Topological phonon analysis of the 2D buckled honeycomb lattice: an application to real materials

By means of group theory, topological quantum chemistry, first-principles and Monte Carlo calculations, we analyze the topology of the 2D buckled honeycomb lattice phonon spectra. Taking the pure crystal structure as an input, we show that eleven distinct phases are possible, five of which necessarily have non-trivial topology according to topological quantum chemistry. Another four of them are also identified as topological using Wilson loops in an analytical model that includes all the symmetry allowed force constants up to third nearest neighbors, making a total of nine topological phases. We then compute the ab initio phonon spectra for the two-dimensional crystals of Si, Ge, P, As and Sb in this structure and construct its phase diagram. Despite the large proportion of topological phases found in the analytical model, all of the crystals lie in a trivial phase. By analyzing the force constants space using Monte Carlo calculations, we elucidate why topological phonon phases are physically difficult to realize in real materials with this crystal structure

Switchable chiral transport in charge-ordered kagome metal CsV3Sb5

When electric conductors differ from their mirror image, unusual chiral transport coefficients appear that are forbidden in achiral metals, such as a non-linear electric response known as electronic magnetochiral anisotropy (eMChA)$^{1–6}$. Although chiral transport signatures are allowed by symmetry in many conductors without a centre of inversion, they reach appreciable levels only in rare cases in which an exceptionally strong chiral coupling to the itinerant electrons is present. So far, observations of chiral transport have been limited to materials in which the atomic positions strongly break mirror symmetries. Here, we report chiral transport in the centrosymmetric layered kagome metal CsV$_{3}$Sb$_{5}$ observed via second-harmonic generation under an in-plane magnetic field. The eMChA signal becomes significant only at temperatures below $${T}^{{\prime} }\approx$$ 35 K, deep within the charge-ordered state of CsV$_{3}$Sb$_{5}$ (T$_{CDW}$ ≈ 94 K). This temperature dependence reveals a direct correspondence between electronic chirality, unidirectional charge order$^{7}$ and spontaneous time-reversal symmetry breaking due to putative orbital loop currents$^{8–10}$. We show that the chirality is set by the out-of-plane field component and that a transition from left- to right-handed transport can be induced by changing the field sign. CsV$_{3}$Sb$_{5}$ is the first material in which strong chiral transport can be controlled and switched by small magnetic field changes, in stark contrast to structurally chiral materials, which is a prerequisite for applications in chiral electronics

Distinct switching of chiral transport in the kagome metals KV3_3Sb5_5 and CsV3_3Sb5_5

The kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) present an ideal sandbox to study the interrelation between multiple coexisting correlated phases such as charge order and superconductivity. So far, no consensus on the microscopic nature of these states has been reached as the proposals struggle to explain all their exotic physical properties. Among these, field-switchable electric magneto-chiral anisotropy (eMChA) in CsV$_3$Sb$_5$ provides intriguing evidence for a rewindable electronic chirality, yet the other family members have not been likewise investigated. Here, we present a comparative study of magneto-chiral transport between CsV$_3$Sb$_5$ and KV$_3$Sb$_5$. Despite their similar electronic structure, KV$_3$Sb$_5$ displays negligible eMChA, if any, and with no field switchability. This is in stark contrast to the non-saturating eMChA in CsV$_3$Sb$_5$ even in high fields up to 35 T. In light of their similar band structures, the stark difference in eMChA suggests its origin in the correlated states. Clearly, the V kagome nets alone are not sufficient to describe the physics and the interactions with their environment are crucial in determining the nature of their low-temperature state

Real-space observation of ultraconfined in-plane anisotropic acoustic terahertz plasmon polaritons

Thin layers of in-plane anisotropic materials can support ultraconfined polaritons, whose wavelengths depend on the propagation direction. Such polaritons hold potential for the exploration of fundamental material properties and the development of novel nanophotonic devices. However, the real-space observation of ultraconfined in-plane anisotropic plasmon polaritons (PPs)-which exist in much broader spectral ranges than phonon polaritons-has been elusive. Here we apply terahertz nanoscopy to image in-plane anisotropic low-energy PPs in monoclinic Ag2Te platelets. The hybridization of the PPs with their mirror image-by placing the platelets above a Au layer-increases the direction-dependent relative polariton propagation length and the directional polariton confinement. This allows for verifying a linear dispersion and elliptical isofrequency contour in momentum space, revealing in-plane anisotropic acoustic terahertz PPs. Our work shows high-symmetry (elliptical) polaritons on low-symmetry (monoclinic) crystals and demonstrates the use of terahertz PPs for local measurements of anisotropic charge carrier masses and damping.The work was financially supported by the Spanish Ministry of Science and Innovation under the María de Maeztu Units of Excellence Program (CEX2020-001038-M/MCIN/AEI/10.13039/501100011033) (R.H., A.C., L.E.H. and E.A.); Projects PID2021-123949OB-I00 (R.H.), PID2019-109905GB-C21 (M.G.V. and I.E.), RTI2018-094861-B-100 (L.E.H.), PID2019-107432GB-I00 (J.A.) and PID2019-107338RB-C61 (E.A.) funded by MCIN/AEI/10.13039/501100011033 and by ‘ERDF—A Way of Making Europe’; the National Natural Science Foundation of China (NSFC) (52225207 and 11934005) and the Shanghai Pilot Program for Basic Research—Fudan University 21TQ1400100 (21TQ006) (F.X.X.); NSFC grant no. 61988102 and the Science and Technology Commission of Shanghai Municipality (nos. 23010503400 and 23ZR1443500) (S.C.); the Czech Science Foundation GACR under the Junior Star grant no. 23-05119M (A.K.); the European Research Council (ERC) under grant agreement no. 101020833 (M.G.V.); the German Research Foundation (DFG) under project nos. 467576442 (I.N.) and GA 3314/1-1–FOR 5249 (QUAST) (M.G.V.); the Gipuzkoa Council (Spain) in the frame of the Gipuzkoa Fellows Program (B.M.-G.); and the University groups of the Basque Government (IT1526-22) (J.A.).Peer reviewe

Nurses' perceptions of aids and obstacles to the provision of optimal end of life care in ICU

Contains fulltext : 172380.pdf (publisher's version ) (Open Access

Topological quantum chemistry on phonon spectra: an application to the buckled honeycomb lattice

Resumen del trabajo presentado al APS March Meeting, celebrado de forma virtual del 15 al 19 de marzo de 2021.Recently the systematic application of the methods of Topological Quantum Chemistry (TQC) has enormously enlarged the number of known topological materials and led to new and more refined methods of classifying their topology. We use the methods of TQC to analyse the topology of phonon spectra. Using only the structure as input, a general recipe to find whether a material can host topological phonons is described. Applying these methods to the buckled honeycomb lattice we show how eleven phases arise, nine of which have non trivial topology. Using an analytical model consistent with the system symmetries we are able to compute Wilson loop spectrum fully characterizing all the possible topological phases. We compute the phonon spectra with DFT for Si, Ge, P, As and Sb placing them in the phase diagram, these results are justified with a Monte Carlo analysis of the phase space that shows why topological phases are physically difficult to realize.This work has been supported in part by Basque Government Grant IT979-16.Peer reviewe

Three-dimensional Fermi surfaces from charge order in layered CsV3 Sb5

The cascade of electronic phases in CsV3Sb5 raises the prospect to disentangle their mutual interactions in a clean, strongly interacting kagome lattice. When the kagome planes are stacked into a crystal, its electronic dimensionality encodes how much of the kagome physics and its topological aspects survive. The layered structure of CsV3Sb5 reflects in Brillouin-zone-sized quasi-two-dimensional Fermi surfaces and significant transport anisotropy. Yet here we demonstrate that CsV3Sb5 is a three-dimensional (3D) metal within the charge density wave (CDW) state. Small 3D pockets play a crucial role in its low-temperature magneto- and quantum transport. Their emergence at TCDW≈93K results in an anomalous sudden increase of the in-plane magnetoresistance by four orders of magnitude. The presence of these 3D pockets is further confirmed by quantum oscillations under in-plane magnetic fields, demonstrating their closed nature. These results emphasize the impact of interlayer coupling on the kagome physics in 3D materials.This work was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Rnnovation program (MiTopMat, Grant No. 715730). This project received funding from the Swiss National Science Foundation (Grant No. PP00P2_176789). M.G.V., I.E., and M.G.A. acknowledge the Spanish Ministerio de Ciencia e Innovacion (Grant No. PID2019-109905GBC21). M.G.V. acknowledges support from Programa Red Guipuzcoana de Ciencia Tecnología e Innovación 2021, No. 2021-CIEN-000070-01 Gipuzkoa Next, and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) through GA 3314/1-1–FOR 5249 (QUAST). This work has been supported in part by the Basque Government (Grant No. IT979-16). This work was also supported by the European Research Council (Advanced Grant No. 742068 TOPMAT), the Deutsche Forschungsgemeinschaft (Project ID No. 258499086 SFB 1143), and the DFG through the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147, Project ID No. 39085490).Peer reviewe

Transport and optical properties of the chiral semiconductor Ag3AuSe2

Previous band structure calculations predicted Ag3AuSe2 to be a semiconductor with a band gap of approximately 1 eV. Here, we report single crystal growth of Ag3AuSe2 and its transport and optical properties. Single crystals of Ag3AuSe2 were synthesized by slow-cooling from the melt, and grain sizes were confirmed to be greater than 2 mm using electron backscatter diffraction. Optical and transport measurements reveal that Ag3AuSe2 is a highly resistive semiconductor with a band gap and activation energy around 0.3 eV. Our first-principles calculations show that the experimentally determined band gap lies between the predicted band gaps from GGA and hybrid functionals. We predict band inversion to be possible by applying tensile strain. The sensitivity of the gap to Ag/Au ordering, chemical substitution, and heat treatment merit further investigation.11Nsciescopu

Switchable chiral transport in charge-ordered Kagome CsV3Sb5 [Dataset]

When electric conductors differ from their mirror image, unusual chiral transport coefficients appear that are forbidden in achiral metals, such as a non-linear electric response known as electronic magneto-chiral anisotropy (eMChA). While chiral transport signatures are by symmetry allowed in many conductors without a center of inversion, it reaches appreciable levels only in rare cases when an exceptionally strong chiral coupling to the itinerant electrons is present. So far, observations of chiral transport have been limited to materials in which the atomic positions strongly break mirror symmetries. Here, we report chiral transport in the centro-symmetric layered Kagome metal CsV3Sb5, observed via second harmonic generation under in-plane magnetic field. The eMChA signal becomes significant only at temperatures below T'~ 35 K, deep within the charge ordered state of CsV3Sb5 (TCDW ~ 94 K). This temperature dependence reveals a direct correspondence between electronic chirality, unidirectional charge order, and spontaneous time-reversal-symmetry breaking due to putative orbital loop currents. We show that the chirality is set by the out-of-plane field component and that a transition from left- to right-handed transport can be induced by changing the field sign. CsV3Sb5 is the first material in which strong chiral transport can be controlled and switched by small magnetic-field changes, in stark contrast to structurally chiral materials -- a prerequisite for their applications in chiral electronics.Peer reviewe