arXiv subjects

Zdenek Sofer

Publications and source records attributed to Zdenek Sofer.

At least 55 records · Page 3Linked to original sources

arXiv2025

Magnetic Correlation Spectroscopy in CrSBr

CrSBr is an air-stable magnetic van der Waals semiconductor with strong magnetic anisotropy, where the interaction of excitons with the magnetic order enables the optical identification of different magnetic phases. Here, we study the magnetic anisotropy of multi-layer CrSBr inside a three-axis vector magnet and correlate magnetic order and optical transitions in emission and absorption. We identify layer by layer switching of the magnetization through drastic changes of the optical emission and absorption energy and strength as a function of the applied magnetic field. We correlate optical transitions in reflection spectra with photoluminescence (PL) emission using a transfer-matrix analysis and find that ferromagnetic and antiferromagnetic order between layers can coexist in the same crystal. In the multi-peak PL emission the intensity of energetically lower lying transitions reduces monotonously with increasing field strength whereas energetically higher lying transitions around the bright exciton $X_B$ brighten close to the saturation field. Using this contrasting behavior we can therefore correlate transitions with each other.

cond-mat.mtrl-sci↗

arXiv2025

Electric-Field Control of Magnetic Skyrmion Chirality in a Centrosymmetric 2D van der Waals Magnet

Two-dimensional van der Waals magnets hosting topological magnetic textures, such as skyrmions, show promise for applications in spintronics and quantum computing. Electrical control of these topological spin textures would enable novel devices with enhanced performance and functionality. Here, using electron microscopy combined with in situ electric and magnetic biasing, we show that the skyrmion chirality, whether left-handed or right-handed, in insulating Cr2Ge2Te6, is controlled by external electric field direction applied during magnetic field cooling process. The electric-field-tuned chirality remains stable, even amid variations in magnetic and electric fields. Our theoretical investigation reveals that nonzero Dzyaloshinskii-Moriya interactions between the nearest neighbors, induced by the external electric field, change their sign upon reversing the electric field direction, thereby facilitating chirality selection. The electrical control of magnetic chirality demonstrated in this study can be extended to other non-metallic centrosymmetric skyrmion-hosting magnets, opening avenues for future device designs in topological spintronics and quantum computing.

cond-mat.mes-hall↗

arXiv2025

Electric-field switchable chirality in rhombohedral graphene Chern insulators stabilized by tungsten diselenide

Chern insulators host topologically protected chiral edge currents with quantized conductance characterized by their Chern number. Switching the chirality of a Chern insulator, namely, the direction of the edge current, is highly challenging due to topologically forbidden backscattering but is of considerable importance for the design of topological devices. Nevertheless, this can be achieved by reversing the sign of the Chern number through a topological phase transition. Here, we report electrically switchable chirality in rhombohedral multilayer graphene-based Chern insulators. By introducing moire superlattices in rhombohedral heptalayer graphene, we observed a cascade of topological phase transitions at quarter electron filling of a moire band with the Chern number tunable from -1, 1 to 2. Furthermore, integrating monolayer tungsten diselenide at the moireless interface of rhombohedral decalayer graphene/h-BN superlattices stabilizes the Chern insulators, enabling quantized anomalous Hall resistance of h/2e^2. Remarkably, the Chern number can be switched from -1 to 2 using displacement fields. Our work establishes rhombohedral multilayer graphene moire superlattices as a versatile platform for topological engineering, with switchable chirality offering significant promise for integrating chiral edge currents into topological electronic circuits.

cond-mat.mes-hall↗

arXiv2024

Gate-tunable spin Hall effect in trilayer graphene/group-IV monochalcogenide van der Waals heterostructures

Spintronic devices require materials that facilitate effective spin transport, generation, and detection. In this regard, graphene emerges as an ideal candidate for long-distance spin transport owing to its minimal spin-orbit coupling, which, however, limits its capacity for effective spin manipulation. This problem can be overcome by putting spin-orbit coupling materials in close contact to graphene leading to spin-orbit proximity and, consequently, efficient spin-to-charge conversion through mechanisms such as the spin Hall effect. Here, we report and quantify the gate-dependent spin Hall effect in trilayer graphene proximitized with tin sulfide (SnS), a group-IV monochalcogenide which has recently been predicted to be a viable alternative to transition-metal dichalcogenides for inducing strong spin-orbit coupling in graphene. The spin Hall angle exhibits a maximum around the charge neutrality point of graphene up to room temperature. Our findings expand the library of materials that induce spin-orbit coupling in graphene to a new class, group-IV monochalcogenides, thereby highlighting the potential of two-dimensional materials to pave the way for the development of innovative spin-based devices and future technological applications.

cond-mat.mes-hall↗

arXiv2024

Tunable Nanostructuring for van der Waals Materials

Van der Waals (vdW) materials are becoming increasingly popular in scientific and industrial applications because of their unique mixture of record electronic, optical, and mechanical properties. However, nanostructuring of vdW materials is still in its infancy and strongly depends on the specific vdW crystal. As a result, the universal self-assembled technology of vdW materials nanostructuring opens vast technological prospects. This work demonstrates an express and universal synthesis method of vdW nanoparticles with well-defined geometry using femtosecond laser ablation and fragmentation. The disarming simplicity of the technique allows us to create nanoparticles from over 50 vdW precursor materials covering transition metal chalcogenides, MXenes, and other vdW materials. Obtained nanoparticles manifest perfectly defined crystalline structures and diverse shapes, from nanospheres to nanocubes and nanotetrahedrons. Thus, our work provides a new paradigm for vdW nanostructuring with a vast potential of tunability for size, shape, and materials specific to the particular application.

physics.app-ph↗

arXiv2024

Raman Polarization Switching in CrSBr

Semiconducting CrSBr is a layered A-type antiferromagnet, with individual layers antiferromagnetically coupled along the stacking direction. Due to its unique orthorhombic crystal structure, CrSBr exhibits highly anisotropic mechanical and optoelectronic properties acting itself as a quasi-1D material. CrSBr demonstrates complex coupling phenomena involving phonons, excitons, magnons, and polaritons. Here we show through polarization-resolved resonant Raman scattering the intricate interaction between the vibrational and electronic properties of CrSBr. For samples spanning from few-layer to bulk thickness, we observe that the polarization of the A$_g^2$ Raman mode can be rotated by 90 degrees, shifting from alignment with the crystallographic a (intermediate magnetic) axis to the b (easy magnetic) axis, depending on the excitation energy. In contrast, the A$_g^1$ and A$_g^3$ modes consistently remain polarized along the b axis, regardless of the laser energy used. We access real and imaginary parts of the Raman tensor in our analysis, uncovering resonant electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2024

Using Electrical Impedance Spectroscopy to Separately Quantify the Effect of Strain on Nanosheet and Junction Resistance in Printed Nanosheet Networks

Many printed electronic applications require strain-independent electrical properties to ensure deformation-independent performance. Thus, developing printed, flexible devices using 2D and other nanomaterials will require an understanding of the effect of strain on the electrical properties of nano-networks. Here we introduce novel AC electrical techniques to fully characterise the effect of strain on the resistance of high mobility printed networks, fabricated from of electrochemically exfoliated MoS2 nanosheets. These devices were initially characterised using DC piezoresistance measurements and showed good cyclability and a linear strain response, consistent with a low gauge factor of G~3. However, AC impedance spectroscopy measurements, performed as a function of strain, allowed the measurement of the effects of strain on both the nanosheets and the inter-nanosheet junctions separately. The junction resistance was found to increase linearly with strain, while the nanosheet resistance remained constant. This response is consistent with strain-induced sliding of the highly-aligned nanosheets past one another, without any strain being transferred to the sheets themselves. Our approach allows us to individually estimate the contributions of dimensional factors (G~1.4) and intrinsic factors (G~1.9) to the total gauge factor. This novel technique may provide insight into other piezoresistive systems.

cond-mat.mes-hall↗

arXiv2024

Production of Ultra-Thin and High-Quality Nanosheet Networks via Layer-by-Layer Assembly at Liquid-Liquid Interfaces

Solution-processable 2D materials are promising candidates for a range of printed electronics applications. Yet maximising their potential requires solution-phase processing of nanosheets into high-quality networks with carrier mobility (μNet) as close as possible to that of individual nanosheets (μNS). In practise, the presence of inter-nanosheet junctions generally limits electronic conduction, such that the ratio of junction resistance (RJ) to nanosheet resistance (RNS), determines the network mobility via . Hence, achieving RJ/RNS<1 is a crucial step for implementation of 2D materials in printed electronics applications. In this work, we utilise an advanced liquid-interface deposition process to maximise nanosheet alignment and network uniformity, thus reducing RJ. We demonstrate the approach using graphene and MoS2 as model materials, achieving low RJ/RNS values of 0.5 and 0.2, respectively. The resultant graphene networks show a high conductivity of σNet = 5 \times 104 S/m while our semiconducting MoS2 networks demonstrate record mobility of μNet = 30 cm2/Vs, both at extremely low network thickness (tNet <10 nm). Finally, we show that the deposition process is compatible with non-layered quasi-2D materials such as silver nanosheets (AgNS), achieving network conductivity close to bulk silver for networks <100 nm thick. We believe this work is the first to report nanosheet networks with RJ/RNS<1 and serves to guide future work in 2D materials-based printed electronics.

cond-mat.mes-hall↗

arXiv2024

Magneto-optics in a van der Waals magnet tuned by self-hybridized polaritons

Controlling quantum materials with light is of fundamental and technological importance. By utilizing the strong coupling of light and matter in optical cavities (1-3), recent studies were able to modify some of their most defining features (4-6). In this work, we study the magneto-optical properties of a van der Waals magnet that supports strong coupling of photons and excitons even in the absence of external cavity mirrors. In this material - the layered magnetic semiconductor CrSBr - emergent light-matter hybrids called polaritons are shown to significantly increase the spectral bandwidth of correlations between the magnetic, electronic, and optical properties, enabling largely tunable optical responses to applied magnetic fields and magnons. Our results highlight the importance of exciton-photon self-hybridization in van der Waals magnets and motivate novel directions for the manipulation of quantum material properties by strong light-matter coupling.

cond-mat.mes-hall↗

arXiv2024

Room-temperature Ferroelectric Control of 2D Layered Magnetism

Electrical tuning of magnetism is crucial for developing fast, compact, ultra-low power electronic devices. Multiferroics offer significant potential due to their ability to control magnetic via an electric field through magnetoelectric coupling, especially in layered ferroelectric/ferromagnet heterostructures. A key challenge is achieving reversible and stable switching between distinct magnetic states using a voltage control. In this work, we present ferroelectric tuning of room-temperature magnetism in a 2D layered ferromagnet. The energy-efficient control consumes less than 1 fJ per operation which is normally in the order of several aJ, resulting in a ~43% change in magnetization. This tunable multiferroic interface and associated devices provide promising opportunities for next-generation reconfigurable communication systems, spintronics, sensors and memories.

cond-mat.mes-hall↗

arXiv2024

Doping-control of excitons and magnetism in few-layer CrSBr

Magnetism in two-dimensional materials reveals phenomena distinct from bulk magnetic crystals, with sensitivity to charge doping and electric fields in monolayer and bilayer van der Waals magnet CrI3. Within the class of layered magnets, semiconducting CrSBr stands out by featuring stability under ambient conditions, correlating excitons with magnetic order and thus providing strong magnon-exciton coupling, and exhibiting peculiar magneto-optics of exciton-polaritons. Here, we demonstrate that both exciton and magnetic transitions in bilayer and trilayer CrSBr are sensitive to voltage-controlled field-effect charging, exhibiting bound exciton-charge complexes and doping-induced metamagnetic transitions. Moreover, we demonstrate how these unique properties enable optical probes of local magnetic order, visualizing magnetic domains of competing phases across metamagnetic transitions induced by magnetic field or electrostatic doping. Our work identifies few-layer CrSBr as a rich platform for exploring collaborative effects of charge, optical excitations, and magnetism.

cond-mat.mes-hall↗

arXiv2024

Probing the band splitting near the $Γ$ point in the van der Waals magnetic semiconductor CrSBr

This study investigates the electronic band structure of Chromium Sulfur Bromide (CrSBr) through comprehensive photoluminescence (PL) characterization. We clearly identify low-temperature optical transitions between two closely adjacent conduction-band states and two different valence-band states. The analysis of the PL data robustly unveils energy splittings, bandgaps and excitonic transitions across different thicknesses of CrSBr, ranging from monolayer to bulk. Temperature-dependent PL measurements elucidate the stability of the band splitting below the Néel temperature, suggesting that magnons coupled with excitons are responsible for the symmetry breaking and brightening of the transitions from the secondary conduction band minimum (CBM2) to the global valence band maximum (VBM1). Collectively, these results not only reveal band splitting in both the conduction and valence bands, but also point to an intricate interplay between the optical, electronic and magnetic properties of antiferromagnetic two-dimensional van der Waals crystals.

cond-mat.mes-hall↗

arXiv2024

Strong Exciton-Phonon Coupling as a Fingerprint of Magnetic Ordering in van der Waals Layered CrSBr

The layered, air-stable van der Waals antiferromagnetic compound CrSBr exhibits pronounced coupling between its optical, electronic, and magnetic properties. As an example, exciton dynamics can be significantly influenced by lattice vibrations through exciton-phonon coupling. Using low-temperature photoluminescence spectroscopy, we demonstrate the effective coupling between excitons and phonons in nanometer-thick CrSBr. By careful analysis, we identify that the satellite peaks predominantly arise from the interaction between the exciton and an optical phonon with a frequency of 118 cm-1 (~14.6 meV) due to the out-of-plane vibration of Br atoms. Power-dependent and temperature-dependent photoluminescence measurements support exciton-phonon coupling and indicate a coupling between magnetic and optical properties, suggesting the possibility of carrier localization in the material. The presence of strong coupling between the exciton and the lattice may have important implications for the design of light-matter interactions in magnetic semiconductors and provides new insights into the exciton dynamics in CrSBr. This highlights the potential for exploiting exciton-phonon coupling to control the optical properties of layered antiferromagnetic materials.

cond-mat.mtrl-sci↗

arXiv2023

Quantifying the contribution of material and junction resistances in nano-networks

Networks of nanowires and nanosheets are important for many applications in printed electronics. However, the network conductivity and mobility are usually limited by the inter-particle junction resistance, a property that is challenging to minimise because it is difficult to measure. Here, we develop a simple model for conduction in networks of 1D or 2D nanomaterials, which allows us to extract junction and nanoparticle resistances from particle-size-dependent D.C. resistivity data of conducting and semiconducting materials. We find junction resistances in porous networks to scale with nanoparticle resistivity and vary from 5 Ohm for silver nanosheets to 25 GOhm for WS2 nanosheets. Moreover, our model allows junction and nanoparticle resistances to be extracted from A.C. impedance spectra of semiconducting networks. Impedance data links the high mobility (~7 cm2/Vs) of aligned networks of electrochemically exfoliated MoS2 nanosheets to low junction resistances of ~670 kOhm. Temperature-dependent impedance measurements allow us to quantitatively differentiate intra-nanosheet phonon-limited band-like transport from inter-nanosheet hopping for the first time.

cond-mat.mes-hall↗

arXiv2023

Ferromagnetic interlayer coupling in CrSBr crystals irradiated by ions

Layered magnetic materials are becoming a major platform for future spin-based applications. Particularly the air-stable van der Waals compound CrSBr is attracting considerable interest due to its prominent magneto-transport and magneto-optical properties. In this work, we observe a transition from antiferromagnetic to ferromagnetic behavior in CrSBr crystals exposed to high-energy, non-magnetic ions. Already at moderate fluences, ion irradiation induces a remanent magnetization with hysteresis adapting to the easy-axis anisotropy of the pristine magnetic order up to a critical temperature of 110 K. Structure analysis of the irradiated crystals in conjunction with density functional theory calculations suggest that the displacement of constituent atoms due to collisions with ions and the formation of interstitials favors ferromagnetic order between the layers.

cond-mat.mtrl-sci↗

arXiv2023

Intrinsic magnetic properties of the layered antiferromagnet CrSBr

Van der Waals magnetic materials are an ideal platform to study low-dimensional magnetism. Opposed to other members of this family, the magnetic semiconductor CrSBr is highly resistant to degradation in air, which, besides its exceptional optical, electronic, and magnetic properties, is the reason the compound is receiving considerable attention at the moment. For many years, its magnetic phase diagram seemed to be well-understood. Recently, however, several groups observed a magnetic transition in magnetometry measurements at temperatures of around 40 K that is not expected from theoretical considerations, causing a debate about the intrinsic magnetic properties of the material. In this letter, we report the absence of this particular transition in magnetization measurements conducted on high-quality CrSBr crystals, attesting to the extrinsic nature of the low-temperature magnetic phase observed in other works. Our magnetometry results obtained from large bulk crystals are in very good agreement with the magnetic phase diagram of CrSBr previously predicted by the mean-field theory; A-type antiferromagnetic order is the only phase observed below the Néel temperature at TN = 131 K. Moreover, numerical fits based on the Curie-Weiss law confirm that strong ferromagnetic correlations are present within individual layers even at temperatures much larger than TN.

cond-mat.mtrl-sci↗

arXiv2023

In-plane magnetocrystalline anisotropy in the van der Waals antiferromagnet FePSe$_3$ probed by magneto-Raman scattering

Magnon gap excitations selectively coupled to phonon modes have been studied in FePSe$_3$ layered antiferromagnet with magneto-Raman scattering experiments performed at different temperatures. The bare magnon excitation in this material has been found to be split (by $\approx~1.2$ cm$^{-1}$) into two components each being selectively coupled to one of the two degenerated, nearby phonon modes. Lifting the degeneracy of the fundamental magnon mode points out toward the biaxial character of the FePS$_3$ antiferromagnet, with an additional in-plane anisotropy complementing much stronger, out-of-plane anisotropy. Moreover, the tunability, with temperature, of the phonon- versus the magnon-like character of the observed coupled modes has been demonstrated.

cond-mat.mtrl-sci↗

arXiv2023

Charge transfer-induced Lifshitz transition and magnetic symmetry breaking in ultrathin CrSBr crystals

Ultrathin CrSBr flakes are exfoliated \emph{in situ} on Au(111) and Ag(111) and their electronic structure is studied by angle-resolved photoemission spectroscopy. The thin flakes' electronic properties are drastically different from those of the bulk material and also substrate-dependent. For both substrates, a strong charge transfer to the flakes is observed, partly populating the conduction band and giving rise to a highly anisotropic Fermi contour with an Ohmic contact to the substrate. The fundamental CrSBr band gap is strongly renormalized compared to the bulk. The charge transfer to the CrSBr flake is substantially larger for Ag(111) than for Au(111), but a rigid energy shift of the chemical potential is insufficient to describe the observed band structure modifications. In particular, the Fermi contour shows a Lifshitz transition, the fundamental band gap undergoes a transition from direct on Au(111) to indirect on Ag(111) and a doping-induced symmetry breaking between the intra-layer Cr magnetic moments further modifies the band structure. Electronic structure calculations can account for non-rigid Lifshitz-type band structure changes in thin CrSBr as a function of doping and strain. In contrast to undoped bulk band structure calculations that require self-consistent $GW$ theory, the doped thin film properties are well-approximated by density functional theory if local Coulomb interactions are taken into account on the mean-field level and the charge transfer is considered.

cond-mat.str-el↗