arXiv subjects

Eve Emmanouilidou

Publications and source records attributed to Eve Emmanouilidou.

15 recordsLinked to original sources

arXiv2024

Fermiology and transport properties of the proposed topological crystalline insulator SrAg4Sb2

Compared to time-reversal symmetry-protected Z2 topological insulators and Dirac/Weyl semimetals, there are significantly fewer candidates for topological crystalline insulators. SrAg4Sb2 is predicted to exhibit topological crystalline insulator behavior when considering spin-orbit coupling. In this study, we systematically investigate single crystals of SrAg4Sb2 using electrical transport and magnetic torque measurements, along with first-principles calculations. Our transport data reveals its compensated semimetal nature with a magnetoresistance up to around 700% at 2 K and 9 T. Analysis of de Haas-van Alphen oscillations uncovers a Fermi surface consisting of three distinct Fermi pockets with light effective masses. Comparison between the three-dimensional fermiology obtained from our oscillation data and the first-principles calculations demonstrates excellent agreement. This confirms the accuracy of the calculations, which indicate a band inversion centered at the Γ point and identify the existence of nontrivial tube and needle hole Fermi pockets at Γ, alongside one trivial diamond electron pocket at the T point in the Brillouin zone. Furthermore, symmetry and topology analysis results in two potential sets of topological invariants, suggesting the emergence of two-dimensional gapless Dirac surface states either on the ab planes or on both the ab planes and mirror planes, protected by crystal symmetries. Therefore, SrAg4Sb2 emerges as a promising candidate topological crystalline insulator.

cond-mat.mtrl-sci↗

arXiv2022

Exchange-biased quantum anomalous Hall effect

The quantum anomalous Hall (QAH) effect is characterized by a dissipationless chiral edge state with a quantized Hall resistance at zero magnetic field. Manipulating the QAH state is of great importance in both the understanding of topological quantum physics and the implementation of dissipationless electronics. Here, we realized the QAH effect in the magnetic topological insulator Cr-doped (Bi,Sb)2Te3 (CBST) grown on an uncompensated antiferromagnetic insulator Al-doped Cr2O3. Through polarized neutron reflectometry (PNR), we find a strong exchange coupling between CBST and Al-Cr2O3 surface spins fixing interfacial magnetic moments perpendicular to the film plane. The interfacial coupling results in an exchange-biased QAH effect. We further demonstrate that the magnitude and sign of the exchange bias can be effectively controlled using a field training process to set the magnetization of the Al-Cr2O3 layer. Our work demonstrates the use of the exchange bias effect to effectively manipulate the QAH state, opening new possibilities in QAH-based spintronics.

cond-mat.mes-hall↗

arXiv2022

Unconventional pressure-driven metamagnetic transitions in topological van der Waals magnets

Activating metamagnetic transitions between ordered states in van der Waals magnets and devices bring great opportunities in spintronics. We show that external pressure, which enhances the interlayer hopping without introducing chemical disorders, triggers multiple metamagnetic transitions upon cooling in the topological van der Waals magnets Mn(Bi$_{1-x}$Sb$_x$)$_4$Te$_7$, where the antiferromagnetic interlayer superexchange coupling competes with the ferromagnetic interlayer coupling mediated by the antisite Mn spins. The temperature-pressure phase diagrams reveal that while the ordering temperature from the paramagnetic to ordered states is almost pressure-independent, the metamagnetic transitions show non-trivial pressure and temperature dependence, even re-entrance. For these highly anisotropic magnets, we attribute the former to the ordering temperature being only weakly dependent on the intralayer parameters, the latter to the parametrically different pressure and temperature dependence of the two interlayer couplings. Our independent probing of these disparate magnetic interactions paves an avenue for efficient magnetic manipulations in van der Waals magnets.

cond-mat.str-el↗

arXiv2020

Structural distortion and incommensurate noncollinear magnetism in EuAg4As2

Layered pnictide materials have provided a fruitful platform to study various emergent phenomena, including superconductivity, magnetism, charge density waves, etc. Here we report the observation of structural distortion and noncollinear magnetism in layered pnictide EuAg$_4$As$_2$ via transport, magnetization, single crystal X-ray and neutron diffraction data. EuAg$_4$As$_2$ single crystal shows a structural distortion at 120 K, where two sets of superlattice peaks with the propagation vectors of $q_1=\pm$(0, 0.25, 0.5) and $q_2=\pm$(0.25, 0, 1) emerge. Between 9 K to 15 K, the hexagonal Eu$^{2+}$ sub-lattice enters an unpinned state, with magnetic Bragg reflections pictured as circular-sectors. Below 9 K, it orders in an incommensurate noncollinear antiferromagnetic state with a well-defined propagation wavevector of (0, 0.1, 0.12), where the magnetic structure is helical along the $c$ axis and cycloidal along the $b$ axis with a moment of 6.4 $μ_B$/Eu$^{2+}$. Furthermore, rich magnetic phases under magnetic fields, large magnetoresistance, and strong coupling between charge carriers and magnetism in EuAg$_4$As$_2$ are revealed.

cond-mat.str-el↗

arXiv2020

A Néel-type antiferromagnetic order in the spin 1/2 rare-earth honeycomb YbCl$_3$

Most of the searches for Kitaev materials deal with $4d/5d$ magnets with spin-orbit-coupled ${J=1/2}$ local moments such as iridates and $α$-RuCl$_3$. Here we propose the monoclinic YbCl$_3$ with a Yb$^{3+}$ honeycomb lattice for the exploration of Kiteav physics. We perform thermodynamic, $ac$ susceptibility, angle-dependent magnetic torque and neutron diffraction measurements on YbCl$_3$ single crystal. We find that the Yb$^{3+}$ ion exhibits a Kramers doublet ground state that gives rise to an effective spin ${J_{\text{eff}}=1/2}$ local moment. The compound exhibits short-range magnetic order below 1.20 K, followed by a long-range Néel-type antiferromagnetic order at 0.60 K, below which the ordered Yb$^{3+}$ spins lie in the $ac$ plane with an angle of 16(11)$^{\circ}$ away from the $a$ axis. These orders can be suppressed by in-plane and out-of-plane magnetic fields at around 6 and 10 T, respectively. Moreover, the Néel temperature varies non-monotonically under the out-of-plane magnetic fields. The in-plane magnetic anisotropy and the reduced order moment 0.8(1) $μ_B$ at 0.25 K indicate that YbCl$_3$ could be a two-dimensional spin system to proximate the Kitaev physics.

cond-mat.str-el↗

arXiv2019

A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling

Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall (QAH) effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~ 0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena.

cond-mat.mtrl-sci↗

arXiv2019

Non-trivial topology in a layered Dirac nodal-line semimetal candidate SrZnSb$_2$ with distorted Sb square nets

Dirac states hosted by Sb/Bi square nets are known to exist in the layered antiferromagnetic AMnX$_2$ (A = Ca/Sr/Ba/Eu/Yb, X=Sb/Bi) material family the space group to be P4/nmm or I4/mmm. In this paper, we present a comprehensive study of quantum transport behaviors, angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations on SrZnSb2, a nonmagnetic analogue to AMnX2, which crystallizes in the pnma space group with distorted square nets. From the quantum oscillation measurements up to 35 T, three major frequencies including F$_1$ = 103 T, F$_2$ = 127 T and F$_3$ = 160 T, are identified. The effective masses of the quasiparticles associated with these frequencies are extracted, namely, m*$_1$ = 0.1 m$_e$, m*$_2$ = 0.1 m$_e$ and m*$_3$ = 0.09m$_e$, where m$_e$ is the free electron mass. From the three-band Lifshitz-Kosevich fit, the Berry phases accumulated along the cyclotron orbit of the quasiparticles are 0.06$π$, 1.2$π$ and 0.74$π$ for F$_1$, F$_2$ and F$_3$, respectively. Combined with the ARPES data and the first-principles calculations, we reveal that F2 and F3 are associated with the two nontrivial Fermi pockets at the Brillouin zone edge while F1 is associated with the trivial Fermi pocket at the zone center. In addition, the first-principles calculations further suggest the existence of Dirac nodal line in the band structure of SrZnSb$_2$.

cond-mat.mtrl-sci↗

arXiv2018

Significant change in the electronic behavior associated with structural distortions in the single crystalline SrAg4As2

We report a combined study of transport and thermodynamic measurements on the layered pnictide material SrAg4As2. Upon cooling, a drop in electrical and Hall resistivity, a jump in heat capacity and an increase in susceptibility and magnetoresistance (MR) are observed around 110 K. These observations suggest that non-magnetic phase transitions emerge at around 110 K, that are likely associated with structural distortions. In sharp contrast with the first-principles calculations based on the crystal structure at room temperature, quantum oscillations reveal small Fermi pockets with light effective masses, suggesting a significant change in the Fermi surface topology caused by the low temperature structural distortion. No superconductivity emerges in SrAg$_4$As$_2$ down to 2 K and under pressures up to 2.13 GPa; instead, the low temperature structural distortion increases linearly with temperature at a rate of ~13 K/GPa above 0.89 GPa.

cond-mat.str-el↗

arXiv2018

Observation of superconductivity in pressurized Weyl semimetal candidate TaIrTe4

Here we report the observation of superconductivity in pressurized type-II Weyl semimetal (WSM) candidate TaIrTe4 by means of complementary high-pressure transport and synchrotron X-ray diffraction measurements. We find that TaIrTe4 shows superconductivity with transition temperature (TC) of 0.57 K at the pressure of ~23.8 GPa. Then, the TC value increases with pressure and reaches ~2.1 K at 65.7 GPa. In situ high-pressure Hall coefficient (RH) measurements at low temperatures demonstrate that the positive RH increases with pressure until the critical pressure of the superconducting transition is reached, but starts to decrease upon further increasing pressure. Above the critical pressure, the positive magnetoresistance effect disappears simultaneously. Our high pressure X-ray diffraction measurements reveal that, at around the critical pressure the lattice of the TaIrTe4 sample is distorted by the application of pressure and its volume is reduced by ~19.2%, the value of which is predicted to result in the change of the electronic structure significantly. We propose that the pressure-induced distortion in TaIrTe4 is responsible for the change of topology of Fermi surface and such a change favors the emergence of superconductivity. Our results clearly demonstrate the correlation among the lattice distortion, topological physics and superconductivity in the WSM.

cond-mat.supr-con↗

arXiv2018

Spin-flop phase transition in the orthorhombic antiferromagnetic topological semimetal Cu0.95MnAs

The orthorhombic antiferromagnetic compound CuMnAs was recently predicted to be an antiferromagnetic Dirac semimetal if both the Ry gliding and S2z rotational symmetries are preserved in its magnetic ordered state. In our previous work on Cu0.95MnAs and Cu0.98Mn0.96As, we showed that in their low temperature commensurate antiferromagnetic state the b axis is the magnetic easy axis, which breaks the S2z symmetry. As a result, while the existence of Dirac fermions is no longer protected, the polarized surface state makes this material potentially interesting for antiferromagnetic spintronics. In this paper, we report a detailed study of the anisotropic magnetic properties and magnetoresistance of Cu0.95MnAs and Cu0.98Mn0.96As. Our study shows that in Cu0.95MnAs the b axis is the easy axis and the c axis is the hard axis. Furthermore, it reveals that Cu0.95MnAs features a spin-flop phase transition at high temperatures and low fields when the field is applied along the easy b axis, resulting in canted antiferromagnetism. However, no metamagnetic transition is observed in Cu0.98Mn0.96As, indicating that the magnetic interactions in this system are very sensitive to Cu vacancies and Cu/Mn site mixing.

cond-mat.str-el↗

arXiv2017

Coexistence of Tunable Weyl Points and Topological Nodal Lines in Ternary Transition-Metal Telluride TaIrTe4

We report a combined theoretical and experimental study on TaIrTe4, a potential candidate of the minimal model of type-II Weyl semimetals. Unexpectedly, an intriguing node structure with twelve Weyl points and a pair of nodal lines protected by mirror symmetry was found by first-principle calculations, with its complex signatures such as the topologically non-trivial band crossings and topologically trivial Fermi arcs cross-validated by angle-resolved photoemission spectroscopy. Through external strain, the number of Weyl points can be reduced to the theoretical minimum of four, and the appearance of the nodal lines can be switched between different mirror planes in momentum space. The coexistence of tunable Weyl points and nodal lines establishes ternary transition-metal tellurides as a unique test ground for topological state characterization and engineering.

cond-mat.mtrl-sci↗

arXiv2017

Magnetic order induces symmetry breaking in the single crystalline orthorhombic CuMnAs semimetal

Recently, orthorhombic CuMnAs has been proposed to be a magnetic material where topological fermions exist around the Fermi level. Here we report the magnetic structure of the orthorhombic Cu0.95MnAs and Cu0.98Mn0.96As single crystals. While Cu0.95MnAs is a commensurate antiferromagnet (C-AFM) below 360 K with a propagation vector of k = 0, Cu0.98Mn0.96As undergoes a second-order paramagnetic to incommensurate antiferromagnetic (IC-AFM) phase transition at 320 K with k = (0.1,0,0), followed by a second-order IC-AFM to C-AFM phase transition at 230 K. In the C-AFM state, the Mn spins order parallel to the b-axis but antiparallel to their nearest-neighbors with the easy axis along the b axis. This magnetic order breaks Ry gliding and S2z rotational symmetries, the two crucial for symmetry analysis, resulting in finite band gaps at the crossing point and the disappearance of the massless topological fermions. However, the spin-polarized surface states and signature induced by non-trivial topology still can be observed in this system, which makes orthorhombic CuMnAs promising in antiferromagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2017

Topological surface electronic states in candidate nodal-line semimetal CaAgAs

We investigate systematically the bulk and surface electronic structure of the candidate nodal-line semimetal CaAgAs by angle resolved photoemission spectroscopy and density functional calculations. We observed a metallic, linear, non-$k_z$-dispersive surface band that coincides with the high-binding-energy part of the theoretical topological surface state, proving the topological nontriviality of the system. An overall downshift of the experimental Fermi level points to a rigid-band-like $p$-doping of the samples, due possibly to Ag vacancies in the as-grown crystals.

cond-mat.mtrl-sci↗

arXiv2017

Magneto-transport properties of the "hydrogen atom" nodal-line semimetal candidates CaTX (T=Ag, Cd, X=As, Ge)

Topological semimetals are characterized by protected crossings between conduction and valence bands. These materials have recently attracted significant interest because of the deep connections to high-energy physics, the novel topological surface states, and the unusual transport phenomena. While Dirac and Weyl semimetals have been extensively studied, the nodal-line semimetal remains largely unexplored due to the lack of an ideal material platform. In this paper, we report the magneto-transport properties of two nodal-line semimetal candidates CaAgAs and CaCdGe. First, our single crystalline CaAgAs supports the first "hydrogen atom" nodal-line semimetal, where only the topological nodal-line is present at the Fermi level. Second, our CaCdGe sample provides an ideal platform to perform comparative studies because it features the same topological nodal line but has a more complicated Fermiology with irrelevant Fermi pockets. As a result, the magnetoresistance of our CaCdGe sample is more than 100 times larger than that of CaAgAs. Through our systematic magneto-transport and first-principles band structure calculations, we show that our CaTX compounds can be used to study, isolate, and control the novel topological nodal-line physics in real materials.

cond-mat.str-el↗

arXiv2016

Coexistence of superconductivity and antiferromagnetism in Ca$_{0.74(1)}$La$_{0.26(1)}$(Fe$_{1-x}$Co$_{x}$)As$_{2}$ single crystals

We report the transport, thermodynamic, $μ$SR and neutron study of the Ca$_{0.74(1)}$La$_{0.26(1)}$(Fe$_{1-x}$Co$_{x}$)As$_{2}$ single crystals, mapping out the temperature-doping level phase diagram. Upon Co substitution on the Fe site, the structural/magnetic phase transitions in this 112 compound are suppressed and superconductivity up to 20 K occurs. Our measurements of the superconducting and magnetic volume fractions show that these two phases coexist microscopically in the underdoped region, in contrast to the related 10-3-8 Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_x$)$_{2}$As$_{2}$)$_{5}$ compound, where coexistence is absent. Supported by model calculations, we discuss the differences in the phase diagrams of the 112 and 10-3-8 compounds in terms of the FeAs interlayer coupling, whose strength is affected by the character of the spacer layer, which is metallic in the 112 and insulating in the 10-3-8.

cond-mat.supr-con↗