arXiv subjects

M. Tas

Publications and source records attributed to M. Tas.

9 recordsLinked to original sources

arXiv2025

Itinerant versus localized magnetism in spin gapped metallic half-Heusler compounds: Stoner criterion and magnetic interactions

Spin gapped metals have recently emerged as promising candidates for spintronic and nanoelectronic applications, enabling functionalities such as sub-60mV/dec switching, negative differential resistance, and non-local spin-valve effects in field-effect transistors. Realizing these functionalities, however, requires a deeper understanding of their magnetic behavior, which is governed by a subtle interplay between localized and itinerant magnetism. This interplay is particularly complex in spin gapped metallic half-Heusler compounds, whose magnetic properties remain largely unexplored despite previous studies of their electronic structure. In this work, we systematically investigate the magnetic behavior of spin gapped metallic half-Heusler compounds XYZ (X = Fe, Co, Ni, Rh, Ir, Pd, Pt; Y = Ti, V, Zr, Hf, Nb, Ta; Z = In, Sn, Sb), revealing clear trends. Co- and Ni-based compounds predominantly exhibit itinerant magnetism, whereas Ti-, V-, and Fe-based systems may host localized moments, itinerant moments, or a coexistence of both. To uncover the origin of magnetism, we apply the Stoner model, with the Stoner parameter I estimated from Coulomb interaction parameters (Hubbard U and Hund's exchange J) computed using the constrained random phase approximation (cRPA). Our analysis shows that compounds not satisfying the Stoner criterion tend to remain non-magnetic. On the contrary compounds, which satisfy the Stoner criterion, generally exhibit magnetic ordering highlighting the crucial role of electronic correlations and band structure effects in the emergence of magnetism. For compounds with magnetic ground states, we compute Heisenberg exchange parameters, estimate Curie temperatures (T_C), and analyze spin-wave properties, including magnon dispersions and stiffness constants.

cond-mat.mtrl-sci↗

arXiv2025

Correlation effects in two-dimensional MX_2 and MA_2Z_4 (M= Nb, Ta; X= S, Se, Te; A=Si, Ge; Z=N, P) cold metals: Implications for device applications

Cold metals, characterized by their distinctive band structures, hold promise for innovative electronic devices such as tunnel diodes with negative differential resistance (NDR) effect and field-effect transistors (FETs) with sub-60 mV/dec subthreshold swing (SS). In this study, we employ the GW approximation and HSE06 hybrid functional to investigate the correlation effects on the electronic band structure of two-dimensional (2D) cold metallic materials, specifically focusing on MX_2 and MA_2Z_4 (M=Nb, Ta; X=S, Se, Te; A=Si, Ge; Z= N, P) compounds in 1H structure. These materials exhibit a unique band structure with an isolated metallic band around the Fermi energy, denoted as W_m, as well as two energy gaps: the internal gap E^I_g below the Fermi level and the external gap E^E_g above the Fermi level. These three electronic structure parameters play a decisive role in determining the current-voltage (I-V) characteristics of tunnel diodes, the nature of the NDR effect, and the transfer characteristics and SS value of FETs. Our calculations reveal that both GW and HSE06 methods yield consistent electronic structure properties for all studied compounds. We observed a consistent increase in both internal and external band gaps, as well as metallic bandwidths, across all pn-type cold metal systems. Notably, the internal band gap E^I_g exhibits the most substantial enhancement, highlighting the sensitivity of these materials to correlation effects. In contrast, the changes in the metallic bandwidth W_m and external band gap E^E_g are relatively modest. These findings offer valuable insights for designing and optimizing cold metal-based devices. Materials like NbSi_2N_4, NbGe_2N_4, and TaSi_2N_4 show particular promise for high-performance NDR tunnel diodes and sub-60 mV/dec SS FETs.

cond-mat.mtrl-sci↗

arXiv2024

Spin gapped metals: A novel class of materials for multifunctional spintronic devices

Gapped metals, a recently proposed class of materials, possess a band gap slightly above or below the Fermi level, behaving as intrinsic p- or n-type semiconductors without requiring external doping. Inspired by this concept, we propose a novel material class: "spin gapped metals". These materials exhibit intrinsic p- or n-type character independently for each spin channel, similar to dilute magnetic semiconductors but without the need for transition metal doping. A key advantage of spin gapped metals lies in the absence of band tails that exist within the band gap of conventional p- and n-type semiconductors. Band tails degrade the performance of devices like tunnel field-effect transistors (causing high subthreshold slopes) and negative differential resistance tunnel diodes (resulting in low peak-to-valley current ratios). Here, we demonstrate the viability of spin gapped metals using first-principles electronic band structure calculations on half-Heusler compounds. Our analysis reveals compounds displaying both gapped metal and spin gapped metal behavior, paving the way for next-generation multifunctional devices in spintronics and nanoelectronics.

cond-mat.mtrl-sci↗

arXiv2022

Ab-initio calculation of the Hubbard $U$ and Hund exchange $J$ in local moment magnets: The case of Mn-based full Heusler compounds

Mn-based full Heusler compounds possess well-defined local atomic Mn moments, and thus the correlation effects between localized d electrons are expected to play an important role in determining the electronic and magnetic properties of these materials. Employing ab-initio calculations in conjunction with the constrained random-phase approximation (cRPA) method, we calculate the strength of the effective on-site Coulomb interaction parameters (Hubbard U and Hund exchange J) in the case of X2MnZ full Heusler compounds with X being one of Ni, Pd or Cu, and Z being one of In, Sn, Sb or Te. We show that the Z element (or sp element) in Heusler compounds significantly reduces the strength of the Hubbard U parameter for Mn 3d electrons compared to the elementary bulk Mn. On the contrary, the effect of the sp-atom on the strength of the U parameter of Ni, Cu or Pd valence d electrons is not so substantial with respect to the elementary bulk values. The U values for all transition metal atoms decrease with increasing sp electron number in the In-Sn-Sb-Te sequence. Our cRPA calculations reveal that despite their well-defined local magnetic moments, the Mn-based full Heusler alloys fall into the category of the weakly correlated materials.

cond-mat.mtrl-sci↗

arXiv2017

A first-principles DFT+GW study of spin-filter and spin-gapless semiconducting Heusler compounds

Among Heusler compounds, the ones being magnetic semiconductors (also known as spin-filter materials) are widely studied as they offer novel functionalities in spintronic/magnetoelectronic devices. The spin-gapless semiconductors are a special case. They possess a zero or almost-zero energy gap in one of the two spin channels. We employ the $GW$ approximation, which allows an elaborate treatment of the electronic correlations, to simulate the electronic band structure of these materials. Our results suggest that in most cases the use of $GW$ self energy instead of the usual density functionals is important to accurately determine the electronic properties of magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2017

Design of L2_1-type antiferromagnetic semiconducting full-Heusler compounds: A first principles DFT+GW study

Antiferromagnetic spintronics is an on-going growing field of research. Employing both standard density functional theory and the $GW$ approximation within the framework of the FLAPW method, we study the electronic and magnetic properties of seven potential antiferromagnetic semiconducting Heusler compounds with 18 (or 28 when Zn is present) valence electrons per unit cell. We show that in these compounds G-type antiferromagnetism is the ground state and that they are all either emiconductors (Cr$_2$ScP, Cr$_2$TiZn, V$_2$ScP, V$_2$TiSi, and V$_3$Al) or semimetals (Mn$_2$MgZn and Mn$_2$NaAl). The many-body corrections have a minimal effect on the electronic band structure with respect to the standard electronic structure calculations.

cond-mat.mtrl-sci↗

arXiv2016

Quasiparticle band structure of the almost-gapless transition-metal-based Heusler semiconductors

Transition-metal-based Heusler semiconductors are promising materials for a variety of applications ranging from spintronics to thermoelectricity. Employing the $GW$ approximation within the framework of the FLAPW method, we study the quasi-particle band structure of a number of such compounds being almost gapless semiconductors. We find that in contrast to the \textit{sp}-electron based semiconductors such as Si and GaAs, in these systems the many-body corrections have a minimal effect on the electronic band structure and the energy band gap increases by less than 0.2~eV, which makes the starting point density functional theory (DFT) a good approximation for the description of electronic and optical properties of these materials. Furthermore, the band gap can be tuned either by the variation of the lattice parameter or by the substitution of the \emph{sp}-chemical element.

cond-mat.mtrl-sci↗

arXiv2003

The influence of the mutual drag of carrier-phonon system on the thermopower and the transverse Nernst-Ettingshausen effect

The thermopower and Nernst-Ettingshausen (NE) effect in degenerate semiconductors and semimetals placed in high electric and magnetic fields are calculated by taking into account the heating of both electrons and phonons as well as their thermal and mutual drags. The magnetic and electric field dependences of the thermoelectric power and the transverse NE voltage are found in analytical forms. It is shown that in weak and high transverse magnetic fields, the electronic and phonon parts of NE coefficients change their sign for some scattering mechanisms.

cond-mat↗

arXiv2003

Thermoelectric power of nondegenerate Kane semiconductors under the conditions of mutual electron-phonon drag in a high electric field

The thermoelectric power of nondegenerate Kane semiconductors with due regard for the electron and phonon heating, and their thermal and mutual drags is investigated. The electron spectrum is taken in the Kane two-band form. It is shown that the nonparabolicity of electron spectrum significantly influences the magnitude of the thermoelectric power and leads to a change of its sign and dependence on the heating electric field. The field dependence of the thermoelectric power is determined analytically under various drag conditions.

cond-mat.stat-mech↗