arXiv subjects

Bogdan J. Kowalski

Publications and source records attributed to Bogdan J. Kowalski.

6 recordsLinked to original sources

arXiv2026

Electronic structure of Sn(1-x)Mn(x)Te semiconducting solid solution: a resonant photoemission study

Manganese-doped tin telluride, Sn(1-x)Mn(x)Te, initially investigated as diluted magnetic semiconductor, has recently attracted considerable attention as a prospective thermoelectric material. The introduction of Mn was found to modify the valence band electronic structure, resulting in an improvement in the Seebeck coefficient and thus, the figure of merit (ZT). In the paper, we present a synchrotron radiation study of the electronic band structure of Sn(0.9)Mn(0.1)Te by resonant photoemission. The contribution of the Mn3d electrons to the valence band (VB), calculated as the difference between the Energy Distribution Curves (EDCs) taken at the maximum and minimum of the Fano resonance for the Mn3p - Mn3d absorption threshold, shows a contribution at the VB edge, a dominant maximum at 4 eV, as well as a wide structure between 7 and 11 eV. Moreover, comparison with undoped SnTe reveals strong renormalization of the Sn5p and Te5p electronic states, which certainly influences the shape of the upper part of the valence band and electron effective mass.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic Frustration Enforced Electronic Reconstruction in Ni intercalated NbSe$_{2}$: Suppression of Electronic Orders

We investigate the single crystals of Ni$_{0.19}$NbSe$_2$, revealing that Ni intercalation profoundly alters the physical properties of NbSe$_2$. Magnetic measurements clearly show that the system is magnetically frustrated with antiferromagnetic ordering below 23.5\,K, with an irreversibility temperature near 10\,K, and a magnetic hysteresis with a small net magnetic moment. Overall, the system can be described as an inhomogeneous antiferromagnetic phase with magnetic disorder and magnetic frustration. We found two Curie-Weiss temperatures of -80\,K for the field in the {\it ab}-plane and -137\,K for the field out of plane, which are a consequence of anisotropic interactions in spin space and favor an orientation of the spin along the {\it c}-axis. Temperature-dependent resistivity shows a complete suppression of both charge density waves and superconducting order down to 300\,mK. Angle-resolved photoemission spectroscopy at 84\,K reveals a $\overlineΓ$-centered electron pocket in Ni$_{0.19}$NbSe$_2$, which is absent in pristine NbSe$_2$. The electronic structure results show a shift of the van Hove singularity (VHS), which is the main cause of the suppression of the electronic orders. These results align with recent theoretical predictions that Ni intercalation with cationic disorder favors frustrated antiferromagnetic stripe states, shifts the VHS and reconstructs the Fermi surface in NbSe$_2$. Our findings position Ni$_{0.19}$NbSe$_2$ within a magnetically frustrated, non-superconducting regime, highlighting how partial intercalation and disorder drive complex magnetic order and the Fermi surface reconstruction in low-dimensional quantum materials.

cond-mat.mtrl-sci↗

arXiv2024

Revealing Polytypism in 2D Boron Nitride with UV Photoluminescence

Boron nitride exhibits diverse crystal structures, predominantly a layered arrangement with strong intraplanar covalent bonds and weak interplanar van der Waals bonds. While commonly referred to as hexagonal BN (hBN), the sp$^2$-bonded BN atomic planes can also arrange in other configurations like Bernal (bBN) or rhombohedral (rBN) stacking orders. Variations in the orientation and translation of successive atomic layers lead to changes in crystal symmetry, potentially resulting in piezoelectric, pyroelectric or ferroelectric effects. However, distinguishing between different polytypes using conventional methods like X-ray diffraction or Raman spectroscopy presents a significant challenge. In this work, we demonstrate that the optical response of the 4.1 eV defect can serve as an indicator of the polytype. To this end, we study BN samples grown by metalorganic vapor phase epitaxy (MOVPE), which contain different polytypes. The identification of the polytypes was achieved by X-ray diffraction and transmission electron microscopy. Photoluminescence and cathodoluminescence measurements with a high spatial resolution allowed for the deconvolution of the signal into two components from which we can extract a zero-phonon line (ZPL) at 4.096 eV (302.6 nm) for hBN and 4.143 eV (299.2 nm) for rBN. We performed calculations that enable us to identify the defect as a carbon dimer CBCN (C2) and show that the ZPL shift reflects differences in the crystal environment for different polytypes. Furthermore, we demonstrate that different polytypic composition ratios of hBN and rBN can be achieved by MOVPE, which could pave the way for future applications in large-area van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Systemic Consequences of Disorder in Magnetically Self-Organized Topological MnBi$_{2}$Te$_{4}/$(Bi$_{2}$Te$_{3}$)$_{n}$ Superlattices

MnBi$_{2}$Te$_{4}/$(Bi$_{2}$Te$_{3}$)$_{n}$ materials system has recently generated strong interest as a natural platform for realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic MnBi$_{2}$Te$_{4}/$(Bi$_{2}$Te$_{3}$)$_{n}$ it is found that migration of Mn between MnBi$_{2}$T$e_{4}$ septuple layers (SLs) and otherwise non-magnetic Bi$_{2}$Te$_{3}$ quintuple layers (QLs) has systemic consequences - it induces ferromagnetic coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL separation (n $\gtrsim$ 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and density functional theory studies show that Mn disorder within an SL causes delocalization of electron wavefunctions and a change of the surface bandstructure as compared to the ideal MnBi$_{2}$Te$_{4}/$(Bi$_{2}$Te$_{3}$)$_{n}$. These findings highlight the critical importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topological magnets.

cond-mat.mtrl-sci↗

arXiv2021

Near-infrared emission from spatially indirect excitons in type II ZnTe/CdSe/(Zn,Mg)Te core/double-shell nanowires

ZnTe/CdSe/(Zn,Mg)Te core/double-shell nanowires are grown by molecular beam epitaxy by employing the vapor-liquid-solid growth mechanism assisted with gold catalysts. A photoluminescence study of these structures reveals the presence of an optical emission in the near infrared. We assign this emission to the spatially indirect exciton recombination at the ZnTe/CdSe type II interface. This conclusion is confirmed by the observation of a significant blue-shift of the emission energy with an increasing excitation fluence induced by the electron-hole separation at the interface. Cathodoluminescence measurements reveal that the optical emission in the near infrared originates from nanowires and not from two dimensional residual deposits between them. Moreover, it is demonstrated that the emission energy in the near infrared depends on the average CdSe shell thickness and the average Mg concentration within the (Zn,Mg)Te shell. The main mechanism responsible for these changes is associated with the strain induced by the (Zn,Mg)Te shell in the entire core/shell nanowire heterostructure.

cond-mat.mes-hall↗

arXiv2017

Fragility of the Dirac Cone Splitting in Topological Crystalline Insulator Heterostructures

The 'double Dirac cone' 2D topological interface states found on the (001) faces of topological crystalline insulators such as Pb$_{1-x}$Sn$_{x}$Se feature degeneracies located away from time reversal invariant momenta, and are a manifestation of both mirror symmetry protection and valley interactions. Similar shifted degeneracies in 1D interface states have been highlighted as a potential basis for a topological transistor, but realizing such a device will require a detailed understanding of the intervalley physics involved. In addition, the operation of this or similar devices outside of ultra-high vacuum will require encapsulation, and the consequences of this for the topological interface state must be understood. Here we address both topics for the case of 2D surface states using angle-resolved photoemission spectroscopy. We examine bulk Pb$_{1-x}$Sn$_{x}$Se(001) crystals overgrown with PbSe, realizing trivial/topological heterostructures. We demonstrate that the valley interaction that splits the two Dirac cones at each $\bar{X}$ is extremely sensitive to atomic-scale details of the surface, exhibiting non-monotonic changes as PbSe deposition proceeds. This includes an apparent total collapse of the splitting for sub-monolayer coverage, eliminating the Lifshitz transition. For a large overlayer thickness we observe quantized PbSe states, possibly reflecting a symmetry confinement mechanism at the buried topological interface.

cond-mat.mes-hall↗