[Paper Review] Pseudogap and weak multifractality in disordered Mott charge-density-wave insulator

Jianhua Gao, Jae Whan Park+6 more|arXiv (Cornell University)|Apr 9, 2019

2D Materials and Applications45 references4 citations

TL;DR

This study investigates Se-substituted 1T-TaS2 using scanning tunneling microscopy/spectroscopy to reveal that disorder induces a global pseudogap and weak multifractality in the electronic states, transforming the Mott-CDW insulator into a disordered correlated metal. The pseudogap forms independently of local Se concentration or CDW domain walls, and the diverging spatial correlation of the local density of states (LDOS) at the Fermi level, along with weak multifractal wave functions, supports a disordered Mott insulator scenario as the origin of emergent superconductivity in this 2D correlated system.

ABSTRACT

The competition, coexistence and cooperation of various orders in low-dimensional materials like spin, charge, topological orders and charge-density-wave has been one of the most intriguing issues in condensed matter physics. In particular, layered transition metal dichalcogenides provide an ideal platform for studying such an interplay with a notable case of 1${T}$-TaS$_{2}$ featuring Mott-insulating ground state, charge-density-wave, spin frustration and emerging superconductivity together. We investigated local electronic states of Se-substituted 1${T}$-TaS$_{2}$ by scanning tunneling microscopy/spectroscopy (STM/STS), where superconductivity emerges from the unique Mott-CDW state. Spatially resolved STS measurements reveal that an apparent V-shape pseudogap forms at the Fermi Level (E$_{F}$), with the origin of the electronic states splitting and transformation from the Mott states, and the CDW gaps are largely preserved. The formation of the pseudogap has little correlation to the variation of local Se concentration, but appears to be a global characteristics. Furthermore, the correlation length of local density of states (LDOS) diverges at the Fermi energy and decays rapidly at high energies. The spatial correlation shows a power-law decay close to the Fermi energy. Our statistics analysis of the LDOS indicates that our system exhibits weak multifractal behavior of the wave functions. These findings strongly support a correlated metallic state induced by disorder in our system, which provides an new insight into the novel mechanism of emerging superconductivity in the two-dimensional correlated electronic systems.

Motivation & Objective

• To understand the microscopic origin of emergent superconductivity in disordered 1T-TaS2 with Se substitution.
• To investigate how electron correlation, charge-density-wave (CDW) order, and disorder interplay in a Mott-insulating state.
• To determine whether the pseudogap and metallic behavior arise from local defects, CDW domain walls, or global electronic correlations.
• To test the hypothesis that disorder-induced multifractality and correlated metallic states drive unconventional superconductivity in 2D correlated systems.

Proposed method

• Scanning tunneling microscopy/spectroscopy (STM/STS) with sub-angstrom and meV resolution was used to map atomic structure, charge modulations, and local electronic states.
• Spatially resolved dI/dV spectra were acquired at 4.3 K to probe the local density of states (LDOS) and identify pseudogap formation near the Fermi level.
• Statistical analysis of LDOS fluctuations was performed to quantify multifractal behavior of the wave functions.
• The spatial correlation length of LDOS was extracted by analyzing the decay of spatial correlations around the Fermi energy.
• Density functional theory (DFT) calculations with an on-site Coulomb U correction (1.5 eV for Ta 5d orbitals) were used to model electronic structure and validate experimental observations.
• The study compared experimental results with theoretical models of disordered Mott insulators and multifractal wave functions to assess their relevance to superconductivity.

Experimental results

Research questions

• RQ1 Does the pseudogap in Se-substituted 1T-TaS2 correlate with local Se concentration or CDW domain walls?
• RQ2 What is the spatial correlation structure of the local density of states (LDOS) near the Fermi energy, and does it indicate a metallic or insulating state?
• RQ3 To what extent do the electronic wave functions exhibit multifractal characteristics, and what does this imply about the nature of the electronic state?
• RQ4 Is the emergence of superconductivity in this system driven by a disordered Mott insulator scenario rather than competing CDW or metallic domain wall networks?
• RQ5 How does disorder in a Mott-insulating system lead to a correlated metallic state with pseudogap and enhanced superconductivity?

Key findings

• A V-shaped pseudogap forms at the Fermi level in Se-substituted 1T-TaS2, indicating a global insulator-to-metal transition independent of local Se concentration.
• The CDW gaps are largely preserved, indicating that the melting of the Mott gap is not driven by CDW order fluctuations or domain wall networks.
• The spatial correlation length of the local density of states (LDOS) diverges at the Fermi energy and decays as a power law near E_F, signaling long-range quantum coherence.
• The wave functions exhibit weak multifractal behavior, with statistical analysis confirming non-universal scaling of LDOS fluctuations.
• The pseudogap formation is not correlated with isolated metallic clusters or specific S-Se configurations, contradicting models based on periodic superstructures.
• The results strongly support a disordered Mott insulator scenario as the origin of emergent superconductivity, consistent with theoretical proposals linking multifractality to enhanced superconductivity.