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Zdenek Sofer

Publications and source records attributed to Zdenek Sofer.

At least 37 records · Page 2Linked to original sources
arXiv2026

Galvanic intercalation of molecular cations into van der Waals materials

The intercalation of molecular species between the layers of van der Waals (vdW) crystals is a powerful approach to combine the remarkable physical properties of vdW materials with the chemical versatility of organic molecules. However, the full transformative potential of molecular intercalation remains underexplored, largely due to the lack of simple, broadly applicable methods that preserve high crystalline quality down to the few-layer limit. Here, we introduce a simple galvanic approach to intercalate different molecules into various vdW materials under ambient conditions, leveraging the low reduction potential of selected metals. We employ our method, which is particularly well-suited for the in-situ intercalation of few-layer-thick crystals, to intercalate nine vdW materials, including magnets and superconductors, with molecules ranging from conventional alkylammonium ions to metallorganic and bio-inspired chiral cations. Notably, intercalation leads to an unprecedented transition from antiferromagnetic to ferrimagnetic ordering in α-RuCl3 and to a molecule-dependent enhancement of the superconducting transition in 2H-TaS2. These results establish our approach as a versatile technique for engineering atomically thin quantum materials and heterostructures, unlocking the transformative effects of molecular intercalation.

cond-mat.mtrl-sci↗
arXiv2026

Visible and Terahertz Nonlinear Responses in the Topological Noble Metal Dichalcogenide PdTe2

Nonlinear processes can offer pathways to next-generation sensors and frequency mixing devices to overcome modern imaging, detection, and communication challenges. In this article, we report on strong second and third-order nonlinear optical responses in visible and terahertz (THz) light in single crystals of the noble metal dichalcogenide PdTe$_2$. We find that buried conduction and valence topological surface states of PdTe$_2$ lead to resonant optical second-harmonic generation. On the other hand, although the nonlinear responses obtained with THz excitation are not close to this resonance, they can be clearly observed in reflection geometry, even in the presence of broadband excitation, where optical filters are not necessary to observe the enhanced odd-order higher harmonic output. By carefully considering the radiative photocurrent framework of stimulated THz emission, we are able to extract fingerprints of both second- and third-order processes in the THz regime, and show that PdTe$_2$ is a promising material candidate for radio frequency rectification, frequency mixing, and beam focusing.

cond-mat.mtrl-sci↗
arXiv2026

Large-scale Integration of Experimental and Computational Data for 2D Materials

The past decade has seen rapid growth in the number of experimentally realized two-dimensional (2D) materials with diverse chemical and physical properties. However, information on their crystal structure, synthesis routes, and measured or predicted properties, remains scattered across thousands of publications. Here we consolidate this fragmented knowledge by establishing X2DB - an open infrastructure that integrates experimental and computational data on 2D materials. Using extensive literature mining and direct community uploads, we identify 370 unique 2D materials that have been realized in monolayer or few-layer form, and link them to their digital counterparts in computational databases, enabling consistent ab initio characterization of their properties across monolayer, bilayer and bulk forms. We describe the structure and content of the database highlighting its support for community uploads, illustrate how it can be used to generate new scientific insight and introduce a hierarchical classification of the known set of 2D materials. Our work provides a foundation for the integration and cross-fertilization of experimental and theoretical knowledge, opening new avenues for data-driven, predictive synthesis of novel 2D materials.

cond-mat.mtrl-sci↗
arXiv2026

A Sliding Ferroelectric Resonant Tunnel Junction

Ferroelectric tunnel junctions (FTJs) leverage polarization-dependent tunneling through ultrathin barriers to enable two-terminal, non-volatile memory and logic. Although conceptually appealing, the practical implementation of conventional FTJs has been hindered by high coercive voltages, low readout currents, limited cycling endurance, and significant device-to-device variability. Here, we overcome these bottlenecks by introducing the sliding ferroelectric resonant tunnel (SFeRT) junction, integrating three cooperative mechanisms: (i) spontaneous interfacial polarization of atomically thin, depolarization-resilient barriers; (ii) superlubric sliding of shear-solitons, enabling ultra-low-friction, wear-free switching; and (iii) momentum-conserving, elastic resonant tunneling between lattice-aligned graphitic electrodes, providing sensitive readouts at both positive and negative biases. We demonstrate nanometer-scale SFeRT junctions using polar polytypes of hexagonal boron nitride (hBN) or transition metal dichalcogenides (TMDs) as barriers, achieving configurable writing voltages below $0.5$ V and tunable reading biases under $0.1$ V. These devices yield current densities exceeding $50$ nA $μ$m$^{-2}$, with a robust room-temperature ON/OFF ratio $> 7$. The crystalline and polarization integrity of sliding van der Waals (vdW) polytypes, down to the atomically thin limit, ensures exceptional device uniformity and performance that remains scalable down to sub-$0.1$ $μ$m$^{2}$ footprints. Furthermore, we provide a predictive model for SFeRT performance across diverse doping levels, temperatures, electrodes, and polytype configurations. Integrated within a Superlubric Array of Polytypes (SLAP) architecture, SFeRT junctions enable switching energies below $1$ fJ, establishing a scalable and durable foundation for low-energy ``slidetronic'' logic and memory.

cond-mat.other↗
arXiv2026

Atomic imaging of 2D transition metal dihalides

Transition metal di-iodides such as FeI2, NiI2 and CoI2 are an emerging class of 2D magnets exhibiting rich and diverse magnetic behaviour, but their study at the monolayer limit has been severely hindered by fabrication challenges due to their air-sensitivity. Here, we introduce a polymer-free method for clean, rapid, and high-yield assembly of hermetically encapsulated suspended samples of air-sensitive monolayers. Applying it to di-iodides enables atomic resolution characterisation of thin samples - down to the monolayer limit - for the first time. Our imaging, combined with complementary first-principles calculations, reveals an unusually small energy barrier between alternate stable stacking polytypes in few-layer films, enabling extrinsic control of the stacking phase. We also observe stable isolated iodine vacancies that do not aggregate to form extended structures, and identify and verify the stability of the various edge configurations of thin samples. These results establish the unique structural characteristics of these materials in the thin limit, and more broadly demonstrate the utility of our transfer platform for creating atomically clean suspended vdW heterostructures.

cond-mat.mtrl-sci↗
arXiv2026

Stabilizing van der Waals NbOI2 by SiO2 encapsulation for Photonic Applications

Niobium oxide diiodide (NbOI2) is an emerging material for photonics and electronics, distinguished by its exceptional second-order nonlinearity and pronounced in-plane ferroelectricity, both originating from its highly anisotropic ABC-stacked crystal structure. Its broken inversion symmetry enables its optical nonlinear efficiency to scale with thickness, making multilayer NbOI2 highly promising for nonlinear frequency conversion like second harmonic generation or and spontaneous parametric down-conversion in bulk or waveguides. However, under ambient conditions NbOI2 degrades into an amorphous oxide within weeks, severely diminishing its nonlinear response. To overcome this, we investigate SiO2 encapsulation via physical vapor deposition to protect NbOI2 multilayers from environmental degradation. Our systematic study reveals that encapsulation preserves structural integrity and nonlinear optical performance, establishing NbOI2 as a stable candidate for heterogeneous integration in foundry-compatible photonic platforms and quantum technologies.

physics.optics↗
arXiv2026

Van der Waals CrSBr alloys with tunable magnetic and optical properties

Van der Waals magnets are attracting a lot of attention for their potential integration in spintronic and magnonic technologies. CrSBr is an A-type antiferromagnet that shows a coupling between its electronic band structure and magnetic properties. This property is appealing for applications and it also offers the possibility to investigate magnetic ground states and GHz magnons using visible optics techniques. Using Raman scattering and (magneto)-optical experiments, we describe the magnetic and optical properties of alloys of CrSBr$_{(1-x)}$Cl$_x$ with $x<0.46$. Similar to CrSBr, these alloys are direct band gap semiconductors with a coupling of their electronic and magnetic properties. Exciton energies evolve weakly with composition and we describe the large changes in the saturation magnetic fields and their implications on the magnetic properties. We show that both the interlayer magnetic exchange and electronic interactions are modified by the halogen mixing, offering the possibility to tune magnon energies with alloy composition.

cond-mat.mtrl-sci↗
arXiv2025

Wafer-Scale Films of Two-Dimensional Materials via Roll-to-Roll Mechanical Exfoliation

In this study, we demonstrate an improved version of the roll-to-roll mechanical exfoliation method, incorporating a controlled sliding motion into the exfoliation process to achieve uniform nanosheet films of two-dimensional materials at wafer-scale. This scalable technique enables the fabrication of high-quality films suitable for electronic and optoelectronic applications. We validate the process by fabricating WSe2 phototransistors directly on the exfoliated films, achieving performance metrics comparable to the best reported devices based on electrochemically exfoliated material. The all-dry transfer method employed ensures minimal contamination and preserves the intrinsic properties of the material. This work highlights the potential of high-throughput mechanical exfoliation as a cost-effective and reliable route for large-scale production of 2D material-based devices.

cond-mat.mtrl-sci↗
arXiv2025

Tunable magnetism in 2D organic-ion-intercalated MnPS3 via molecule-dependent vacancy generation

The magnetic properties of van der Waals materials are profoundly influenced by structural defects. The layered antiferromagnet MnPS3 offers a unique opportunity to explore defect-related magnetism, as Mn2+ vacancies can be generated by the intercalation of specific guest molecules. However, the effectiveness of this process in atomically thin flakes and the extent of the magnetic tunability remain unclear. Here, we show that the magnetic properties of MnPS3 can be tailored through the intercalation of different guest molecules. Notably, the insertion of four alkylammonium ions introduces different populations of Mn2+ vacancies, leading to a transition from the pristine antiferromagnetic state to more complex magnetic textures, including a ferrimagnetic state displaying a magnetic saturation of 1 uB/atom. Moreover, we show that the intercalation of few-nm-thick flakes also leads to the emergence of a ferrimagnetic response. This in-flake intercalation, which can be monitored in real time using optical microscopy, can be interrupted before completion, generating lateral heterostructures between pristine and intercalated areas. This approach opens the way to the use of partial intercalation to define regions with distinct magnetic properties within a single flake.

cond-mat.mtrl-sci↗
arXiv2025

Twist-tuned exchange and hysteresis in a bilayer van der Waals magnet

Moiré superlattices in twisted bilayers enable profound reconstructions of the electronic bandstructure, giving rise to correlated states with remarkable tunability. Extending this paradigm to van der Waals magnets, twisting creates spatially varying interlayer exchange interactions that stabilize emergent spin textures and the coexistence of ferromagnetic and antiferromagnetic domains. Here, we demonstrate the emergence of robust magnetic hysteresis in bilayer CrSBr upon twisting by an angle of ~ 3°. This is observed as the corresponding hysteretic evolution of the exciton energy, that directly correlates with the bilayer magnetic state, in magnetic field dependent photoluminescence measurements. A two-sublattice model captures this behavior, attributing it to the twist-induced reduction of interlayer exchange that stabilizes both parallel and antiparallel spin configurations across a broad field range. Comparison with experiment enables quantitative extraction of the effective exchange strength. Remarkably, the system exhibits coherent averaging across the moiré supercell, yielding an effective monodomain response characterized by switching into the antiferromagnetic state, rather than forming spin textures or fragmented domains. Spatially resolved measurements further uncover local variations in hysteresis loops, consistent with position-dependent modulation of the average exchange interaction. Our results establish twist engineering as a powerful route to programmable magnetic memories in two-dimensional magnets, harnessing the robustness of antiferromagnetic order.

cond-mat.mtrl-sci↗
arXiv2025

Giant optical anisotropy in CrSBr from giant exciton oscillator strength

The interplay between dimensionality and electronic correlations in van der Waals (vdW) materials offers a powerful toolkit for engineering light-matter interactions at the nanoscale. Excitons, bound electron-hole pairs, are central to this endeavor, yet maximizing their oscillator strength, which dictates the interaction cross-section, remains a challenge. Conventional wisdom suggests a trade-off, where the observable oscillator strength often decreases in strongly bound systems due to population dynamics. Here, we unveil a colossal oscillator strength associated with the quasi-one-dimensional (quasi-1D) excitons in the layered magnetic semiconductor CrSBr, which fundamentally defies this established scaling law. Through comprehensive optical characterization and ab initio calculations, we establish that this anomalous enhancement originates directly from the reduced dimensionality, which enforces an increased electron-hole wavefunction overlap. Moreover, we find a close connection between fundamental exciton and local spin fluctuations that contribute to the opening of the gap in the electronic spectrum. The resulting optical anisotropy shows a giant in-plane birefringence (Delta_n = 1.45) and profoundly anisotropic waveguiding, which we directly visualize using nano-optical imaging. Leveraging this extreme response, we realize a true zero-order quarter-wave plate with an unprecedented wavelength-to-thickness ratio (lambda/t) exceeding 3.4, surpassing the limits of current miniaturization technologies, including state-of-the-art metasurfaces. Our findings underscore the profound impact of dimensionality engineering in magnetic vdW materials for realizing novel regimes of light-matter coupling and developing next-generation ultracompact photonic architectures.

cond-mat.mtrl-sci↗
arXiv2025

Interplay of vibrational, electronic, and magnetic states in CrSBr

The van der Waals antiferromagnet CrSBr exhibits coupling of vibrational, electronic, and magnetic degrees of freedom, giving rise to distinctive quasi-particle interactions. We investigate these interactions across a wide temperature range using polarization-resolved Raman spectroscopy at various excitation energies, complemented by optical absorption and photoluminescence excitation (PLE) spectroscopy. Under 1.96 eV excitation, we observe pronounced changes in the A$_g^1$, A$_g^2$, and A$_g^3$ Raman modes near the Néel temperature, coinciding with modifications in the oscillator strength of excitonic transitions and clear resonances in PLE. The distinct temperature evolution of Raman tensor elements and polarization anisotropy for Raman modes indicates that they couple to different excitonic and electronic states. The suppression of the excitonic state's oscillation strength above the Néel temperature could be related to the magnetic phase transition, thereby connecting these excitonic states and Raman modes to a specific spin alignment. These observations make CrSBr a versatile platform for probing quasi-particle interactions in low-dimensional magnets and provide insights for applications in quantum sensing and quantum communication.

cond-mat.mtrl-sci↗
arXiv2025

Spin light-emitting devices in a 2D magnet

Emerging two-dimensional (2D) magnetic semiconductors represent transformative platforms to explore magneto-optics and opto-spintronic applications. Though 2D opto-spintronics has attracted tremendous research efforts in spin-dependent photodetectors and non-volatile memory components, the realization of one core application - spin-modulated light-emitting device (spin-LED) - remains elusive so far. Here we successfully realize prototype spin-LED integrated with a 2D semiconducting magnet CrSBr, demonstrating considerable electroluminescence (EL) down to bilayers. Intriguingly, the EL of the spin-LED is discovered to be directly manipulated by spin-flip and spin-canting transitions. Notably, spin-flip transitions enable unprecedented hysteretic behaviors of EL characteristics, while spin-canting transitions induce EL continuous modulation with robust anisotropy. This versatile manipulation is originated from the synergy of magnetic-order mediated excitonic transitions and spintronic transport. The prototype demonstration of spin-LED establishes an indispensable scheme of opto-spintronic devices leveraging 2D spin transitions and strong excitonic effects, presenting a critical step towards integrated 2D opto-spintronics.

physics.app-ph↗
arXiv2025

Exciton transport driven by spin excitations in an antiferromagnet

A new class of optical quasiparticles called magnetic excitons recently emerged in magnetic van der Waals materials. Akin to the highly effective strategies developed for electrons, the strong interactions of these excitons with the spin degree of freedom may provide innovative solutions for long-standing challenges in optics, such as steering the flow of energy and information. Here, we demonstrate transport of excitons by spin excitations in the van der Waals antiferromagnetic semiconductor CrSBr. Key results of our study are the observations of ultrafast, nearly isotropic exciton propagation substantially enhanced at the Neel temperature, transient contraction and expansion of the exciton clouds at low temperatures, as well as superdiffusive behavior in bilayer samples. These signatures largely defy description by commonly known exciton transport mechanisms and are related to the currents of incoherent magnons induced by laser excitation instead. We propose that the drag forces exerted by these currents can effectively imprint characteristic properties of spin excitations onto the motion of excitons. The universal nature of the underlying exciton-magnon scattering promises driving of excitons by magnons in other magnetic semiconductors and even in non-magnetic materials by proximity in heterostructures, merging the rich physics of magneto-transport with optics and photonics.

cond-mat.mes-hall↗
arXiv2025

Defect complexes in CrSBr revealed through electron microscopy and deep learning

Atomic defects underpin the properties of van der Waals materials, and their understanding is essential for advancing quantum and energy technologies. Scanning transmission electron microscopy is a powerful tool for defect identification in atomically thin materials, and extending it to multilayer and beam-sensitive materials would accelerate their exploration. Here we establish a comprehensive defect library in a bilayer of the magnetic quasi-1D semiconductor CrSBr by combining atomic-resolution imaging, deep learning, and ab-initio calculations. We apply a custom-developed machine learning work flow to detect, classify and average point vacancy defects. This classification enables us to uncover several distinct Cr interstitial defect complexes, combined Cr and Br vacancy defect complexes and lines of vacancy defects that extend over many unit cells. We show that their occurrence is in agreement with our computed structures and binding energy densities, reflecting the intriguing layer interlocked crystal structure of CrSBr. Our ab-initio calculations show that the interstitial defect complexes give rise to highly localized electronic states. These states are of particular interest due to the reduced electronic dimensionality and magnetic properties of CrSBr and are furthermore predicted to be optically active. Our results broaden the scope of defect studies in challenging materials and reveal new defect types in bilayer CrSBr that can be extrapolated to the bulk and to over 20 materials belonging to the same FeOCl structural family.

cond-mat.mtrl-sci↗
arXiv2025

All-electrical near-field injection of excitons in a van der Waals antiferromagnet

Van der Waals materials have become a promising building block for future electronics and photonics. The two-dimensional magnet CrSBr came into the spotlight of solid state research due to its intriguing combination of antiferromagnetic order, strong light-matter coupling and unusual quasi-1D electronic bandstructure. This study reports the electrical excitation of excitons in CrSBr layers from cryogenic temperatures up to room temperature. By exploiting the energy transfer via tunneling electrons in a graphene tunnel junction strongly bound excitons are excited in proximate CrSBr layers. This facilitates electrically-excited emission from CrSBr crystals ranging in thickness from a bilayer up to 250 nm, in which the strong linear polarization of the electroluminescence confirms the excitonic origin. For thicker layers, clear evidence for the electrically excited emission from self-hybridized exciton polaritons is observed, highlighting the strong coupling between optical excitations and confined photon modes in CrSBr. These results pave the way for future applications in spintronic and optical readout of magnetic properties.

cond-mat.mes-hall↗
arXiv2025

Magnetically Modulated Electrical Switching in an Antiferromagnetic Transistor

A spin version of transistor, where magnetism is used to influence electrical behaviors of the semiconductor, has been a long-pursued device concept in spintronics. In this work, we experimentally study a field-effect transistor with CrSBr, a van der Waals (vdW) antiferromagnetic semiconductor, as the channel material. Unlike the weak magnetic tunability of in-plane currents previously reported in vdW magnets, the channel current of our transistor is efficiently tuned by both gate voltage and magnetic transitions, achieving a magnetoresistance ratio as high as 1500%. Combining measurement and theoretical modeling, we reveal magnetically modulated carrier concentration as the origin of the large magnetoresistance. The strategy of using both magnetic ordering and electric field in the same device to control ON/OFF states of a transistor opens a new avenue of energy-efficient spintronics for memory, logic and magnetic sensing applications.

cond-mat.mtrl-sci↗