arXiv subjects

Yuval Nitzav

Publications and source records attributed to Yuval Nitzav.

10 recordsLinked to original sources

arXiv2025

Light-induced trion-exciton competition revealed by ultrafast photoemission

Strong Coulomb interactions in low-dimensional quantum materials give rise to emergent bound states such as excitons and trions. Trions are conventionally secondary excitations, requiring both optical excitation and charge doping. In quasi-one-dimensional Ta$_2$NiS$_5$, however, an exceptionally large binding energy exceeding the single-particle band gap stabilizes an equilibrium trion gas under surface doping alone. Here, using time- and angle-resolved photoemission spectroscopy, we show that trions can also be generated purely optically, without external charge. Following photoexcitation of pristine Ta$_2$NiS$_5$ we observe a bright, momentum-localized, in-gap feature with a slow, fluence-dependent relaxation. Rate-equation modeling identifies it as a mixed population of trions and excitons, with the trions formed via an unconventional single-particle pathway that requires no photoexcited hole. The trion-exciton composition is controlled by pump fluence, with trions dominating the late-time relaxation. In surface doped samples, the same model with unchanged rates reproduces the full dynamics, linking the light-induced trions to their equilibrium counterparts through pump-induced dissociation and recapture. These results establish trARPES as a direct probe of charged quasiparticles far from equilibrium and open routes to optical control of neutral and charged excitations in correlated materials.

cond-mat.str-el↗

arXiv2025

Trion gas on the surface of a failed excitonic insulator

Trions, three-body bound states composed of an exciton and an additional charge, are typically fragile and require external excitation to form. Here, we report the spontaneous emergence of a stable trion gas at the surface of the layered semiconductor Ta2NiS5, revealed through angle-resolved photoemission spectroscopy. We observe a sharp, highly localized in-gap feature that cannot be explained by conventional band-theory. Instead, we argue that it arises from the formation of negative trions, stabilized by surface-induced band bending and the material's quasi-one-dimensional geometry. Unlike excitons, these trions form without optical pumping and persist at equilibrium, marking a rare example of an interaction-driven surface state in a nominally conventional semiconductor. Our findings establish Ta2NiS5 as a unique platform for exploring many-body physics at surfaces and open new avenues for studying and controlling collective excitations in low-dimensional systems.

cond-mat.str-el↗

arXiv2025

Mixed Triplet-Singlet Order Parameter in Decoupled Superconducting 1H Monolayers of Transition-Metal Dichalcogenides

Understanding the emergence of unconventional superconductivity, where the order parameter deviates from simple isotropic s-wave pairing, is a central puzzle in condensed matter physics. Transition-metal dichalcogenides (TMDCs), though generally regarded as conventional superconductors, display signatures of this unusual behavior and thus provide a particularly intriguing platform to explore how exotic states arise. Here we investigate the misfit compound (SnS)$_{1.15}$(TaS$_2$), a heterostructure composed of alternating SnS and 1H-TaS$_2$ layers. Using transport, photoemission, and scanning tunneling spectroscopy, we demonstrate that the SnS layers effectively decouple the TaS$_2$ into electronically isolated 1H sheets. In this limit, the tunneling density of states reveals a clear two-gap superconducting spectrum with T$_c \sim$ 3.1 K. A theoretical model based on lack of inversion symmetry and finite-range attraction reproduces the observed multi-gap structure as a mixed singlet-triplet state. These results establish misfit compounds as a powerful platform for studying unconventional superconductivity in isolated 1H layers and for realizing multiple uncoupled superconductors within a single crystal.

cond-mat.supr-con↗

arXiv2025

Chiral charge density wave in 4Hb- and 1T-TaS$_2$: The Role of interlayer coupling

We use micro-angle-resolved photoemission spectroscopy (micro-ARPES) to investigate chiral charge density waves (CDWs) in 4Hb-TaS$_2$ with micron-scale spatial resolution. In the 1T layers of 4Hb-TaS$_2$, we uncover coexisting left- and right-handed CDW domains and resolve four distinct spectral patterns arising from the interplay of chirality and rotational stacking. In contrast, bulk 1T-TaS$_2$ exhibits a uniform chirality. In addition, 4Hb-TaS$_2$ shows negligible out-of-plane dispersion of the 1T-derived bands, in contrast to the pronounced interlayer coupling observed in bulk 1T-TaS$_2$. Density functional theory (DFT) calculations corroborate this picture, revealing that the interlayer interaction of the chiral order in 4Hb-TaS$_2$ is nearly two orders of magnitude weaker than in the 1T polytype. Our findings establish 4Hb-TaS$_2$ as a quasi-two-dimensional platform for exploring tunable chiral CDW phenomena.

cond-mat.str-el↗

arXiv2024

Ising superconductivity in the bulk incommensurate layered material (PbS)$_{1.13}$(TaS$_2$)

Exploiting the spin-valley degree of freedom of electrons in materials is a promising avenue for energy-efficient information storage and quantum computing. A key challenge in utilizing spin-valley polarization is the realization of spin-valley locking in bulk systems. Here, we report a comprehensive study of the noncentrosymmetric bulk misfit compound (PbS)$_{1.13}$(TaS$_2$), showing a strong spin-valley locking. Our investigation reveals Ising superconductivity with a transition temperature of 3.14 K, closely matching that of a monolayer of TaS$_2$. Notably, the absence of charge density wave (CDW) signatures in transport measurements suggests that the PbS layers primarily act as spacers between the dichalcogenide monolayers. This is further supported by angle-resolved photoemission spectroscopy (ARPES), which shows negligible interlayer coupling, a lack of dispersion along the $k_{\perp}$ direction and significant charge transfer from the PbS to the TaS$_2$ layers. Spin resolved ARPES shows strong spin-valley locking of the electronic bands. Muon spin rotation experiments conducted in the vortex phase reveal an isotropic superconducting gap. However, the temperature dependence of the upper critical field and low-temperature specific heat measurements suggest the possibility of multigap superconductivity. These findings underscore the potential of misfit compounds as robust platforms for both realizing and utilizing spin-valley locking in bulk materials, as well as exploring proximity effects in two-dimensional structures.

cond-mat.supr-con↗

arXiv2024

Emergence of a Fermi-surface in the current-driven Hidden state of 1T-TaS$_2$

The origin of the insulating state in 1T-TaS$_2$ has long been a subject of debate. A short current pulse transforms this insulating state into a metastable metallic phase. Using micro-ARPES, we investigate the electronic structure of this phase and uncover spatially dependent modifications caused by the current pulse. In some regions of the sample, a Fermi surface emerges, while others remain gapped. Detailed band structure analysis reveals that the metallic regions exhibit an electronic structure similar to that observed in the high-temperature phase of 1T-TaS$_2$, characterized by suppressed energy gaps and bands crossing the Fermi level. Furthermore, the metallic and insulating regions display distinct dispersions along the out-of-plane direction. These observations suggest a scenario in which the current pulse breaks the star-of-David dimers characteristic of the insulating phase, implicating these dimers as the likely origin of the insulating behavior in 1T-TaS$_2$.

cond-mat.str-el↗

arXiv2024

Charge transfer and Spin-Valley locking in 4Hb-TaS$_{2}$

4Hb-TaS$_2$ is a superconductor that exhibits unique characteristics such as time-reversal symmetry breaking, hidden magnetic memory, and topological edge modes. It is a naturally occurring heterostructure comprising of alternating layers of 1H-TaS$_2$ and 1T-TaS$_2$. The former is a well-known superconductor, while the latter is a correlated insulator with a possible non-trivial magnetic ground state. In this study, we use angle resolved photoemission spectroscopy to investigate the normal state electronic structure of this unconventional superconductor. Our findings reveal that the band structure of 4H-TaS$_2$ fundamentally differs from that of its constituent materials. Specifically, we observe a significant charge transfer from the 1T layers to the 1H layers that drives the 1T layers away from half-filling. In addition, we find a substantial reduction in inter-layer coupling in 4Hb-TaS$_2$ compared to the coupling in 2H-TaS$_2$ that results in a pronounced spin-valley locking within 4Hb-TaS$_2$

cond-mat.supr-con↗

arXiv2023

Hall anomaly and vortex charge in Bi$_2$Sr$_2$CaCu$_2$O$_x$

We present a systematic study of the Hall conductance in Bi$_2$Sr$_2$CaCu$_2$O$_x$ (Bi2212) thin films over a large range of doping. We find that in a large part of the phase diagram the Hall coefficient changes sign as a function of temperature in the flux-flow regime. By comparing data from many samples, we show that the sign reversal is tied to the superconducting transition and is not a result of a competing order. We then compare our data to the predictions of the Bardeen-Stephan model and show that in all samples there is an additional negative contribution to the Hall conductivity. We extract from the negative excess Hall a vortex-charge that is found to be strongly doping dependent.

cond-mat.supr-con↗

arXiv2020

Observation of Josephson-like tunneling junction characteristics and positive magnetoresistance in Oxygen deficient Nickelate films of $Nd_{0.8}Sr_{0.2}NiO_{3-\delta}$

Nickelate films have recently attracted broad attention due to the observation of superconductivity in the infinite layer phase of $Nd_{0.8}Sr_{0.2}NiO_2$ (obtained by reducing Sr doped $NdNiO_3$ films) and their similarity to the cuprates high temperature superconductors. Here we report on the observation of a new type of transport in oxygen poor $Nd_{0.8}Sr_{0.2}NiO_{3-\delta}$ films. At high temperatures, variable range hopping is observed while at low temperatures a novel tunneling behavior is found where Josephson-like tunneling junction characteristic with serial resistance is revealed. We attribute this phenomenon to coupling between superconductive (S) surfaces of the grains in our Oxygen poor films via the insulating (I) grain boundaries, which yields SIS junctions in series with the normal (N) resistance of the grains themselves. The similarity of the observed conductance spectra to tunneling junction characteristic with Josephson-like current is striking, and seems to support the existence of superconductivity in our samples.

cond-mat.supr-con↗

arXiv2019

Band inversion and topology of the bulk electronic structure in FeSe${}_{0.45}$Te${}_{0.55}$

FeSe${}_{0.45}$Te${}_{0.55}$ (FeSeTe) has recently emerged as a promising candidate to host topological superconductivity, with a Dirac surface state and signatures of Majorana bound states in vortex cores. However, correlations strongly renormalize the bands compared to electronic structure calculations, and there is no evidence for the expected bulk band inversion. We present here a comprehensive angle resolved photoemission (ARPES) study of FeSeTe as function of photon energies ranging from 15 - 100 eV. We find that although the top of bulk valence band shows essentially no $k_z$ dispersion, its normalized intensity exhibits a periodic variation with $k_z$. We show, using ARPES selection rules, that the intensity oscillation is a signature of band inversion indicating a change in the parity going from $\Gamma$ to Z. Thus we provide the first direct evidence for a topologically non-trivial bulk band structure that supports protected surface states.

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