Chirality manipulation of ultrafast phase switchings in a correlated CDW-Weyl semimetal

A recently emerging concept for quantum phase discovery is the controlled gapping of linear band crossings in topological semimetals. For example, achieving topological superconducting and charge-density-wave (CDW) gapping could introduce Majorana zero modes and axion electrodynamics, respectively. Light engineering of correlation gaps in topological materials provides a new avenue of achieving exotic topological phases inaccessible by conventional tuning methods such as doping and straining. Here we demonstrate a light control of correlation gaps and ultrafast phase switchings in a model CDW and polaron insulator (TaSe$_4$)$_2$I recently predicted to be an axion insulator. Our ultrafast terahertz photocurrent spectroscopy reveals a two-step, non-thermal melting of polarons and electronic CDW gap via studying the fluence dependence of a {\em longitudinal} circular photogalvanic current. The helicity-dependent photocurrent observed along the propagation of light reveals continuous ultrafast switchings from the polaronic state, to the CDW (axion) phase, and finally to a hidden Weyl phase as the pump fluence increases. Other distinguishing features corroborating with the light-induced switchings include: mode-selective coupling of coherent phonons to polaron and CDW modulation, and the emergence of a {\em non-thermal} chiral photocurrent above pump threshold of CDW-related phonons. The ultrafast chirality control of correlated topological states revealed here is important to realize axion electrodynamics and quantum computing.Comment: 9 pages, 4 figure

Is it possible to stabilize the 1144-phase pnictides with tri-valence cations?

A lately discovered 1144 phase has generated significant interest for its high superconducting temperatures, disorder-free doping, and various chemical substitutions. However, it has only been found in iron arsenides ( A B Fe 4 As 4 ), and cations are limited to +1 or +2 valence states (e.g., alkali metals, alkaline earth elements, and Eu). Whether more 1144 phases could be stabilized and whether intriguing properties exist are questions of general interest. In this work, we investigate 1144 iron and cobalt arsenides with tri-valence cations (La, Y, In, Tl, Sm, Gd). We study phase stability among other competing phases: 122 solution phase and phase decomposition. With La as the cation, we predict room-temperature stable 1144 structures: La A Fe 4 As 4 ( A = K , Rb, and Cs). Other La-contained 1144 structures tend to form solution phase. The solubility of La is estimated and compared with the experiment. By contrast, we do not find stable 1144 structures with Y as the cation. For In and Tl as cations, two 122-phase compounds are remarkably stable: InCo 2 As 2 and TlCo 2 As 2 , which adds to our knowledge about the In(Tl)-Co-As phase diagram. Stable 1144 phases are found in InKCo 4 As 4 and InRbCo 4 As 4 . With Sm and Gd as cations, 1144- or 122-phase iron arsenides are generally unstable. Among structures investigated, we recognize two critical factors for 1144-phase stability: size effect and charge balance, which yields a merging picture with the rule found in previous 1144 systems. Moreover, La A Fe 4 As 4 ( A = K , Rb, and Cs), InCo 2 As 2 , and TlCo 2 As 2 are exhibiting semimetal features and a two-dimensional Fermi surface, similar to iron superconductors

Ultrafast nonthermal terahertz electrodynamics and possible quantum energy transfer in the Nb3Sn superconductor

We report terahertz (THz) electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to manipulate quasiparticle (QP) transport. In the Martensitic normal state, we observe a photo enhancement in the THz conductivity using optical pulses, while the opposite is observed for the THz pump. This demonstrates wavelength-selective nonthermal control of conductivity distinct from sample heating. The photo enhancement persists up to an additional critical temperature, above the SC one, from a competing electronic order. In the SC state, the fluence dependence of pair-breaking kinetics together with an analytic model provides an implication for a “one photon to one Cooper pair” nonresonant energy transfer during the 35-fs laser pulse; i.e., the fitted photon energy ℏω absorption to create QPs set by 2ΔSC/ℏω=0.33%. This is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron-phonon coupling and possible influence of phonon condensation

Bulk observables at topological phase transitions and material prediction

Topologies in band structures are defined under the frame of homotopy theory or otherwise known as K-theory. The idea is to classify bands with their distortions: if a band can continuously (without gap closing) distort into a second, meanwhile respecting all the pristine symmetries, the two bands are topologically equivalent. Therefore, the band topology, from the beginning, is defined “relatively”, i.e., there must be a second topology as “reference”. That is why the observables for topology are usually at the edge where topologies crossover; otherwise, deep in the bulk, a lonely topology will lose its identity. This fact is often referred to as “bulk-edge correspondence”: the topology (characterized by topological indices usually obtained in k-space) is for bulk, while its observable is on the edge, i.e., the dimension decreases by one. This dissertation is poised to extend this conventional wisdom by revealing topology’s consequences in the same dimension as the topology itself is defined, i.e., bulk topological observables. Two changes might lead to progress (i) moving from static topology to dynamic topological phase transition (TPT), (ii) from equilibrium to non-equilibrium. Intuitively speaking, we add the time-axis, to make every spatial point in the bulk will meet different topologies, such that the edge confinement is circumvented. However, this is different from defining observables on a “temporal edge”. Bulk topological effect (BTE) is examined from three aspects. First, we discover a bulk inter-band charge pumping caused by TPT. It features a fractional pumping probability p_G=1/2 at gap closing point. It is robust to energetic details, and the robustness roots from an unprecedented source in math, such as number theory. The second aspect is about proving a rigorous theoretical tool to handle non-equilibrium and non-perturbation situations required by (BTE). That is, we generalize Liouville’s theorem (LT), which was only valid for classical mechanics, to quantum, namely quantum Liouville’s theorem. In view of the fundamental role played by LT in statistical mechanics, quantum LT helps reveals the relationship between topology and statistics. The third aspect is about transport, which follows the position shift during the topological charge pumping. A long-standing challenge was the r matrix is diverging (in diagonals), which prevents evaluating transport currents via〈r ̇ 〉. We resolve the well-known divergence problem with so-called N-th Weyl algebra, paving the way in evaluating 〈r ̂ 〉 and thus the current