arXiv subjects

Adam Kaminski

Publications and source records attributed to Adam Kaminski.

At least 37 records · Page 2Linked to original sources

arXiv2021

Visualizing band selective enhancement of quasiparticle lifetime in a metallic ferromagnet

Electrons navigate more easily in a background of ordered magnetic moments than around randomly oriented ones. This fundamental quantum mechanical principle is due to their Bloch wave nature and also underlies ballistic electronic motion in a perfect crystal. As a result, a paramagnetic metal that develops ferromagnetic order often experiences a sharp drop in the resistivity. Despite the universality of this phenomenon, a direct observation of the impact of ferromagnetic order on the electronic quasiparticles in a magnetic metal is still lacking. Here we demonstrate that quasiparticles experience a significant enhancement of their lifetime in the ferromagnetic state of the low-density magnetic semimetal EuCd2As2, but this occurs only in selected bands and specific energy ranges. This is a direct consequence of the magnetically induced band splitting and the multi-orbital nature of the material. Our detailed study allows to disentangle different electronic scattering mechanisms due to non-magnetic disorder and magnon exchange. Such high momentum and energy dependence quasiparticle lifetime enhancement can lead to spin selective transport and potential spintronic applications.

cond-mat.str-el↗

arXiv2021

Electronic structure of $A$Co$_2$As$_2$ ($A=$ Ca, Sr, Ba, Eu) studied using angle-resolved photoemission spectroscopy and theoretical calculations

We present a comprehensive study of the low-energy band structure and Fermi surface (FS) topology of $A$Co$_2$As$_2$ ($A=$ Ca, Sr, Ba, Eu) using high-resolution angle-resolved photoemission spectroscopy. The experimental FS topology and band dispersion data are compared with theoretical full-potential linearized augmented-plane-wave (FP-LAPW) calculations, which yielded reasonably good agreement. We demonstrate that the FS maps of $A$Co$_2$As$_2$ are significantly different from those of the parent compounds of Fe-based high-temperature superconductors. Further, the FSs of CaCo$_2$As$_2$ do not show significant changes across its antiferromagnetic transition temperature. The band dispersions extracted in different momentum $(k_{\it x}, k_{\it y})$ directions show a small electron pocket at the center and a large electron pocket at the corner of the Brillouin zone (BZ). The absence of the hole FS in these compounds does not allow nesting between pockets at the Fermi energy ({\it E}$_{\rm F}$), which is in contrast to $A$Fe$_2$As$_2$-type parent compounds of the iron-based superconductors. Interestingly, we find that the hole bands are moved 300--400~meV below $E_{\rm F}$ depending on the $A$ element. Moreover, the existence of nearly flat bands in the vicinity of $E_{\rm F}$ are consistent with the large density of states at $E_{\rm F}$. These results are important to understand the physical properties as well as the possibility of the emergence of superconductivity in related materials.

cond-mat.supr-con↗

arXiv2020

Discovery of a weak topological insulating state and van Hove singularity in triclinic RhBi2

Time reversal symmetric (TRS) invariant topological insulators (TIs) fullfil a paradigmatic role in the field of topological materials, standing at the origin of its development. Apart from TRS protected 'strong' TIs, it was realized early on that more confounding weak topological insulators (WTI) exist. WTIs depend on translational symmetry and exhibit topological surface states only in certain directions making it significantly more difficult to match the experimental success of strong TIs. We here report on the discovery of a WTI state in RhBi2 that belongs to the optimal space group P1, which is the only space group where symmetry indicated eigenvalues enumerate all possible invariants due to absence of additional constraining crystalline symmetries. Our ARPES, DFT calculations, and effective model reveal topological surface states with saddle points that are located in the vicinity of a Dirac point resulting in a van Hove singularity (VHS) along the (100) direction close to the Fermi energy. Due to the combination of exotic features, this material offers great potential as a material platform for novel quantum effects.

cond-mat.str-el↗

arXiv2020

Evidence for large Rashba splitting in PtPb4 from Angle-Resolved Photoemission Spectroscopy

We studied the electronic structure of PtPb$_{4}$ using laser angle-resolved photoemission spectroscopy(ARPES) and density functional theory(DFT) calculations. This material is closely related to PtSn$_{4}$, which exhibits exotic topological properties such as Dirac node arcs. Fermi surface(FS) of PtPb$_{4}$ consists of two electron pockets at the center of the Brillouin zone(BZ) and several hole pockets around the zone boundaries. Our ARPES data reveals significant Rashba splitting at the $Γ$ point in agreement with DFT calculations. The presence of Rashba splitting may render this material of potential interest for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Manipulating of magnetism in the topological semimetal EuCd2As2

Magnetic Weyl semimetals are expected to have extraordinary physical properties such as a chiral anomaly and large anomalous Hall effects that may be useful for future, potential, spintronics applications. However, in most known host materials, multiple pairs of Weyl points prevent a clear manifestation of the intrinsic topological effects. Our recent density functional theory (DFT) calculations study suggest that EuCd$_{2}$As$_{2}$ can host Dirac fermions in an antiferromagnetically (AFM) ordered state or a single pair of Weyl fermions in a ferromagnetically (FM) ordered state. Unfortunately, previously synthesized crystals ordered antiferromagnetically with $T_{\textrm{N}}$\,$\simeq$\,9.5\,K. Here, we report the successful synthesis of single crystals of EuCd$_{2}$As$_{2}$ that order ferromagnetically (FM) or antiferromagnetically (AFM) depending on the level of band filling, thus allowing for the use of magnetism to tune the topological properties within the same host. We explored their physical properties via magnetization, electrical transport, heat capacity, and angle resolved photoemission spectroscopy (ARPES) measurements and conclude that EuCd$_{2}$As$_{2}$ is an excellent, tunable, system for exploring the interplay of magnetic ordering and topology.

cond-mat.mtrl-sci↗

arXiv2019

Intrinsic axion insulating behavior in antiferromagnetic MnBi$_6$Te$_{10}$

A striking feature of time reversal symmetry (TRS) protected topological insulators (TIs) is that they are characterized by a half integer quantum Hall effect on the boundary when the surface states are gapped by time reversal breaking perturbations. While time reversal symmetry (TRS) protected TIs have become increasingly under control, magnetic analogs are still largely unexplored territories with novel rich structures. In particular, topological magnetic insulators can also host a quantized axion term in the presence of lattice symmetries. Since these symmetries are naturally broken on the boundary, the surface states can develop a gap without external manipulation. In this work, we combine theoretical analysis, density functional calculations and experimental evidence to reveal intrinsic axion insulating behavior in MnBi6Te10. The quantized axion term arises from the simplest possible mechanism in the antiferromagnetic regime where it is protected by inversion symmetry and a fractional translation symmetry. The anticipated gapping of the Dirac surface state at the edge is subsequently experimentally established using Angle Resolved Spectroscopy. As a result, this system provides the magnetic analogue of the simplest TRS protected TI with a single, gapped Dirac cone at the surface.

cond-mat.mtrl-sci↗

arXiv2019

Gapless Dirac surface states in the antiferromagnetic topological insulator MnBi2Te4

We use high-resolution, tunable angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic properties of single crystals of MnBi2Te4, a material that was predicted to be the first intrinsic antiferromagnetic (AFM) topological insulator. We observe both bulk and surface bands in the electronic spectra, in reasonable agreement with the DFT calculations results. In striking contrast to the earlier literatures showing a full gap opening between two surface band manifolds along (0001) direction, we observed a gapless Dirac cone remain protected in MnBi2Te4 across the AFM transition (TN = 24 K). Our data also reveal the existence of a second Dirac cone closer to the Fermi level, predicted by band structure calculations. Whereas the surface Dirac cones seem to be remarkably insensitive to the AFM ordering, we do observe splitting of the bulk band that develops below the TN . Having a moderately high ordering temperature, MnBi2Te4 provides a unique platform for studying the interplay between topology and magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2019

Fragility of Fermi arcs in Dirac semimetals

We use tunable, vacuum ultraviolet laser-based angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic properties of Dirac semimetal candidate cubic PtBi${}_{2}$. In addition to bulk electronic states we also find surface states in PtBi${}_{2}$ which is expected as PtBi${}_{2}$ was theoretical predicated to be a candidate Dirac semimetal. The surface states are also well reproduced from DFT band calculations. Interestingly, the topological surface states form Fermi contours rather than double Fermi arcs that were observed in Na$_3$Bi. The surface bands forming the Fermi contours merge with bulk bands in proximity of the Dirac points projections, as expected. Our data confirms existence of Dirac states in PtBi${}_{2}$ and reveals the fragility of the Fermi arcs in Dirac semimetals. Because the Fermi arcs are not topologically protected in general, they can be deformed into Fermi contours, as proposed by [Kargarian {\it et al.}, PNAS \textbf{113}, 8648 (2016)]. Our results demonstrate validity of this theory in PtBi${}_{2}$.

cond-mat.mtrl-sci↗

arXiv2019

A Single Pair of Weyl Fermions in Half-metallic EuCd2As2 Semimetal

An ideal Weyl semimetal with a single pair of Weyl points (WPs) may be generated by splitting a single Dirac point (DP) through the breaking of time-reversal symmetry by magnetic order. However, most known Dirac semimetals possess a pair of DPs along an axis that is protected by crystalline symmetry. Here, we demonstrate that a single pair of WPs may also be generated from a pair of DPs. Using first-principles band structure calculations, we show that inducing ferromagnetism in the AFM Dirac semimetal EuCd2As2 generates a single pair of WPs due to its half-metallic nature. Analysis with a low-energy effective Hamiltonian shows that this ideal Weyl semimetal is obtained in EuCd2As2 because the DPs are very close to the zone center and the ferromagnetic exchange splitting is large enough to push one pair of WPs to merge and annihilate at Gamma while the other pair survives. Furthermore, we predict that alloying with Ba at the Eu site can stabilize the ferromagnetic configuration and generate a single pair of Weyl points without application of a magnetic field.

cond-mat.mtrl-sci↗

arXiv2018

Electronic structure of topological superconductor candidate Au${}_{2}$Pb

We use magnetization measurements, high-resolution angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations to study the electronic properties of Au${}_{2}$Pb, a topological superconductor candidate. The magnetization measurements reveal three discontinuities at 40, 51, and 99~K that agree well with reported structural phase transitions. ARPES measurements of the Au${}_{2}$Pb (111) surface at 110~K shows a shallow hole pocket at the center and flower-petal-like surface states at the corners of the Brillouin zone. These observations match the results of DFT calculations relatively well. The flower-petal-like surface states appear to originate from a Dirac like dispersion close to the zone corner. For the Au${}_{2}$Pb (001) surface at 150~K, ARPES reveals at least one electron pocket between the $Γ$ and $M$ points, consistent with the DFT calculations. Our results provide evidence for the possible existence of Dirac state in this material.

cond-mat.str-el↗

arXiv2018

In-plane magnetic penetration depth of superconducting CaKFe$_4$As$_4$

The temperature dependence of the in-plane magnetic penetration depth ($λ_{ab}$) in an extensively characterized sample of superconducting CaKFe$_4$As$_4$ ($T_{\rm c}\simeq35$ K) was investigated using muon-spin rotation ($μ$SR). A comparison of $λ_{ab}^{-2}(T)$ measured by $μ$SR with the one inferred from ARPES data confirms the presence of multiple gaps at the Fermi level. An agreement between $μ$SR and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterized by small supercondcting gaps with an average zero-temperature value of $Δ_{0} =$ 2.4(2) meV. Our data suggest that in CaKFe$_4$As$_4$ the $s^\pm$ order parameter symmetry acquires a more sophisticated form by allowing a sign change not only between electron and hole pockets, but also within pockets of similar type.

cond-mat.supr-con↗

arXiv2017

Extremely large magnetoresistance and Kohler's rule in PdSn4: a complete study of thermodynamic, transport and band structure properties

The recently discovered material PtSn$_4$ is known to exhibit extremely large magnetoresistance (XMR) that also manifests Dirac arc nodes on the surface. PdSn$_4$ is isostructure to PtSn$_4$ with same electron count. We report on the physical properties of high quality single crystals of PdSn$_4$ including specific heat, temperature and magnetic field dependent resistivity and magnetization, and electronic band structure properties obtained from angle resolved photoemission spectroscopy (ARPES). We observe that PdSn$_4$ has physical properties that are qualitatively similar to those of PtSn$_4$, but find also pronounced differences. Importantly, the Dirac arc node surface state of PtSn$_4$ is gapped out for PdSn$_4$. By comparing these similar compounds, we address the origin of the extremely large magnetoresistance in PdSn$_4$ and PtSn$_4$; based on detailed analysis of the magnetoresistivity, $ρ(H,T)$, we conclude that neither carrier compensation nor the Dirac arc node surface state are primary reason for the extremely large magnetoresistance. On the other hand, we find that surprisingly Kohler's rule scaling of the mangnetoresistance, which describes a self-similarity of the field induced orbital electronic motion across different length scales and is derived for a simple electronic response of metals to applied in a magnetic field is obeyed over the full range of temperatures and field strengths that we explore.

cond-mat.str-el↗

arXiv2017

The effects of Moire lattice on the electronic properties of graphene

We study structural and electronic properties of graphene grown on SiC substrate using scanning tunneling microscope (STM), spot-profile-analysis low energy electron diffraction (SPA-LEED) and angle resolved photoemission spectroscopy (ARPES). We find several new replicas of Dirac cones in the Brillouin zone (BZ). Their locations can be understood in terms of combination of basis vectors linked to SiC 6x6 and graphene 6xsqrt(3) x 6sqrt(3) reconstruction. Therefore these new features originate from the Moie caused by the lattice mismatch between SiC and graphene. More specifically, Dirac cones replicas are caused by underlying weak modulation of the ionic potential by the substrate that is then experienced by the electrons in the graphene. We also demonstrate that this effect is equally strong in single and tri-layer graphene, therefore the additional Dirac cones are intrinsic features rather than result of photoelectron diffraction. These new features in the electronic structure are very important for the interpretation of recent transport measurements and can assist in tuning the properties of graphene for practical applications.

cond-mat.mes-hall↗

arXiv2017

Electronic structure of RSb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu) studied by angle-resolved photoemission spectroscopy

We use high resolution angle-resolved photoemission spectroscopy (ARPES) and electronic structure calculations to study the electronic properties of rare-earth monoantimonides RSb (R = Y, Ce, Gd, Dy, Ho, Tm, Lu). The experimentally measured Fermi surface (FS) of RSb consists of at least two concentric hole pockets at the $Γ$ point and two intersecting electron pockets at the $X$ point. These data agree relatively well with the electronic structure calculations. Detailed photon energy dependence measurements using both synchrotron and laser ARPES systems indicate that there is at least one Fermi surface sheet with strong three-dimensionality centered at the $Γ$ point. Due to the "lanthanide contraction", the unit cell of different rare-earth monoantimonides shrinks when changing rare-earth ion from CeSb to LuSb. This results in the differences in the chemical potentials in these compounds, which is demonstrated by both ARPES measurements and electronic structure calculations. Interestingly, in CeSb, the intersecting electron pockets at the $X$ point seem to be touching the valence bands, forming a four-fold degenerate Dirac-like feature. On the other hand, the remaining rare-earth monoantimonides show significant gaps between the upper and lower bands at the $X$ point. Furthermore, similar to the previously reported results of LaBi, a Dirac-like structure was observed at the $Γ$ point in YSb, CeSb, and GdSb, compounds showing relatively high magnetoresistance. This Dirac-like structure may contribute to the unusually large magnetoresistance in these compounds.

cond-mat.mtrl-sci↗

arXiv2017

Direct observation of self energy signatures of the resonant collective mode in Bi2Sr2CaCu2O8

We use high resolution ARPES to study the resonant, collective excitation mode in the superconducting state of Bi2212. By collecting very high quality data we found new features in the self energy in the antinodal region, where the interaction of electrons with the mode is the strongest. This interaction leads to pronounced peak in the scattering rate and we demonstrate that this feature is directly responsible for well known hump-dip-peak structure in cuprates. By studying how the weight of this peak changes with temperature we unequivocally demonstrate that interaction of electrons with resonant mode in cuprates vanishes at Tc and is very much localized in the momentum space close to the antinode. These findings present a consistent picture of line shape and self energy signatures of the electron-boson coupling in cuprates and resolve long standing controversy surrounding this issue. The momentum dependence of the strength of electron-mode interaction enables development of quantitative theory of this phenomenon in cuprates.

cond-mat.supr-con↗

arXiv2017

Phonon-Induced Topological Transition to a Type-II Weyl Semimetal

Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied, as exemplified in NbNiTe2, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For NbNiTe2 structure in space group 53 (Pmna) - having an inversion center arising from two glide planes and one mirror plane with a 2-fold rotation and screw axis - a full gap opening exists between two band manifolds near the Fermi energy. Upon atomic displacements by optical phonons, the symmetry lowers to space group 28 (Pma2), eliminating one glide plane along c, the associated rotation and screw axis, and the inversion center. As a result, twenty Weyl points emerge, including four type-II Weyl points in the G-X direction at the boundary between a pair of adjacent electron and hole bands. Thus, optical phonons may offer control of the transition to a Weyl fermion state.

cond-mat.mtrl-sci↗

arXiv2017

Three-dimensionality of the bulk electronic structure in WTe2

We use temperature- and field-dependent resistivity measurements [Shubnikov--de Haas (SdH) quantum oscillations] and ultrahigh resolution, tunable, vacuum ultraviolet (VUV) laser-based angle-resolved photoemission spectroscopy (ARPES) to study the three-dimensionality (3D) of the bulk electronic structure in WTe2, a type-II Weyl semimetal. The bulk Fermi surface (FS) consists of two pairs of electron pockets and two pairs of hole pockets along the X-Gamma-X direction as detected by using an incident photon energy of 6.7 eV, which is consistent with the previously reported data. However, if using an incident photon energy of 6.36 eV, another pair of tiny electron pockets is detected on both sides of the Gamma point, which is in agreement with the small quantum oscillation frequency peak observed in the magnetoresistance. Therefore, the bulk, 3D FS consists of three pairs of electron pockets and two pairs of hole pockets in total. With the ability of fine tuning the incident photon energy, we demonstrate the strong three-dimensionality of the bulk electronic structure in WTe2. The combination of resistivity and ARPES measurements reveal the complete, and consistent, picture of the bulk electronic structure of this material.

cond-mat.mtrl-sci↗

arXiv2016

Anisotropic physical properties and pressure dependent magnetic ordering of CrAuTe$_4$

Systematic measurements of temperature dependent magnetization, resistivity and angle-resolved photoemission spectroscopy (ARPES) at ambient pressure as well as resistivity under pressures up to 5.25 GPa were conducted on single crystals of CrAuTe$_4$. Magnetization data suggest that magnetic moments are aligned antiferromagnetically along the crystallographic $c$-axis below $T_\textrm{N}$ = 255 K. ARPES measurements show band reconstruction due to the magnetic ordering. Magnetoresistance data show clear anisotropy, and, at high fields, quantum oscillations. The Neel temperature decreases monotonically under pressure, decreasing to $T_\textrm{N}$ = 236 K at 5.22 GPa. The pressure dependencies of (i) $T_\textrm{N}$, (ii) the residual resistivity ratio, and (iii) the size and power-law behavior of the low temperature magnetoresistance all show anomalies near 2 GPa suggesting that there may be a phase transition (structural, magnetic, and/or electronic) induced by pressure. For pressures higher than 2 GPa a significantly different quantum oscillation frequency emerges, consistent with a pressure induced change in the electronic states.

cond-mat.str-el↗