P2 single crystals and the influence of low temperatures." Acta Materialia 274 (2024): 119989. doi: https://doi.org/10.1016/j.actamat.2024.119989
Design and Analysis of Rotary Positive Displacement Mechanism for Oil-Less Compression
No full textAuthor(s): Holger Roser University of Technology Sydney, Sydney, NSW, Australia In this paper, a simple positive displacement mechanism is investigated, which comprises two counter-rotating meshing rotors within a casing. Although considered for various applications more than a century ago, the basic geometry of this mechanism has not been further explored or adapted to modern gas compressor technology
Charge density waves in kagome-lattice extended Hubbard models at the van Hove filling
Get PDFThe Hubbard model on the kagome lattice is presently often considered as a minimal model to describe the rich low-temperature behavior of AV3 Sb5 compounds (with A = K, Rb, Cs), including charge density waves (CDWs), superconductivity, and possibly broken time-reversal symmetry. Here, we investigate, via variational Jastrow-Slater wave functions, the properties of its ground state when both on-site U and nearest-neighbor V Coulomb repulsions are considered at the van Hove filling. Our calculations reveal the presence of different interaction-driven CDWs and, contrary to previous renormalization-group studies, the absence of ferromag- netism and charge- or spin-bond order. No signatures of chiral phases are detected. Remarkably, the CDWs triggered by the nearest-neighbor repulsion possess charge disproportionations that are not compatible with the ones observed in AV3Sb5. As an alternative mechanism to stabilize charge-bond order, we consider the electron-phonon interaction, modeled by coupling the hopping amplitudes to quantum phonons, as in the Su-Schrieffer-Heeger model. Our results show the instability towards a trihexagonal distortion with 2 × 2 periodicity, in closer agreement with experimental findings
Tuning superconductivity and spin-vortex instabilities in CaKFe4As4 through in-plane antisymmetric strains
Get PDFLattice strains of appropriate symmetry have served as an excellent tool to explore the interaction of superconductivity in the iron-based superconductors with orthorhombic-nematic and stripe spin-density-wave (SSDW) order. In this Letter, we contribute to a broader understanding of the coupling of strain to superconductivity and competing normal-state orders by studying CaKFe4As4 under large, in-plane strains of 1 and 2 symmetry. In contrast to the majority of iron-based superconductors, pure CaKFe4As4 exhibits superconductivity with a relatively high transition temperature of ∼35K in proximity of a noncollinear, tetragonal, hedgehog spin-vortex crystal (SVC) order. Through experiments and calculations, we demonstrate an anisotropic in-plane strain response of and the favored SVC configuration in CaKFe4As4. This supports a scenario, in which the change in spin fluctuations dominates the strain response of superconducting . Overall, by suggesting moderate 2 strains as an effective parameter to change the stability of SVC and SSDW, we outline a pathway to a unified phase diagram of iron-based superconductivity.This article is published as Valadkhani, Adrian, Belén Zúñiga Céspedes, Salony Mandloi, Mingyu Xu, Juan Schmidt, Sergey L. Bud'Ko, Paul C. Canfield, Roser Valentí, and Elena Gati. "Tuning superconductivity and spin-vortex instabilities in CaKFe 4 As 4 through in-plane antisymmetric strains." Physical Review B 109, no. 18 (2024): L180503. doi: https://doi.org/10.1103/PhysRevB.109.L180503
Effects of Fe substitution on the electronic, transport, and magnetic properties of ZnGa2O4: A systematic ab initio study
No full textWe present a density functional study of Fe doped into the tetrahedral and octahedral cation sites of the wide-band-gap spinel ZnGa2O4. We calculate the electronic structure for different substitutions and discuss the magnetic and transport properties for each case considering different approximations for the exchange-correlation potential. We show that for certain doped cases, significant differences in the predicted behavior are obtained depending on the exchange-correlation potential adopted. Possible applications of the doped systems as magnetic semiconductors are outlined
Electron correlations in the kagome flat band metal CsCr3Sb5
Get PDFKagome metals offer a unique platform for investigating robust electron-correlation effects because of their lattice geometry, flat bands, and multiorbital nature. In the cases with active flat bands, recent theoretical studies have pointed to a rich phase diagram that contains not only electronic orders but also quantum criticality. Very recently, CsCr3Sb5 has emerged as a strong candidate for exploring such new physics. Here, using effective tight-binding models obtained from ab initio calculations, we study the effects of electronic correlations and symmetries on the electronic structure of CsCr3Sb5. The effective tight-binding model and Fermi surface comprise multiple Cr− orbitals and Sb− orbitals. The introduction of Hubbard-Kanamori interactions leads to orbital-selective band renormalization dominated by the band, concurrently producing emergent flat bands very close to the Fermi level. Our analysis sets the stage for further investigations into the electronic properties of CsCr3Sb5, including electronic orders, quantum criticality, and unconventional superconductivity, which promise to shed much new light into the electronic materials with frustrated lattices and bring about new connections with the correlation physics of a variety of strongly correlated systems.This article is published as Xie, Fang, Yuan Fang, Ying Li, Yuefei Huang, Lei Chen, Chandan Setty, Shouvik Sur, Boris Yakobson, Roser Valentí, and Qimiao Si. "Electron correlations in the kagome flat band metal CsCr 3 Sb 5." Physical Review Research 7, no. 2 (2025): L022061. doi: https://doi.org/10.1103/PhysRevResearch.7.L022061
Construction of A-B heterolayer intermetallic crystals: Case studies of the 1144-phase TM-phosphides AB(TM)(4)P-4 (TM=Fe, Ru, Co, Ni)
Get PDFThe discovery of the 1144 phase, e.g., CaKFe4As4, creates opportunities to build novel intermetallics with alternative stacking of two parent compounds. Here we formalize the idea by defining a class of bulk crystalline solids with A-B stacking (including 1144 phases and beyond), which is a generalization of heterostructures from few-layer or thin-film semiconductors to bulk intermetallics. Theoretically, four families of phosphides AB(TM)(4)P-4 (TM=Fe, Ru, Co, Ni) are investigated by first-principles calculations, wherein configurational, vibrational, and electronic degrees of freedom are considered. It predicts a variety of stable 1144 phases (especially Ru- and Fe-phosphides). Stability rules are found and structural/electronic properties are discussed. Experimentally, we synthesize high-purity CaKRu4P4 as a proof of principle example. The synthetic method is simple and easily applied. Moreover, it alludes to a strategy to explore complex multicomponent compounds, facilitated by a phase diagram coordinated by collective descriptors.This article is published as Song, B. Q., Mingyu Xu, Vladislav Borisov, Olena Palasyuk, C. Z. Wang, Roser Valentí, Paul C. Canfield, and K. M. Ho. "Construction of A− B heterolayer intermetallic crystals: Case studies of the 1144-phase TM-phosphides AB (TM) 4 P 4 (TM= Fe, Ru, Co, Ni)." Physical Review Materials 5, no. 9 (2021): 094802.
DOI: 10.1103/PhysRevMaterials.5.094802.
Copyright 2021 American Physical Society
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Charge orders in organic charge-transfer salts
Get PDFMotivated by recent experimental suggestions of charge-order-driven ferroelectricity in organic charge-transfer salts, such as κ-(BEDT-TTF)2Cu[N(CN)2]Cl, we investigate magnetic and charge-ordered phases that emerge in an extended two-orbital Hubbard model on the anisotropic triangular lattice at 3/4 filling. This model takes into account the presence of two organic BEDT-TTF molecules, which form a dimer on each site of the lattice, and includes short-range intramolecular and intermolecular interactions and hoppings. By using variational wave functions and quantum Monte Carlo techniques, we find two polar states with charge disproportionation inside the dimer, hinting to ferroelectricity. These charge-ordered insulating phases are stabilized in the strongly correlated limit and their actual charge pattern is determined by the relative strength of intradimer to interdimer couplings. Our results suggest that ferroelectricity is not driven by magnetism, since these polar phases can be stabilized also without antiferromagnetic order and provide a possible microscopic explanation of the experimental observations. In addition, a conventional dimer-Mott state (with uniform density and antiferromagnetic order) and a nonpolar charge-ordered state (with charge-rich and charge-poor dimers forming a checkerboard pattern) can be stabilized in the strong-coupling regime. Finally, when electron–electron interactions are weak, metallic states appear, with either uniform charge distribution or a peculiar 12-site periodicity that generates honeycomb-like charge order
Emergent lattices with geometrical frustration in doped extended Hubbard models
No full textSpontaneous charge ordering occurring in correlated systems may be considered as a possible route to generate effective lattice structures with unconventional couplings. For this purpose we investigate the phase diagram of doped extended Hubbard models on two lattices: (i) the honeycomb lattice with on-site U and nearest-neighbor V Coulomb interactions at 3/4 filling (n=3/2) and (ii) the triangular lattice with on-site U, nearest-neighbor V, and next-nearest-neighbor V' Coulomb interactions at 3/8 filling (n=3/4). We consider various approaches including mean-field approximations, perturbation theory, and variational Monte Carlo. For the honeycomb case (i), charge order induces an effective triangular lattice at large values of U/t and V/t, where t is the nearest-neighbor hopping integral. The nearest-neighbor spin exchange interactions on this effective triangular lattice are antiferromagnetic in most of the phase diagram, while they become ferromagnetic when U is much larger than V. At U/t∼(V/t)^3, ferromagnetic and antiferromagnetic exchange interactions nearly cancel out, leading to a system with four-spin ring-exchange interactions. On the other hand, for the triangular case (ii) at large U and finite V', we find no charge order for small V, an effective kagome lattice for intermediate V, and one-dimensional charge order for large V. These results indicate that Coulomb interactions induce [case (i)] or enhance [case(ii)] emergent geometrical frustration of the spin degrees of freedom in the system, by forming charge order
Konsequente Unterdrückungsstrategien führen zu niedrigeren Gesamtkosten : die Physiker Roser Valenti und Claudius Gros haben sich mit den sozioökonomischen Folgen von »Social Distancing« beschäftigt
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