Researchers Reveal Ta₂₂NiSe₅₅ Excitonic Insulator Stabilised by 5.5 Magnetic Fields

Researchers have long sought to isolate and control the elusive excitonic state of matter, often hampered by its entanglement with structural changes within materials. Now, Giacomo Mazza from the Dipartimento di Fisica dell’Università di Pisa, along with colleagues, demonstrate a pathway to achieving this in tantalum nickel diselenide (Ta₂NiSe₅₅). Their work reveals a metastable excitonic insulating state within the material, decoupled from lattice distortions, and stabilised by the application of magnetic fields. This is significant because it predicts a magnetic field-induced transition from a distorted semiconducting phase to a pristine excitonic insulator exhibiting ground state loop currents, and importantly, suggests a method for detecting a hidden excitonic state via magnetic field softening of associated phonon modes. These findings offer an unbiased route to disentangle competing electronic and structural instabilities in Ta₂NiSe₅₅, and potentially unlock a general mechanism for magnetic field control of such phenomena in other materials.

The research, published recently, details how sufficiently high applied magnetic fields can maintain this excitonic state, transitioning the material from a low-temperature, structurally distorted semiconductor to an undistorted excitonic insulator exhibiting ground state loop currents.

This breakthrough addresses a long-standing challenge in identifying true excitonic insulators, which are often obscured by simultaneous structural changes. The team achieved this by meticulously considering the interplay between excitonic and structural instabilities within Ta2NiSe5. Their approach involved predicting a magnetic field-induced transition, where the structural distortion vanishes, revealing the underlying excitonic insulating behaviour.
Prior to this transition, the existence of a latent excitonic phase is detectable through magnetic field softening of the phonon mode linked to the structural distortion. This magnetic field softening serves as a key indicator of the impending phase change and confirms the presence of the hidden excitonic state.

This study unveils an unbiased route for disentangling coupled excitonic-structural transitions in Ta2NiSe5, and establishes a general mechanism for magnetic field control of competing phases in quantum materials. Researchers employed a minimal model incorporating interacting electrons and phonons, coupled through an electron-phonon interaction, to describe the system.

The model predicts that a perpendicular magnetic field exceeding a critical value, Bc, suppresses the monoclinic distortion, restoring the orthorombic structure and stabilizing the loop-current carrying excitonic insulator. Experiments show that without an applied magnetic field, electron-coupling drives the monoclinic distortion, transforming the excitonic insulator into a simple, structurally distorted semiconductor.

The team’s theoretical framework, built upon a tight-binding Hamiltonian and density-density interactions, accurately captures the low-energy band structure of Ta2NiSe5. By analysing the interplay between electronic and phononic contributions, they pinpointed the conditions necessary to isolate and stabilize the desired excitonic phase. This work opens new avenues for manipulating and controlling correlated electron systems, with potential applications in tunable metal-insulator transitions and novel electronic devices.

Tight-binding Hamiltonian and parameterisation of Ta2NiSe5 electronic structure reveal key features

Scientists investigated the excitonic insulating behaviour in Ta2NiSe5, a material where electronic and structural distortions typically intertwine. The study pioneered a method to decouple these instabilities, revealing a metastable excitonic insulating state stabilised by sufficiently high applied magnetic fields.

Researchers employed a multi-faceted Hamiltonian, H = Hel + Hph + Hel−ph, to model the system, comprising electronic, phononic, and electron-phonon interaction terms. The electronic component, Hel, included a tight-binding Hamiltonian, H0, and a density-density interaction, Hint, describing electron behaviour.

To establish the model, the team defined a tight-binding Hamiltonian, truncating hoppings to nearest neighbours with specific values: tTa+,Ta+ R,R+a = −0.72 eV, tNi,Ni R,R+a = 0.3 eV, tTa+,Ta+ R,R = 2.0 eV, tNi,Ni R,R = 0 eV, and tT±,Ni R,R = −tT±,Ni R,R−a = 35 meV. This configuration generated a low-energy band structure with weakly overlapping conduction and valence bands, predominantly of Ta and Ni character.

The density-density interaction, Hint, incorporated long-range interaction potentials, with intra-chain interactions denoted as Uα and inter-chain Ta+-Ni interactions as V. The bare phonon Hamiltonian, Hph, described the vibrational modes of the Ta± atoms along the chains, utilising bosonic creation and annihilation operators.

Crucially, the electron-phonon Hamiltonian, Hel−ph, captured the linear coupling between Ta± displacement and the Ta±-Ni nearest neighbour hybridisation, expressed as Hα el−ph = g X Rσ XRα c† RNiσcRασ + c† RNiσcR−aασ + h.c., where g represents the dimensional electron-phonon coupling. Researchers solved this model using mean-field decoupling, separating the electronic and phononic components to analyse the system’s behaviour under varying magnetic fields and electron-phonon coupling strengths. This approach enabled the identification of a critical magnetic field, B Bc, above which the monoclinic distortion vanishes, restoring the orthorombic structure and stabilising a loop-current excitonic insulator.

Magnetic field stabilisation of a decoupled excitonic insulating phase in Ta2NiSe5 is observed

Scientists have demonstrated a metastable excitonic insulating phase in the material Ta2NiSe5, decoupled from lattice distortions and stabilizable with applied magnetic fields. Experiments reveal a magnetic field-induced transition from a low-temperature, structurally distorted semiconducting phase to an undistorted excitonic insulator exhibiting ground state loop currents.

Prior to this transition, the existence of a latent excitonic phase was detected through magnetic field softening of the phonon mode linked to the structural distortion. The research focused on tunable metal-insulator transitions and the disentanglement of coupled excitonic-structural transitions within Ta2NiSe5.

Results demonstrate that the material’s transition at critical temperature involves a gap opening and flattening of the valence band around the Γ-point, suggesting an excitonic insulator state. However, this transition concurrently breaks crystal symmetries, coinciding with an orthorombic-to-monoclinic lattice distortion, complicating the identification of the true insulating phase origin.

Researchers propose a solution involving the stabilization of a latent excitonic insulator immune to lattice distortion through the application of a perpendicular magnetic field. The team measured that a magnetic field exceeding a critical value, Bc, eliminates the monoclinic distortion, restoring the orthorombic structure and stabilizing an insulating phase characterized by closed current loops.

This loop-current phase originates from a purely excitonic mechanism, revealing a metastable excitonic insulator destabilized by electron-phonon coupling in the absence of an applied field. The study utilized a minimal model of interacting electrons in Ta and Ni chains coupled to the Ta-shear mode, defining a Hamiltonian including electronic, phononic, and electron-phonon interaction terms.

Calculations, based on tight-binding approximations with specific hopping parameters, tTa+,Ta+ R,R+a = −0.72 eV, tNi,Ni R,R+a = 0.3 eV, and tT±,Ni R,R = −35 meV, produced a low-energy band structure with weakly overlapping Ta and Ni-character bands. These findings uncover a general mechanism for magnetic field control of competing phases in quantum materials and provide an unbiased route to disentangle coupled excitonic-structural transitions.

Magnetic field stabilisation of a loop current bearing excitonic insulator is crucial for device applications

Scientists have demonstrated a metastable excitonic insulating phase in the material Ta₂NiSe₅, which remains independent of lattice distortions and can be stabilised using applied magnetic fields. Their research reveals that this material undergoes a transition from a low-temperature, structurally distorted semiconductor to an undistorted excitonic insulator when subjected to sufficiently strong magnetic fields.

This transition is accompanied by the formation of ground state loop currents. Prior to this transition, a latent excitonic phase is detectable through the softening of a specific phonon mode linked to the structural distortion. The findings highlight a method for separating excitonic and structural instabilities within Ta₂NiSe₅, suggesting a broader mechanism for controlling competing phases in materials using magnetic fields.

The authors acknowledge that their analysis is based on a minimal model and focuses on a single chain within the material’s layered structure, which may not fully capture the complexity of the system. Future research could explore the behaviour of this material under different conditions and investigate the potential for extending these findings to other materials exhibiting similar coupled excitonic-structural transitions. This work establishes a pathway towards understanding and manipulating excitonic insulators, potentially leading to novel electronic devices and materials.