Irradiated Graphene Exhibits Floquet Chern Insulators and Zero Resistance at High Driving Frequencies

The pursuit of novel electronic states in materials receives a significant boost from new research exploring how light can fundamentally alter a material’s properties, and a team led by Youngjae Kim and Kwon Park from the Korea Institute for Advanced Study demonstrates this potential in graphene. Their work predicts that irradiating graphene with circularly polarized light creates two distinct states exhibiting zero electrical resistance, a phenomenon with profound implications for future electronic devices. The researchers calculate how the material responds to light, revealing the emergence of ‘Floquet Chern insulators’ at high frequencies and, surprisingly, a radiation-induced zero-resistance state at lower frequencies, where an uneven flow of current spontaneously develops. This discovery not only expands our understanding of non-equilibrium states of matter, but also draws a compelling parallel to similar effects observed in quantum Hall systems, potentially paving the way for new avenues in low-energy electronics.

Floquet Engineering and Zero Resistance States

This body of work explores the fascinating realm of non-equilibrium phenomena in materials driven by external forces, particularly focusing on Floquet engineering, microwave interactions, and the emergence of zero-resistance states in two-dimensional electron systems and other materials. Researchers investigate how manipulating materials with time-varying fields can unlock new states and functionalities, offering potential for advanced technologies. Central to this research is the concept of Floquet engineering, where time-periodic driving, often using microwaves or light, alters a material’s electronic band structure, creating novel quantum states and controlling its properties. A significant focus lies on understanding zero-resistance states, observed in two-dimensional electron systems when exposed to microwave radiation.

These states arise from a complex interplay of coherent driving, electron scattering, and symmetry changes within the material. Researchers are actively investigating the underlying mechanisms that give rise to these states, pushing the boundaries of our understanding of electron behavior in driven systems. The research extends beyond equilibrium conditions, exploring the properties of steady states created by these driving forces, which require theoretical frameworks beyond traditional statistical mechanics. The investigations encompass a range of materials, including semiconductor heterostructures hosting two-dimensional electron systems, graphene, and other two-dimensional materials.

Theoretical approaches are diverse, employing Floquet theory, a mathematical framework for analyzing time-periodic systems, alongside techniques like density functional theory and tight-binding models. Advanced computational methods, such as dynamical mean-field theory and optimal control theory, are also utilized to model and predict material behavior. This research explores symmetry breaking induced by external driving, the creation of coherent quantum states, and the formation of spatial domains with unique properties. Researchers are also investigating the generation of terahertz radiation through Floquet engineering and exploring the possibility of driving currents in dielectric materials using intense light fields. This vibrant field of research, combining condensed matter physics, materials science, and quantum optics, aims to discover new quantum phenomena and develop innovative electronic and optical devices.

Irradiated Graphene’s Zero-Resistance States Predicted Computationally

Scientists have computationally predicted novel states of matter in irradiated graphene, specifically zero-resistance states arising from exposure to circularly polarized light. The research identifies two distinct states: Floquet Chern insulators and a radiation-induced zero-resistance state characterized by an uneven distribution of electrical current. Researchers employed sophisticated theoretical techniques and computational methods to model the behavior of graphene under irradiation, paving the way for potential applications in advanced electronics. The work began by establishing a detailed model of graphene’s electronic structure, accounting for its unique honeycomb lattice and resulting Dirac cones.

Researchers then modified this model to incorporate the effects of circularly polarized light, effectively altering the momentum of electrons within the material. This led to the formulation of the Floquet Hamiltonian, an effective model that simplifies the analysis of time-periodic systems. Diagonalizing this Hamiltonian reveals Floquet eigenstates, which describe the behavior of electrons in the irradiated material. To account for imperfections in real materials, the team employed the nonequilibrium Green’s function method, a powerful technique for studying systems far from equilibrium. A key step involved a mathematical transformation connecting the behavior of electrons in the original material to that of the Floquet eigenstates. This allowed researchers to calculate the electrical conductivities, revealing quantized anomalous Hall conductivity and near-zero longitudinal conductivity at high driving frequencies, indicative of Floquet Chern insulators. Conversely, at lower frequencies, the calculations revealed irregular behavior, including negative resistance, potentially leading to a catastrophic breakdown and the formation of a zero-resistance state with an uneven current distribution.

Graphene Exhibits Zero Resistance Under Irradiation

Recent research demonstrates that irradiated graphene can exhibit two distinct zero-resistance states when exposed to circularly polarized light: Floquet Chern insulators and a radiation-induced zero-resistance state characterized by an uneven distribution of electrical current. This discovery opens new avenues for electronic device design and provides a deeper understanding of electron behavior in driven systems. Researchers predicted and calculated these states, confirming their existence through sophisticated theoretical modeling. The team mapped the behavior of irradiated graphene by calculating the electrical conductivities as functions of both driving frequency and electric-field strength.

Results show that Floquet Chern insulators emerge at high driving frequencies, specifically those exceeding the material’s band width. These states are characterized by quantized anomalous Hall conductivity and no current flow in the bulk of the material. Conversely, at lower frequencies, the anomalous Hall conductivity deviates from expected values, and the longitudinal conductivity displays highly irregular behavior, including negative resistance. This negative resistance, deemed thermodynamically unstable, is predicted to trigger a breakdown leading to the zero-resistance state with a spontaneous, uneven distribution of electrical current.

The research employed a sophisticated theoretical framework, beginning with a model of graphene’s electronic structure and then applying the Peierls substitution to account for the effects of the circularly polarized light. This led to the formulation of the Floquet Hamiltonian, which effectively describes the time-periodic behavior of electrons in the irradiated material. The team then utilized the nonequilibrium Green’s function method to calculate the electrical conductivities, incorporating the effects of impurity scattering and thermalization. Specifically, the calculations reveal that the chemical potential shifts individually for each Floquet eigenstate, a crucial extension of previous observations in quantum Hall systems. Measurements confirm that the calculated zero-resistance state closely resembles the radiation-induced zero-resistance state observed in quantum Hall systems, suggesting a common underlying physical mechanism. This work provides a detailed theoretical understanding of these phenomena, paving the way for potential applications in low-power electronics and novel materials design.

Graphene Exhibits Two Distinct Zero-Resistance States

This research demonstrates that irradiated graphene can exhibit two distinct zero-resistance states when exposed to circularly polarized light: Floquet Chern insulators and a radiation-induced zero-resistance state arising from an uneven distribution of electrical current. Researchers predict that Floquet Chern insulators emerge at higher driving frequencies, facilitated by the topological protection of edge states within specific bands. Conversely, the radiation-induced state appears at lower frequencies and is characterized by a spontaneous, uneven distribution of current throughout the material, stemming from a unique current-voltage relationship. Crucially, the researchers identified methods to differentiate between these two states through experimental observation.

Floquet Chern insulators are expected to exhibit quantized anomalous Hall conductivity and no current flow in the bulk, while the radiation-induced state displays unquantized conductivity values that vary with electron density and a non-uniform current distribution even without an applied voltage. Calculations suggest that achieving these states requires driving frequencies ranging from the visible to ultraviolet spectrum and electric field strengths readily attainable in laboratory settings. The authors acknowledge that the robustness of these zero-resistance states against impurity scattering has been confirmed across a range of driving frequencies. Future work could explore the potential for manipulating Floquet bands to induce superconductivity.