About
Anshul Kogar is a physicist specializing in condensed matter physics, with a particular focus on the historical and experimental aspects of magnetic and electronic phenomena in materials. His work explores fundamental concepts such as antiferromagnetism, metal-insulator transitions, and the Kondo effect, often revisiting seminal experiments and theoretical breakthroughs that have shaped the understanding of these complex systems. Through detailed analysis of classic studies, Kogar highlights the evolution of ideas in magnetism, electron correlations, and phase transitions, emphasizing the interplay between experimental discoveries and theoretical advancements. Kogar's research interests include the investigation of hidden order in antiferromagnets, the role of soft phonons in structural phase transitions, and the nature of strongly correlated electron systems exemplified by metal-insulator transitions in 3d electron compounds. He also examines the resistance minimum phenomenon in metals due to magnetic impurities, known as the Kondo effect, and the emergence of superfluidity in liquid helium. His approach combines historical context with modern understanding, providing insights into how foundational experiments and theories continue to influence contemporary condensed matter physics. By chronicling key experiments and theoretical developments, Anshul Kogar contributes to a deeper appreciation of the challenges and breakthroughs in condensed matter physics. His work serves as a bridge connecting past discoveries with ongoing research, illustrating the dynamic and evolving nature of the field.
Research topics
- Quantum mechanics
- Physics
- Stereochemistry
- Chemistry
- Geometry
- Atomic physics
- Condensed matter physics
- Optics
- Mathematics
Selected publications
Physical Review Letters · 2025-09-25 · 1 citations
articleOpen accessWe report measurements of anisotropic triple-q charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1T-TiSe_{2} over a large volume of reciprocal space with x-ray diffuse scattering. Above the transition temperature, T_{CDW}, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. In addition, the out-of-plane diffuse scattering is characterized by rodlike structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations with a hierarchy of length scales, leading to intricate fluctuation patterns that go beyond the conventional 2D-to-3D crossover picture.
In-plane anisotropy of charge density wave fluctuations in 1$T$-TiSe$_2$
arXiv (Cornell University) · 2025-01-17
preprintOpen accessWe report measurements of anisotropic triple-$q$ charge density wave (CDW) fluctuations in the transition metal dichalcogenide 1$T$-TiSe$_2$ over a large volume of reciprocal space with X-ray diffuse scattering. Above the transition temperature, $T_{\text{CDW}}$, the out-of-plane diffuse scattering is characterized by rod-like structures which indicate that the CDW fluctuations in neighboring layers are largely decoupled. In addition, the in-plane diffuse scattering is marked by ellipses which reveal that the in-plane fluctuations are anisotropic. Our analysis of the diffuse scattering line shapes and orientations suggests that the three charge density wave components contain independent phase fluctuations. At $T_{\text{CDW}}$, long range coherence is established in both the in-plane and out-of-plane directions, consistent with the large observed value of the CDW gap compared to $T_{\text{CDW}}$, and the predicted presence of a hierarchy of energy scales.
ArXiv.org · 2025-09-05
preprintOpen accessSenior authorAnomalous relaxation is one of the hallmarks of disordered systems. Following perturbation by an external source, many glassy, jammed and amorphous systems relax as a stretched or compressed exponential as a function of time. However, despite their ubiquity, the origins of and the connection between these phenomenological relaxation functions remains to be understood. Here, we observe a tunable crossover from stretched- to compressed-exponential relaxation by photoexciting single crystal Ca3Ru2O7 across a structural phase transition. We present a simple lattice model that shows how spatial inhomogeneity and local, strain-mediated interactions cooperate to produce the dynamical crossover. Our work reveals anomalous relaxation dynamics in an idealized single crystal material and establishes photoexcited solids as promising platforms for probing the mechanisms underlying anomalous relaxation.
Metastable short-range charge order in superconducting CuxTiSe2
ArXiv.org · 2025-04-04
preprintOpen accessIn a vast array of materials, including cuprates, transition metal dichalcogenides (TMDs) and rare earth tritellurides, superconductivity is found in the vicinity of short-range charge density wave (CDW) order. The crossover from long-range to short-range charge order often occurs as quenched disorder is introduced, yet it is unclear how this disorder disrupts the CDW. Here, using x-ray photon correlation spectroscopy (XPCS), we investigate the prototypical TMD superconductor CuxTiSe2 and show that disorder induces substantial CDW dynamics. We observed the CDW phase fluctuation on a timescale of minutes to hours above the nominal transition temperature while the order parameter amplitude remains finite. These long timescale fluctuations prevent the system from finding the global free energy minimum upon cooling and ultimately traps it in a short-range ordered metastable state. Our findings demonstrate how correlated disorder can give rise to a distinct mechanism of domain formation that may be advantageous to the emergence of superconductivity.
Topological phase transition to a hidden charge density wave liquid
ArXiv.org · 2025-05-08
preprintOpen accessSenior authorCharge density waves (CDWs), electronic crystals that form within a host solid, have long been speculated to melt into a spatially textured electronic liquid. Though they have not been previously detected, liquid CDWs may nonetheless be fundamental to the phase diagrams of many correlated electron systems, including high temperature superconductors and quantum Hall states. In one of the most promising candidate materials capable of hosting a liquid CDW, 1T-TaS2, a structural phase transition impedes its observation. Here, by irradiating the material with a femtosecond light pulse, we circumvent the structural phase transition to reveal how topological defect dynamics govern the otherwise invisible CDW correlations. Upon photoexcitation, the CDW diffraction peaks broaden azimuthally, initially revealing a hexatic state. At higher temperatures, photoexcitation completely destroys translational and orientational order and only a ring of diffuse scattering is observed, a key signature of a liquid CDW. Our work provides compelling evidence for a defect-unbinding transition to a CDW liquid and presents a protocol for uncovering states that are hidden by other transitions in thermal equilibrium.
Physical review. B./Physical review. B · 2025-09-25 · 1 citations
articleSenior authorObservation of a hidden charge density wave liquid
Nature Physics · 2025-12-30 · 1 citations
articleSenior authorLight-Induced Reorientation Transition in an Antiferromagnetic Semiconductor
Physical Review X · 2025-02-26 · 3 citations
articleOpen accessBecause of the lack of a net magnetic moment, antiferromagnets possess a unique robustness to external magnetic fields and are thus predicted to play an important role in future magnetic technologies. However, this robustness also makes them quite difficult to control, and the development of novel methods to manipulate these systems with external stimuli is a fundamental goal of antiferromagnetic spintronics. In this work, we report evidence for a metastable reorientation of the order parameter in an antiferromagnetic semiconductor triggered by an ultrafast quench of the equilibrium order via photoexcitation above the band gap. The metastable state forms less than 10 ps after the excitation pulse, and persists for longer than 150 ps before decaying to the ground state via thermal fluctuations. Importantly, this transition cannot be induced thermodynamically, and requires the system to be driven out of equilibrium. Broadly speaking, this phenomenology is ultimately the result of large magnetoelastic coupling in combination with a relatively low symmetry of the magnetic ground state. Since neither of these properties are particularly uncommon in magnetic materials, the observations presented here imply a generic path toward novel device technology enabled by ultrafast dynamics in antiferromagnets.
Light-induced reorientation transition in an antiferromagnetic semiconductor
ArXiv.org · 2025-02-02
preprintOpen accessDue to the lack of a net magnetic moment, antiferromagnets possess a unique robustness to external magnetic fields and are thus predicted to play an important role in future magnetic technologies. However, this robustness also makes them quite difficult to control, and the development of novel methods to manipulate these systems with external stimuli is a fundamental goal of antiferromagnetic spintronics. In this work, we report evidence for a metastable reorientation of the order parameter in an antiferromagnetic semiconductor triggered by an ultrafast quench of the equilibrium order via photoexcitation above the band gap. The metastable state forms less than 10 ps after the excitation pulse, and persists for longer than 150 ps before decaying to the ground state via thermal fluctuations. Importantly, this transition cannot be induced thermodynamically, and requires the system to be driven out of equilibrium. Broadly speaking, this phenomenology is ultimately the result of large magnetoelastic coupling in combination with a relatively low symmetry of the magnetic ground state. Since neither of these properties are particularly uncommon in magnetic materials, the observations presented here imply a generic path toward novel device technology enabled by ultrafast dynamics in antiferromagnets.
Jeff = 1/2 Diamond Magnet CaCo2TeO6: Reimagining Frontiers of Spin Liquids and Quantum Functions
Research Square · 2025-08-05
preprintOpen access
Frequent coauthors
- 52 shared
Alfred Zong
- 43 shared
Peter Abbamonte
- 36 shared
Nuh Gedik
Massachusetts Institute of Technology
- 27 shared
Sean Vig
- 19 shared
Ali Husain
Vancouver General Hospital
- 19 shared
Timm Rohwer
Center for Free-Electron Laser Science
- 18 shared
Melinda Rak
University of Illinois Urbana-Champaign
- 18 shared
Joshua Straquadine
Stanford University
Labs
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