Evolution of the electronic structure of 1T-Cu(x)TiSe(2)

Ultrafast creation of a light-induced semimetallic state in strongly excited 1T-TiSe2

Screening, a ubiquitous phenomenon associated with the shielding of electric fields by surrounding charges, has been widely adopted as a means to modify a material's properties. While most studies have relied on static changes of screening through doping or gating thus far, here we demonstrate that screening can also drive the onset of distinct quantum states on the ultrafast timescale. By using time- and angle-resolved photoemission spectroscopy, we show that intense optical excitation can drive 1T-TiSe2, a prototypical charge density wave material, almost instantly from a gapped into a semimetallic state. By systematically comparing changes in band structure over time and excitation strength with theoretical calculations, we find that the appearance of this state is likely caused by a dramatic reduction of the screening length. In summary, this work showcases how optical excitation enables the screening-driven design of a nonequilibrium semimetallic phase in TiSe2, possibly providing a general pathway into highly screened phases in other strongly correlated materials.

Carrier-Density Control of the Quantum-Confined 1T-TiSe_{2} Charge Density Wave

Using angle-resolved photoemission spectroscopy, combined with first principle and coupled self-consistent Poisson-Schrödinger calculations, we demonstrate that potassium (K) atoms adsorbed on the low-temperature phase of 1T-TiSe_{2} induce the creation of a two-dimensional electron gas (2DEG) and quantum confinement of its charge-density wave (CDW) at the surface. By further changing the K coverage, we tune the carrier density within the 2DEG that allows us to nullify, at the surface, the electronic energy gain due to exciton condensation in the CDW phase while preserving a long-range structural order. Our Letter constitutes a prime example of a controlled exciton-related many-body quantum state in reduced dimensionality by alkali-metal dosing.

Interlayer Chemical Modulation of Phase Transitions in Two-Dimensional Metal Chalcogenides

Two-dimensional metal chalcogenides (2D-MCs) with complex interactions are usually rich in phase transition behavior, such as superconductivity, charge density wave (CDW), and magnetic transitions, which hold great promise for the exploration of exciting physical properties and functional applications. Interlayer chemical modulation, as a renewed surface modification method, presents congenital advantages to regulate the phase transitions of 2D-MCs due to its confined space, strong guest-host interactions, and local and reversible modulation without destructing the host lattice, whereby new phenomena and functionalities can be produced. Herein, recent achievements in the interlayer chemical modulation of 2D-MCs are reviewed from the aspects of superconducting transition, CDW transition, semiconductor-to-metal transition, magnetic phase transition, and lattice transition. We systematically discuss the roles of charge transfer, spin coupling, and lattice strain on the modulation of phase transitions in the guest-host architectures of 2D-MCs established by electrochemical intercalation, solution-processed intercalation, and solid-state intercalation. New physical phenomena, new insight into the mechanism of phase transitions, and derived functional applications are presented. Finally, a prospectus of the challenges and opportunities of interlayer chemical modulation for future research is pointed out.

Coexistence of resistance oscillations and the anomalous metal phase in a lithium intercalated TiSe2 superconductor

Superconductivity and charge density wave (CDW) appear in the phase diagram of a variety of materials including the high-T_c cuprate family and many transition metal dichalcogenides (TMDs). Their interplay may give rise to exotic quantum phenomena. Here, we show that superconducting arrays can spontaneously form in TiSe₂–a TMD with coexisting superconductivity and CDW—after lithium ion intercalation. We induce a superconducting dome in the phase diagram of Li_xTiSe₂ by using the ionic solid-state gating technique. Around optimal doping, we observe magnetoresistance oscillations, indicating the emergence of periodically arranged domains. In the same temperature, magnetic field and carrier density regime where the resistance oscillations occur, we observe signatures for the anomalous metal—a state with a resistance plateau across a wide temperature range below the superconducting transition. Our study not only sheds further insight into the mechanism for the periodic electronic structure, but also reveals the interplay between the anomalous metal and superconducting fluctuations.

The interplay between superconductivity and charge density wave (CDW) gives rise to exotic quantum phenomena. Here, the authors observe magnetoresistance oscillations and an anomalous metal state due to the coexistence of superconductivity and CDW in lithium intercalated TiSe₂.

Angle, Spin, and Depth Resolved Photoelectron Spectroscopy on Quantum Materials

The role of X-ray based electron spectroscopies in determining chemical, electronic, and magnetic properties of solids has been well-known for several decades. A powerful approach is angle-resolved photoelectron spectroscopy, whereby the kinetic energy and angle of photoelectrons emitted from a sample surface are measured. This provides a direct measurement of the electronic band structure of crystalline solids. Moreover, it yields powerful insights into the electronic interactions at play within a material and into the control of spin, charge, and orbital degrees of freedom, central pillars of future solid state science. With strong recent focus on research of lower-dimensional materials and modified electronic behavior at surfaces and interfaces, angle-resolved photoelectron spectroscopy has become a core technique in the study of quantum materials. In this review, we provide an introduction to the technique. Through examples from several topical materials systems, including topological insulators, transition metal dichalcogenides, and transition metal oxides, we highlight the types of information which can be obtained. We show how the combination of angle, spin, time, and depth-resolved experiments are able to reveal "hidden" spectral features, connected to semiconducting, metallic and magnetic properties of solids, as well as underlining the importance of dimensional effects in quantum materials.

Chemical bonds in intercalation compounds CuxTiCh2 (Ch = S, Te)

A thorough study of the chemical bonding between intercalated copper and host lattice TiCh₂ (Ch = S, Te) was performed. In order to separate the contributions of the copper, titanium, and chalcogen states into the electronic structure of the valence band, photoelectron spectroscopy in nonresonant and resonant (Cu 3p-3d and Ti 2p-3d) excitation modes was used. It is shown that the ionicity of the chemical bond between copper and host lattice is decreased in the TiS₂ → TiSe₂ → TiTe₂ row. In Cu_xTiS₂, copper atoms form the chemical bond with TiCh₂ host lattice, while in Cu_xTiTe₂ directly with tellurium atoms.

Orbital- and k_{z}-Selective Hybridization of Se 4p and Ti 3d States in the Charge Density Wave Phase of TiSe_{2}

We revisit the enduring problem of the 2×2×2 charge density wave (CDW) order in TiSe_{2}, utilizing photon energy-dependent angle-resolved photoemission spectroscopy to probe the full three-dimensional high- and low-temperature electronic structure. Our measurements demonstrate how a mismatch of dimensionality between the 3D conduction bands and the quasi-2D valence bands in this system leads to a hybridization that is strongly k_{z} dependent. While such a momentum-selective coupling can provide the energy gain required to form the CDW, we show how additional "passenger" states remain, which couple only weakly to the CDW and thus dominate the low-energy physics in the ordered phase of TiSe_{2}.

Reproduction of the Charge Density Wave Phase Diagram in 1T-TiSe_{2} Exposes its Excitonic Character

Recent experiments suggest that excitonic degrees of freedom play an important role in precipitating the charge density wave (CDW) transition in 1T-TiSe_{2}. Through systematic calculations of the electronic and phonon spectrum based on density functional perturbation theory, we show that the predicted critical doping of the CDW phase overshoots the experimental value by 1 order of magnitude. In contrast, an independent self-consistent many-body calculation of the excitonic order parameter and renormalized band structure is able to capture the experimental phase diagram in extremely good qualitative and quantitative agreement. This demonstrates that electron-electron interactions and the excitonic instability arising from direct electron-hole coupling are pivotal to accurately describe the nature of the CDW in this system. This has important implications to understand the emergence of superconductivity within the CDW phase of this and related systems.

Unveiling the charge density wave inhomogeneity and pseudogap state in 1T-TiSe2

By using scanning tunneling microscopy (STM)/spectroscopy (STS), we systematically characterize the electronic structure of lightly doped 1T-TiSe₂, and demonstrate the existence of the electronic inhomogeneity and the pseudogap state. It is found that the intercalation induced lattice distortion impacts the local band structure and reduce the size of the charge density wave (CDW) gap with the persisted 2 × 2 spatial modulation. On the other hand, the delocalized doping electrons promote the formation of pseudogap. Domination by either of the two effects results in the separation of two characteristic regions in real space, exhibiting rather different electronic structures. Further doping electrons to the surface confirms that the pseudogap may be the precursor for the superconducting gap. This study suggests that the competition of local lattice distortion and the delocalized doping effect contribute to the complicated relationship between charge density wave and superconductivity for intercalated 1T-TiSe₂.

Charge Transfer Effects in Naturally Occurring van der Waals Heterostructures (PbSe)_{1.16}(TiSe_{2})_{m} (m=1, 2)

van der Waals heterostructures (VDWHs) exhibit rich properties and thus has potential for applications, and charge transfer between different layers in a heterostructure often dominates its properties and device performance. It is thus critical to reveal and understand the charge transfer effects in VDWHs, for which electronic structure measurements have proven to be effective. Using angle-resolved photoemission spectroscopy, we studied the electronic structures of (PbSe)_{1.16}(TiSe_{2})_{m} (m=1, 2), which are naturally occurring VDWHs, and discovered several striking charge transfer effects. When the thickness of the TiSe_{2} layers is halved from m=2 to m=1, the amount of charge transferred increases unexpectedly by more than 250%. This is accompanied by a dramatic drop in the electron-phonon interaction strength far beyond the prediction by first-principles calculations and, consequently, superconductivity only exists in the m=2 compound with strong electron-phonon interaction, albeit with lower carrier density. Furthermore, we found that the amount of charge transferred in both compounds is nearly halved when warmed from below 10 K to room temperature, due to the different thermal expansion coefficients of the constituent layers of these misfit compounds. These unprecedentedly large charge transfer effects might widely exist in VDWHs composed of metal-semiconductor contacts; thus, our results provide important insights for further understanding and applications of VDWHs.

Influence of Domain Walls in the Incommensurate Charge Density Wave State of Cu Intercalated 1T-TiSe_{2}

We report a low-temperature scanning tunneling microscopy study of the charge density wave (CDW) order in 1T-TiSe_{2} and Cu_{0.08}TiSe_{2}. In pristine 1T-TiSe_{2} we observe a long-range coherent commensurate CDW (CCDW) order. In contrast, Cu_{0.08}TiSe_{2} displays an incommensurate CDW (ICDW) phase with localized CCDW domains separated by domain walls. Density of states measurements indicate that the domain walls host an extra population of fermions near the Fermi level which may play a role in the emergence of superconductivity in this system. Fourier transform scanning tunneling spectroscopy studies suggest that the dominant mechanism for CDW formation in the ICDW phase may be electron-phonon coupling.

Stripe and Short Range Order in the Charge Density Wave of 1T-Cu_{x}TiSe_{2}

We study the impact of Cu intercalation on the charge density wave (CDW) in 1T-Cu_{x}TiSe_{2} by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x<0.02) and a breaking up of the commensurate order into 2×2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material.

Unconventional Charge-Density-Wave Transition in Monolayer 1T-TiSe2

Reducing the dimension in materials sometimes leads to unexpected discovery of exotic and/or pronounced physical properties such as quantum Hall effect in graphene and high-temperature superconductivity in iron-chalcogenide atomically thin films. Transition-metal dichalcogenides (TMDs) provide a fertile ground for studying the interplay between dimensionality and electronic properties, since they exhibit a variety of electronic phases like semiconducting, superconducting, and charge-density-wave (CDW) states. Among TMDs, bulk 1T-TiSe2 has been a target of intensive studies due to its unusual CDW properties with the periodic lattice distortions characterized by the three-dimensional (3D) commensurate wave vector. Clarifying the ground states of its two-dimensional (2D) counterpart is of great importance not only to pin down the origin of CDW, but also to find unconventional physical properties characteristic of atomic-layer materials. Here, we show the first experimental evidence for the realization of 2D CDW phase without Fermi-surface nesting in monolayer 1T-TiSe2. Our angle-resolved photoemission spectroscopy (ARPES) signifies an electron pocket at the Brillouin-zone corner above the CDW-transition temperature (TCDW ∼ 200 K), while, below TCDW, an additional electron pocket and replica bands appear at the Brillouin-zone center and corner, respectively, due to the back-folding of bands by the 2 × 2 superstructure potential. Similarity in the spectral signatures to bulk 1T-TiSe2 implies a common driving force of CDW, i.e., exciton condensation, whereas the larger energy gap below TCDW in monolayer 1T-TiSe2 suggests enhancement of electron-hole coupling upon reducing dimensionality. The present result lays the foundation for the electronic-structure engineering based with atomic-layer TMDs.

Pressure controlled transition into a self-induced topological superconducting surface state

Ab-initio calculations show a pressure induced trivial-nontrivial-trivial topological phase transition in the normal state of 1T-TiSe2. The pressure range in which the nontrivial phase emerges overlaps with that of the superconducting ground state. Thus, topological superconductivity can be induced in protected surface states by the proximity effect of superconducting bulk states. This kind of self-induced topological surface superconductivity is promising for a realization of Majorana fermions due to the absence of lattice and chemical potential mismatches. For appropriate electron doping, the formation of the topological superconducting surface state in 1T-TiSe2 becomes accessible to experiments as it can be controlled by pressure.

Structural phase transition in IrTe₂: a combined study of optical spectroscopy and band structure calculations

Ir(1-x)Pt(x)Te₂ is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their electronic properties. In particular, we performed optical spectroscopic measurements, in combination with density function calculations, on the undoped compound IrTe₂ in an effort to elucidate the origin of the structural phase transition at 280 K. The measurement revealed a dramatic reconstruction of band structure and a significant reduction of conducting carriers below the phase transition. We elaborate that the transition is not driven by the density wave type instability but caused by the crystal field effect which further splits/separates the energy levels of Te (p(x), p(y)) and Te p(z) bands.

Superconductivity at 5.2 K in ZrTe3 polycrystals and the effect of Cu and Ag intercalation

We report the occurrence of superconductivity in polycrystalline samples of ZrTe(3) at temperature 5.2 K at ambient pressure. The superconducting state coexists with the charge density wave (CDW) phase, which sets in at 63 K. The intercalation of Cu or Ag does not have any bearing on the superconducting transition temperature but suppresses the CDW state. The feature of a CDW anomaly in these compounds is clearly seen in the DC magnetization data. Resistivity data are analyzed in order to estimate the relative loss of carriers and reduction in the nested Fermi surface area upon CDW formation in ZrTe(3) and the intercalated compounds.

Coexistence of bulk superconductivity and charge density wave in CuxZrTe3

We report the coexistence of bulk superconductivity with T(c)=3.8  K and charge density wave (CDW) in Cu intercalated quasi-two-dimensional crystals of ZrTe(3). The Cu intercalation results in the expansion of the unit cell orthogonal to the Zr-Zr metal chains and partial filling of CDW energy gap. We present anisotropic parameters of the superconducting state. We also show that the contribution of CDW to the scattering mechanism is anisotropic in the a-b plane. The dominant scattering mechanism in the normal state for both ZrTe(3) and Cu(0.05)ZrTe(3) along the b axis is the electron-electron umklapp scattering.

On the origin of charge-density waves in select layered transition-metal dichalcogenides

The occurrence of charge-density waves in three selected layered transition-metal dichalcogenides-1T-TaS(2), 2H-TaSe(2) and 1T-TiSe(2)-is discussed from an experimentalist's point of view with a particular focus on the implications of recent angle-resolved photoelectron spectroscopy results. The basic models behind charge-density-wave formation in low-dimensional solids are recapitulated, the experimental and theoretical results for the three selected compounds are reviewed, and their band structures and spectral weight distributions in the commensurate charge-density-wave phases are calculated using an empirical tight-binding model. It is explored whether the origin of charge-density waves in the layered transition-metal dichalcogenides can be understood in a unified way on the basis of a few measured and calculated parameters characterizing the interacting electron-lattice system. It is found that the predictions of the standard mean-field model agree only semi-quantitatively with the experimental data and that there is not one generally dominant factor driving charge-density-wave formation in this family of layer compounds. The need for further experimental and theoretical scrutiny is emphasized.

77Se and 63Cu NMR studies of the electronic correlations in CuxTiSe2 (x = 0.05, 0.07)

We report a (77)Se and (63)Cu nuclear magnetic resonance (NMR) investigation on the charge-density-wave (CDW) superconductor Cu(x)TiSe(2) (x = 0.05 and 0.07). At high magnetic fields where superconductivity is suppressed, the temperature dependence of (77)Se and (63)Cu spin-lattice relaxation rates 1/T(1) follow a linear relation. The slope of (77)Se 1/T(1) versus T increases with the Cu doping. This can be described by a modified Korringa relation which suggests the significance of electronic correlations and the Se 4p- and Ti 3d-band contribution to the density of states at the Fermi level in the studied compounds.

Pressure induced superconductivity in pristine 1T-TiSe2

The interplay between superconductivity and the charge-density wave (CDW) state in pure 1T-TiSe(2) is examined through a high-pressure study extending up to pressures of 10 GPa between sub-Kelvin and room temperatures. At a critical pressure of 2 GPa a superconducting phase sets in and persists up to pressures of 4 GPa. The maximum superconducting transition temperature is 1.8 K. These findings complement the recent discovery of superconductivity in copper-intercalated 1T-TiSe(2). The comparisons of the normal state and superconducting properties of the two systems reveal the possibility that the emergent electronic state qualitatively depends on the manner in which the CDW state is destabilized, making this a unique example where two different superconducting domes are obtained by two different methods from the same parent compound.

Excitonic insulator state in Ta2NiSe5 probed by photoemission spectroscopy

We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

Anisotropic intermediate coupling superconductivity in Cu(0.03)TaS(2)

The anisotropic superconducting state properties in Cu(0.03)TaS(2) have been investigated by magnetization, magnetoresistance and specific heat measurements. They clearly show that Cu(0.03)TaS(2) undergoes a superconducting transition at T(C) = 4.03 K. The obtained superconducting parameters demonstrate that Cu(0.03)TaS(2) is an anisotropic type-II superconductor. Combining specific heat jump ΔC/γ(n)T(C) = 1.6(4), gap ratio 2Δ/k(B)T(C) = 4.0(9) and the estimated electron-phonon coupling constant λ∼0.68, the superconductivity in Cu(0.03)TaS(2) is explained within the intermediate coupling BCS scenario. First-principles electronic structure calculations suggest that copper intercalation of 2H-TaS(2) causes a considerable increase of the Fermi surface volume and the carrier density, which suppresses the CDW fluctuation and favors the raise of T(C).

Quantum and classical mode softening near the charge-density-wave-superconductor transition of CuxTiSe2

Temperature- and x-dependent Raman scattering studies of the charge-density-wave (CDW) amplitude modes in Cu(x)TiSe(2) show that the amplitude mode frequency omega(0) exhibits identical power-law scaling with the reduced temperature T/T(CDW) and the reduced Cu content x/x(c), i.e., omega(0) approximately (1-p)(0.15) for p=T/T(CDW) or x/x(c), suggesting that mode softening is independent of the control parameter used to approach the CDW transition. We provide evidence that x-dependent mode softening in Cu(x)TiSe(2) is associated with the reduction of the electron-phonon coupling constant, and that x-dependent "quantum" (T approximately 0) mode softening suggests the presence of a quantum critical point within the superconductor phase of Cu(x)TiSe(2).

Anomalous Metallic State of Cu0.07TiSe2: an optical spectroscopy study

We report an optical spectroscopy study on the newly discovered superconductor Cu0.07TiSe2. Consistent with the development from a semimetal or semiconductor with a very small indirect energy gap upon doping TiSe2, it is found that the compound has a low carrier density. Most remarkably, the study reveals a substantial shift of the screened plasma edge in reflectance towards high energy with decreasing temperature. This phenomenon, rarely seen in metals, indicates either a sizable increase of the conducting carrier concentration or/and a decrease of the effective mass of carriers with reducing temperature. We attribute the shift primarily to the latter effect.