Defects Unlock Hidden Magnetic Order in Crystals, Creating New Light Responses

A pronounced longitudinal photocurrent is observed in a chiral cubic sillenite by Bumseop Kim and colleagues at University of Pennsylvania. The material globally forbids such a response based on its crystal symmetry. The collaboration between the University of Pennsylvania and Drexel University, The University of Texas at Austin and Russian Academy of Sciences identifies that defects and applied magnetic fields work together to locally lower the material’s symmetry, thereby unmasking latent quantum-geometric responses. The discovery provides a new pathway for activating forbidden nonlinear optical effects and exploring the fundamental quantum properties of chiral materials.

Oxygen vacancies induce magneto-photocurrent via symmetry reduction and quantum geometry

A pronounced longitudinal magneto-photocurrent, forbidden by crystal symmetry, was observed across the entire visible range of 325-647nm in chiral cubic sillenite materials. This finding sharply departs from established theory, as such a response was previously considered impossible due to the material’s symmetry. Remarkably, the effect persists even with sub-band-gap excitation, demonstrating a previously unrecognised capability within these materials. The persistence of the photocurrent below the band gap, where conventional excitation of electron-hole pairs is suppressed, suggests that the underlying mechanism does not simply relate to standard photoexcitation but instead relies on more subtle quantum phenomena. Sillenite materials, known for their complex crystal structures and potential for hosting exotic electronic states, have been extensively studied for their dielectric and nonlinear optical properties. However, the observation of this forbidden photocurrent represents a significant departure from previous understanding.

The team identified that oxygen vacancies within the sillenite structure create defect-enabled spin ordering, effectively lowering the material’s magnetic symmetry when a magnetic field is applied, thereby enabling the photocurrent. First-principles calculations reveal that this field-selected symmetry reduction unmasks latent quantum-geometric responses encoded within the material’s electronic structure. These calculations correlate circular and linear photocurrent channels with regions of high Berry curvature and quantum metric respectively. Berry curvature, a manifestation of the phase acquired by electrons as they move through the crystal lattice, and the quantum metric, which describes the geometric properties of the electronic band structure, are both crucial in determining the material’s response to electromagnetic fields. Detailed analysis of the photocurrent’s polarisation revealed strong helicity selectivity, with the circular component exceeding the linear one and reversing direction when the light’s ‘handedness’ is switched, indicating a strong link between light polarisation and current flow. Furthermore, calculations pinpointed that oxygen vacancies, imperfections within the sillenite structure, generate localised magnetic moments on bismuth-oxygen units, stabilised by strong spin-orbit coupling, a quantum mechanical effect linking an electron’s spin and motion; this localised magnetic ordering is key to the observed effect. Spin-orbit coupling, particularly strong in materials containing heavy elements like bismuth, plays a vital role in dictating the electronic band structure and influencing the material’s optical and transport properties.

Detection of symmetry-breaking photocurrents in chiral cubic sillenites

Frequency- and polarization-resolved photocurrents provide a sensitive method for probing subtle changes in symmetry and electronic band structure within quantum materials. The technique carefully measures the electrical current generated when a material is illuminated with light, while simultaneously exposed to a magnetic field, distinguishing between currents flowing in different directions and with different light polarizations. This allows researchers to map out the material’s response to light and magnetic fields with high precision, revealing details about its internal symmetry and electronic structure. Employing this technique, a response that established theory predicted should not exist given the material’s symmetry was detected, indicating the presence of hidden properties. The Voigt geometry, where the applied magnetic field is perpendicular to both the light propagation direction and the detected current, is particularly sensitive to symmetry-breaking effects.

A chiral cubic sillenite, a quantum material where symmetry typically prevents a specific type of electrical current, a longitudinal odd-in-B magneto-photocurrent in the Voigt geometry, was investigated. However, this current was observed across the visible light spectrum, even below the material’s band gap, and found to be stronger with circular light polarization. This prompted detailed analysis using frequency- and polarization-resolved photocurrents to understand the origin of the signal. Chiral materials, lacking mirror symmetry, exhibit unique optical and electronic properties, including the ability to rotate the polarization of light. Cubic sillenites, characterised by their specific crystal structure, are known for their anisotropic properties and potential for hosting complex electronic states. The observation of a longitudinal photocurrent, which flows parallel to the direction of light propagation, is particularly noteworthy as it violates the selection rules imposed by the material’s symmetry.

Defect engineering reveals hidden quantum currents in chiral sillenites

Researchers are increasingly adept at teasing out hidden properties in complex materials, but fully understanding these quantum behaviours remains a formidable challenge. This research reveals a surprising ability to activate electrical currents in a chiral cubic sillenite, seemingly defying its inherent symmetry. This method, however, relies on carefully induced defects, specifically oxygen vacancies, and applied magnetic fields. It is vital to acknowledge that these electrical currents arise from defects within the material, rather than its pristine structure. The ability to manipulate material properties through defect engineering opens up new avenues for designing materials with tailored functionalities.

These vacancies create localised magnetic properties, altering how the material responds to magnetic fields and effectively lowering its symmetry. This defect-driven mechanism is not a limitation, but a pathway to access and visualise previously hidden quantum properties, correlating photocurrent channels with regions of rich quantum geometry. Oxygen vacancies within chiral cubic sillenite materials enable a surprising electrical response when exposed to both light and magnetic fields. These vacancies create localised magnetic ordering, effectively reducing the material’s symmetry and unlocking a longitudinal magneto-photocurrent previously forbidden by its crystal structure; this current flows in a defined direction within the material. First-principles calculations demonstrate that this field-selected symmetry lowering isn’t merely an activation of a current, but a revelation of inherent quantum-geometric responses encoded within the material’s electronic structure. The implications of this work extend beyond fundamental materials science, potentially leading to the development of novel optoelectronic devices and sensors that exploit these quantum-geometric effects. Further research will focus on optimising the concentration of oxygen vacancies and exploring other defect types to enhance the photocurrent signal and tailor the material’s response to external stimuli.

The research revealed that oxygen vacancies in chiral cubic sillenite materials can generate a longitudinal magneto-photocurrent, despite the material’s crystal symmetry normally prohibiting it. This occurs because the vacancies create localised magnetic ordering, effectively lowering the material’s symmetry when a magnetic field is applied. Importantly, this process doesn’t simply activate a current, but reveals underlying quantum-geometric responses within the material’s electronic structure. Researchers intend to optimise vacancy concentration and explore other defects to further refine the material’s response to external stimuli.