Exploring 2D Frontiers: Physics, Materials & Phenomena
Welcome, science enthusiasts! This Friday, December 5th, 2025, brings a fascinating array of new research, particularly focusing on the intricate world of 2D materials and related phenomena. From controlling critical points in exotic chains to uncovering new features in quantum Hall regimes, the landscape of 2D physics is constantly expanding. We'll dive deep into how geometrical twists can map 1D systems to 2D, the surprising in-plane anomalies observed in 3D quantum Hall effects, and the experimental realization of elusive 2D nitrogen structures. We'll also explore the cutting edge of ultrafast spectroscopy techniques applied to 2D semiconductors and the collective dynamics within 2D quantum magnets. Get ready to explore the cutting edge of condensed matter physics!
Unraveling Complex Systems with 2D Geometries
2D Helical Twist Controls Tricritical Point in an Interacting Majorana Chain - Authors Hekai Zhao and Philip Phillips explore a compelling theoretical model involving interacting Majorana Fermion chains. They introduce a novel concept: a helical geometrical twist that allows them to map 1D chains with finite-range interactions to 2D models. This ingenious approach reveals how this twist can control the tricritical point, a critical juncture separating different phases of matter. Their work shows that the even and odd cases of interaction range exhibit distinct behaviors, with the odd case offering an exactly solvable point where entanglement entropy vanishes. The mapping to 2D models, featuring a unique helical boundary condition, suggests that phase transitions in the 1D system can be understood as finite-size transitions in the 2D counterpart. This perspective, governed by a 2D tri-critical universality class and predicted by finite-size scaling theory, offers a powerful new way to analyze complex interacting systems. This research is particularly relevant for understanding strongly correlated electrons and opens new avenues for theoretical investigations into quantum many-body physics. The ability to relate 1D phenomena to 2D systems via geometric manipulation is a significant conceptual advance.
In-plane Anomalous Features in the 3D Quantum Hall Regime - Ming Lu and Xiao-Xiao Zhang present groundbreaking experimental findings in the realm of 3D quantum Hall effect. While much research has focused on phenomena analogous to the 2D QHE, this study highlights entirely new features that emerge when an in-plane magnetic field is applied to a 3D Weyl semimetal. They observed an unexpected Hall quantum oscillation, distinct from previously known Weyl-orbit oscillations, coexisting with the QHE. Furthermore, an unquantized two-terminal magnetoresistance and unconventional antichiral transmission leading to a disorder-robust negative longitudinal resistance were detected. The quantization of certain transport properties, tunable by lead configuration, adds another layer of complexity and intrigue. The authors attribute these phenomena to unique nonlocal quantum backscattering channels. This work challenges the conventional topological characterization of transport, even when considering 3D Chern numbers, and uncovers hidden transport properties of the 3D QHE. This research is pivotal for understanding mesoscale and nanoscale physics and quantum phenomena, pushing the boundaries of our understanding of topological states of matter and their transport characteristics. The implications for future quantum devices and fundamental physics are substantial.
Emerging Materials and Spectroscopic Techniques
Evidence of a Two-Dimensional Nitrogen Crystalline Structure on Silver Surfaces - Xuegao Hu and colleagues report a significant experimental breakthrough: the first evidence of two-dimensional nitrogen crystalline structures on silver surfaces. Nitrogen, a ubiquitous element, has long been theoretically predicted to form stable 2D materials, dubbed 'nitrogene,' with diverse properties. However, the strong nitrogen-nitrogen triple bond has made its synthesis extremely challenging. Using advanced techniques like scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and first-principles calculations, the researchers demonstrate the formation of these elusive nitrogene-like structures. The observed structure adopts a puckered honeycomb lattice, and theoretical predictions suggest a substantial band gap of up to 7.5 eV. This discovery is monumental for materials science and holds immense promise for applications in ultraviolet optoelectronics and high-k dielectric materials. The experimental realization of 2D nitrogen could revolutionize materials design, offering a new platform for exploring novel electronic and optical properties. This work represents a major leap in synthesizing and characterizing atomically thin materials.
Ultrafast Transient Absorption Spectroscopy of 2D Semiconductors: A Review - Yuri D. Glinka provides a comprehensive review of ultrafast transient absorption spectroscopy (UTAS) as applied to 2D semiconductors. Despite the maturity of UTAS, its application to these atomically thin materials often leads to experimental errors and misinterpretations. Glinka highlights that the unique nature of 2D samples and the diverse experimental configurations and data processing methods contribute to these challenges. A common pitfall is the application of a purely 'molecular' approach, which is fundamentally unsuitable for thin-film semiconductors. This review advocates for a 'solid-state' approach to interpreting UTAS spectra, covering a wide spectral range from THz to UV. Glinka critically examines recent experimental results and pinpoints typical errors in measurement, processing, and interpretation. This work is crucial for researchers in mesoscale and nanoscale physics, providing essential guidance for accurate characterization of 2D semiconductor dynamics. The review serves as a vital resource for advancing our understanding of carrier relaxation dynamics and excited-state properties in these cutting-edge materials.
Quantum Magnetism and Advanced Simulations
Collective Cluster Nucleation Dynamics in 2D Ising Quantum Magnets - Philip Osterholz and an international team present remarkable observations of collective cluster nucleation in 2D quantum Ising systems realized using an atomic Rydberg array. This research delves into the non-equilibrium response of quantum systems, a topic of significant interest in both condensed matter physics and cosmology. They have identified a confined regime where the steady-state cluster size is energy-dependent, and a deconfined regime characterized by kinetically constrained dynamics. These findings represent a significant advancement in quantum simulations with Rydberg arrays and shed light on highly collective non-equilibrium processes within a fundamental model of condensed matter physics. The study bridges concepts from quantum magnets, the kinetics of glass formers, and even cosmology, highlighting the universality of collective phenomena. This work is a key contribution to quantum gases and quantum physics, offering new insights into emergent collective behavior in strongly interacting systems. The ability to observe and control such dynamics in a quantum simulator opens exciting possibilities for future research.
General Spin Models from Noncollinear Spin Density Functional Theory and Spin-Cluster Expansion - Tomonori Tanaka and Yoshihiro Gohda introduce a highly efficient framework for constructing classical spin Hamiltonians. By combining noncollinear spin density functional theory (DFT) with the spin-cluster expansion (SCE), they develop a data-efficient method that fits magnetic torques instead of total energies. This approach significantly reduces the number of DFT calculations required, particularly for large supercells. Applied to chiral magnets, their models accurately extract magnetic exchange tensors, including isotropic and anisotropic exchange, as well as the Dzyaloshinskii-Moriya interaction. They successfully predict helical spin periods and reproduce experimental composition trends. This systematic framework also allows for the analysis of interaction order, revealing how higher-order interactions restore predictive power when lower-order models falter. This advancement is a significant step forward for materials science, enabling the creation of near-DFT-accurate spin models for finite-temperature magnetism and complex textures at a reduced computational cost. The open-source implementation in Julia further democratizes access to these powerful tools for materials design.
Superconductivity at High Temperatures and Novel Phenomena
Superconductivity Onset Above 60 K in Ambient-Pressure Nickelate Films - Guangdi Zhou and colleagues report a monumental achievement in the field of superconductivity: ambient-pressure superconductivity onset above 60 K in epitaxial (La,Pr)$_3$Ni$_2$O$_7$ thin films. This breakthrough surpasses the previous record for nickelates and approaches the performance of established cuprate and iron-based superconductors. The key to this success lies in their advanced gigantic-oxidative atomic-layer-by-layer epitaxy (GAE) method, which operates in an extreme non-equilibrium growth regime, utilizing in situ oxidation to enhance kinetics and achieve full oxygenation. The resulting films exhibit a fascinating evolution in their normal-state resistivity, linking enhanced superconductivity to non-Fermi liquid behavior. Furthermore, mapping the vortex melting phase diagram reveals significantly stronger interlayer coupling than in cuprates, characterizing these nickelates as anisotropic 3D high-Tc systems. This research is a landmark in superconductivity, potentially opening new pathways for high-temperature superconductor development and offering a new paradigm for understanding these complex materials.
Bulk Photovoltaic Effect in MoSe$_2$ and Janus MoSSe Sliding Ferroelectrics - Roumita Roy and Giuseppe Cuono investigate the bulk photovoltaic effect in ferroelectric bilayers of MoSe$_2$ and Janus MoSSe. They explore two Janus configurations: one where intralayer polarizations cancel, yielding photocurrents similar to pristine MoSe$_2$, and another where they add up, leading to strongly enhanced photoresponses. Their findings indicate that photocurrent generation in polar Janus structures is primarily governed by vertical chemical asymmetry, with minimal dependence on the sliding direction. This research highlights complementary design strategies: interlayer sliding allows for sensitivity to external tuning, while Janus intralayer polarization boosts photoresponses in the visible spectrum. The interplay between composition and stacking offers a versatile platform for tailoring light-matter interactions in 2D ferroelectric materials. This work is significant for understanding nonlinear optical properties and designing novel optoelectronic devices.
Exploring Fundamental Physics and Topological States
On Sak's Criterion for Statistical Models with Long-Range Interaction - Tianning Xiao and collaborators tackle a long-standing debate in statistical mechanics regarding the threshold value ($oldsymbol{oldsymbol{oldsymbol{ au}}}_*$) separating short-range and long-range universality classes. Sak's criterion, $oldsymbol{ au}_* = 2 - oldsymbol{oldsymbol{oldsymbol{ au}}}_{ extrm{SR}}$, has been widely accepted, but recent studies have challenged it. This work focuses on the crossover between long-range and short-range criticality in classical 2D statistical models with interactions decaying as $1/r^{2+oldsymbol{ au}}$. Through large-scale Monte Carlo simulations of the 2D LR-Ising model and analysis of key quantities like the Fortuin-Kasteleyn critical polynomial and Binder ratio, they provide convergent evidence that the universality class changes sharply at $oldsymbol{ au} = 2$. Their results, consistent with previous studies on other models, establish a unified scenario: the crossover from LR to SR universality occurs universally at $oldsymbol{ au}_* = 2$ across all studied models. This research is a fundamental contribution to statistical mechanics and provides a crucial clarification for understanding phase transitions in systems with long-range interactions.
In-plane Anomalous Features in the 3D Quantum Hall Regime - This paper, authored by Ming Lu and Xiao-Xiao Zhang, revisits the 3D quantum Hall effect. It builds upon previous studies by emphasizing transport features beyond those mimicking the 2D QHE. The introduction of an in-plane magnetic field to a 3D Weyl semimetal in the quantum Hall regime unveils new phenomena. These include an unexpected Hall quantum oscillation, distinct from the Weyl-orbit oscillation, alongside an unquantized two-terminal magnetoresistance. The paper also discusses unconventional antichiral transmission leading to a peculiar disorder-robust negative longitudinal resistance. Furthermore, it highlights quantization tunable by lead configuration. The underlying mechanism is identified as a unique type of nonlocal quantum backscattering channels. This work underscores a breakdown in the topological characterization of transport, even with 3D Chern numbers, and reveals previously hidden 3D QHE transport properties. It opens up a new avenue for transport measurements and the exploration of novel phenomena in this field, contributing significantly to mesoscale and nanoscale physics and quantum physics.
Tuning the Electronic States of Bi$_2$Se$_3$ Films with Large Spin-Orbit Interaction Using Molecular Heterojunctions - Matthew Rogers and a collaborative team explore novel ways to tune the electronic states and spin-orbit interaction (SOI) in topological insulator films like Bi$_2$Se$_3$. While electric bias can shift the Fermi level, it's challenging to modify SOI without compromising surface topology. This study demonstrates that forming molecular heterojunctions using n-p or p-n molecular diodes creates ordered interfaces. These interfaces facilitate charge transfer and orbital re-hybridization, leading to a reduced carrier density and improved mobility. Significantly, the spin-orbit lifetime in Bi$_2$Se$_3$ decreases substantially upon adding these molecular diodes, indicating a strengthened SOI. This occurs despite the molecules being composed of light elements and increasing the charge carrier mean free path. The researchers suggest changes to the Berry curvature and/or Rashba splitting around the hybridization points. Raman spectroscopy provides evidence that this coupling effect can be controlled by optical irradiation, opening exciting pathways for designing optoelectronically tunable quantum transport in heavy-light element hybrids. This research is vital for materials science and advancements in topological spintronics.
Axionic Tunneling from a Topological Kondo Insulator - Saikat Banerjee and colleagues report direct evidence of axionic physics using scanning tunneling microscopy (STM) on a topological Kondo insulator. While previous STM experiments on SmB$_6$ nanowires were interpreted as evidence for spin-polarized currents from topological surface states, this study posits that the observed spin polarization actually originates from axionic electrodynamics. Their analysis reveals a striking voltage-induced magnetization: very small voltages (~30 meV) induce tip moments of about 0.1 $oldsymbol{oldsymbol{oldsymbol{eta}}}$ that reverse sign with the applied bias. The characteristics of this signal strongly align with an axionic $E oldsymbol{oldsymbol{ au}} B$ coupling, fully explaining the magnetic component of the tip density of states and ruling out static magnetism. This ability to control spin polarization with millivolt-scale voltages offers a new method for probing axionic electrodynamics and opens up possibilities for future STM applications and spintronics. This work is a significant contribution to mesoscale and nanoscale physics and strongly correlated electrons.
Exploring Magnetism and Spin Phenomena
General Spin Models from Noncollinear Spin Density Functional Theory and Spin-Cluster Expansion - Tomonori Tanaka and Yoshihiro Gohda present a data-efficient framework for constructing general classical spin Hamiltonians. By integrating noncollinear spin density functional theory (DFT) with the spin-cluster expansion (SCE), they enable the fitting of magnetic torques, thus significantly reducing the number of required DFT calculations. This approach is particularly beneficial for large supercells. Applied to chiral magnets like Mn$_{1-x}$Fe$_x$Ge and Fe$_{1-y}$Co$_y$Ge, their models accurately extract the full pairwise exchange tensor, including isotropic and anisotropic exchange interactions, as well as the Dzyaloshinskii-Moriya interaction. They also predict helical spin periods via micromagnetic mapping and reproduce experimental composition trends. The systematic nature of the SCE framework allows for a detailed analysis of interaction order, demonstrating how higher-order interactions can restore predictive power. These advancements in materials science pave the way for creating highly accurate spin models for finite-temperature magnetism and complex spin textures with reduced computational cost, offering a direct route to quantitative first-principles parameterization and predictive materials design. An open-source Julia package is available to facilitate adoption.
Magnetocaloric Effect Measurements in Ultrahigh Magnetic Fields up to 120 T - Reon Ogawa and colleagues report proof-of-concept measurements of the magnetocaloric effect (MCE) in extreme magnetic fields up to 120 T, using the classic spin-ice compound Ho$_2$Ti$_2$O$_7$. Employing radio-frequency resistivity measurements with a specialized thermometer, they detected a rapid change in sample temperature linked to a crystal-field level crossing in the high-field region, in addition to a giant MCE observed at lower fields. The study discusses potential temperature response delays and outlines prospects for more precise MCE measurements in destructive pulsed fields. This work is significant for understanding strongly correlated electrons and materials science, pushing the boundaries of experimental techniques for characterizing magnetic materials under unprecedented conditions. The ability to probe MCE at such high fields can reveal subtle magnetic interactions and phase transitions.
Edge Spin Galvanic Effect in Altermagnets - L. E. Golub proposes the edge spin galvanic effect (ESGE) in $d$-wave altermagnets. This effect involves the creation of an electrical current along the sample edge, driven by the spin orientation of charge carriers. The ESGE arises from the altermagnetic spin splitting and carrier scattering at the sample edge. The generated current is sensitive to the edge's orientation relative to the altermagnet's main axes and reverses direction upon reversing the non-equilibrium spin or the Néel vector. The author also introduces a pure spin edge photocurrent, excited by polarized radiation, which can be converted into an electric current along the edge upon application of an external magnetic field. This research is a key contribution to mesoscale and nanoscale physics, exploring novel phenomena in altermagnetic materials and their potential for spintronic applications.
Interplay between Superconductivity and Altermagnetism in Disordered Materials and Heterostructures - Rodrigo de las Heras and collaborators investigate the complex interplay between superconductivity and altermagnetism in disordered systems. Using quantum kinetic transport equations, they derive the Ginzburg-Landau free energy and identify a coupling term between spin and the spatial variation of the superconducting order parameter. This coupling leads to two distinct effects: a magnetoelectric effect where supercurrent induces a spin texture, and a finite magnetization induced by spatial variations in the superconducting order parameter's magnitude. They show these effects compete in Abrikosov vortices. The latter also causes a proximity-induced magnetization (PIM) in superconductor-altermagnet heterostructures. In Josephson junctions, the coexistence of PIM and the magnetoelectric effect predicts $0$-$oldsymbol{oldsymbol{ au}}$ transitions. This work is highly relevant for understanding superconductivity and mesoscale physics, offering new insights into the behavior of hybrid materials with competing magnetic and superconducting orders.
Degrees of Universality in Wave Turbulence - Jiasheng Liu, Vladimir Rosenhaus, and Gregory Falkovich explore wave turbulence, focusing on its universal properties and the transition from weak to strong turbulence. They contrast the turbulence of spin waves in ferromagnets with turbulent cascades in the Nonlinear Schrödinger Equation (NSE) and a related model. A key finding is that vertex renormalization leads to pumping-scale dependence in spin-wave turbulence, a feature absent in the NSE and MMT-like models. This spectral nonlocality in spin-wave turbulence causes it to deviate from weak turbulence even when wave interactions are weak, a phenomenon they term
