arXiv subjects

E. Manousakis

Publications and source records attributed to E. Manousakis.

At least 19 recordsLinked to original sources

arXiv2021

Experimental observation and spin texture of Dirac node arcs in tetradymite topological metals

We report the observation of a non-trivial spin texture in Dirac node arcs, novel topological objects formed when Dirac cones of massless particles extend along an open one-dimensional line in momentum space. We find that such states are present in all the compounds of the tetradymite M$_2$Te$_2$X family (M$=$Ti, Zr or Hf and X$=$P or As), regardless of the weak or strong character of the topological invariant. The Dirac node arcs in tetradymites are thus the simplest possible, textbook example, of a type-I Dirac system with a single spin-polarized node arc.

cond-mat.mes-hall↗

arXiv2018

Detailed study on the Fermi surfaces of the type-II Dirac semimetallic candidates PdTe2 and PtTe2

We present a detailed quantum oscillatory study on the Dirac type-II semimetallic candidates PdTe$_{2}$ and PtTe$_{2}$ \emph{via} the temperature and the angular dependence of the de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) effects. In high quality single crystals of both compounds, i.e. displaying carrier mobilities between $10^3$ and $10^4$ cm$^2$/Vs, we observed a large non-saturating magnetoresistivity (MR) which in PtTe$_2$ at a temperature $T = 1.3$ K, leads to an increase in the resistivity up to $5 \times 10^{4}$ % under a magnetic field $μ_0 H = 62$ T. These high mobilities correlate with their light effective masses in the range of 0.04 to 1 bare electron mass according to our measurements. For PdTe$_{2}$ the experimentally determined Fermi surface cross-sectional areas show an excellent agreement with those resulting from band-structure calculations. Surprisingly, this is not the case for PtTe$_{2}$ whose agreement between calculations and experiments is relatively poor even when electronic correlations are included in the calculations. Therefore, our study provides a strong support for the existence of a Dirac type-II node in PdTe$_2$ and probably also for PtTe$_2$. Band structure calculations indicate that the topologically non-trivial bands of PtTe$_2$ do not cross the Fermi-level ($\varepsilon_F$). In contrast, for PdTe$_2$ the Dirac type-II cone does intersect $\varepsilon_F$, although our calculations also indicate that the associated cyclotron orbit on the Fermi surface is located in a distinct $k_z$ plane with respect to the one of the Dirac type-II node. Therefore it should yield a trivial Berry-phase.

cond-mat.str-el↗

arXiv2018

Bulk Fermi surfaces of the Dirac Type-II Semimetallic Candidates VAl3, NbAl3 and TaAl3

We report a de Haas-van Alphen (dHvA) effect study on the Dirac type-II semimetallic candidates \emph{M}Al$_3$ (where, \emph{M} = V, Nb and Ta). The angular-dependence of their Fermi surface (FS) cross-sectional areas reveals a remarkably good agreement with first-principle calculations. Therefore, dHvA supports the existence of tilted Dirac cones with Dirac type-II nodes located at 100, 230 and 250 meV above the Fermi level $\varepsilon_F$ for VAl$_3$, NbAl$_3$ and TaAl$_3$ respectively, in agreement with the prediction of broken Lorentz invariance in these compounds. However, for all three compounds we find that the cyclotron orbits on their FSs, including an orbit nearly enclosing the Dirac type-II node, yield trivial Berry phases. We explain this $via$ an analysis of the Berry phase where the position of this orbit, relative to the Dirac node, is adjusted within the error implied by the small disagreement between our calculations and the experiments. We suggest that a very small amount of doping could displace $\varepsilon_F$ to produce topologically non-trivial orbits encircling their Dirac node(s).

cond-mat.mtrl-sci↗

arXiv2017

Converting topological insulators into topological metals within the tetradymite family

We report the electronic band structures and concomitant Fermi surfaces for a family of exfoliable tetradymite compounds with the formula $T_2$$Ch_2$$Pn$, obtained as a modification to the well-known topological insulator binaries Bi$_2$(Se,Te)$_3$ by replacing one chalcogen ($Ch$) with a pnictogen ($Pn$) and Bi with the tetravalent transition metals $T$ $=$ Ti, Zr, or Hf. This imbalances the electron count and results in layered metals characterized by relatively high carrier mobilities and bulk two-dimensional Fermi surfaces whose topography is well-described by first principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states. In contrast to Bi$_2$Se$_3$, where the surface Dirac bands are at the $Γ-$point, for (Zr,Hf)$_2$Te$_2$(P,As) there are Dirac cones of strong topological character around both the $\bar Γ$- and $\bar {M}$-points which are above and below the Fermi energy, respectively. For Ti$_2$Te$_2$P the surface state is predicted to exist only around the $\bar {M}$-point. In agreement with these predictions, the surface states that are located below the Fermi energy are observed by angle resolved photoemission spectroscopy measurements, revealing that they coexist with the bulk metallic state. Thus, this family of materials provides a foundation upon which to develop novel phenomena that exploit both the bulk and surface states (e.g., topological superconductivity).

cond-mat.str-el↗

arXiv2017

Bulk Fermi-surface of the Weyl type-II semi-metallic candidate MoTe2

The electronic structure of WTe$_2$ and orthorhombic $γ-$MoTe$_2$, are claimed to contain pairs of Weyl type-II points. A series of ARPES experiments claim a broad agreement with these predictions. We synthesized single-crystals of MoTe$_2$ through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) \emph{via} quantum oscillatory phenomena. We find that the superconducting transition temperature of $γ-$MoTe$_2$ depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to WTe$_2$, the magnetoresistivity of $γ-$MoTe$_2$ does not saturate at high magnetic fields and can easily surpass $10^{6}$ \%. Remarkably, the analysis of the de Haas-van Alphen (dHvA) signal superimposed onto the magnetic torque, indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman-effect precluding the extraction of the Berry-phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level $\varepsilon_F$. Here, we show that a shift of the DFT valence bands relative to $\varepsilon_F$, in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron- and hole-bands eliminates their crossings and, therefore, the Weyl type-II points predicted for $γ-$MoTe$_2$.

cond-mat.mes-hall↗

arXiv2017

Fermi surface of the Weyl type-II metallic candidate WP2

Weyl type-II fermions are massless quasiparticles that obey the Weyl equation and which are predicted to occur at the boundary between electron- and hole-pockets in certain semi-metals, i.e. the (W,Mo)(Te,P)$_2$ compounds. Here, we present a study of the Fermi-surface of WP$_2$ \emph{via} the Shubnikov-de Haas (SdH) effect. Compared to other semi-metals WP$_2$ exhibits a very low residual resistivity, i.e. $ρ_0 \simeq 10$ n$Ω$cm, which leads to perhaps the largest non-saturating magneto-resistivity $(ρ(H))$ reported for any compound. For the samples displaying the smallest $ρ_0$, $ρ(H)$ is observed to increase by a factor of $2.5 \times 10^{7}$ $\%$ under $μ_{0}H = 35$ T at $T = 0.35$ K. The angular dependence of the SdH frequencies is found to be in very good agreement with the first-principle calculations when the electron- and hole-bands are slightly shifted with respect to the Fermi level, thus supporting the existence of underlying Weyl type-II points in WP$_2$.

cond-mat.mes-hall↗

arXiv2017

Electronic Structure and Magnetism in the Layered Triangular Lattice Compound CeAuAl$_4$Ge$_2$

Results are reported for the $f$-electron intermetallic CeAuAl$_4$Ge$_2$, where the atomic arrangement of the cerium ions creates the conditions for geometric frustration. Despite this, magnetic susceptibility measurements reveal that the low temperature magnetic exchange interaction is weak, resulting in marginally frustrated behavior and ordering near $T_{\rm{M}}$ $\approx$ 1.4 K. This occurs within a metallic Kondo lattice, where electrical resistivity and heat capacity measurements show that the Kondo-driven electronic correlations are negligible. Quantum oscillations are detected in ac-magnetic susceptibility measurements and uncover small charge carrier effective masses. Electronic structure calculations reveal that when the experimentally observed antiferromagnetic exchange interaction and the on-$f$-site Coulomb repulsion (Hubbard) U are considered, the $f$-electron bands move away from the Fermi level, resulting in electronic behavior that is dominated by the $s$-, $p$-, and $d$- bands, which are all characterized by light electron masses. Thus, CeAuAl$_4$Ge$_2$ provides a starting point for investigating geometric magnetic frustration in a cerium lattice without strong Kondo hybridization, where calculations provide useful guidance.

cond-mat.mtrl-sci↗

arXiv2016

Uncovering the behavior of Hf2Te2P and the candidate Dirac metal Zr2Te2P

Results are reported for single crystal specimens of Hf$_2$Te$_2$P and compared to its structural analogue Zr$_2$Te$_2$P, which was recently proposed to be a potential reservoir for Dirac physics.[1] Both materials are produced using the iodine vapor transport method and the resulting crystals are exfoliable. The bulk electrical transport and thermodynamic properties indicate Fermi liquid behavior at low temperature for both compounds. Quantum oscillations are observed in magnetization measurements for fields applied parallel but not perpendicular to the $c$-axis, suggesting that the Fermi surfaces are quasi-two dimensional. Frequencies are determined from quantum oscillations for several parts of the Fermi surfaces. Lifshitz-Kosevich fits to the temperature dependent amplitudes of the oscillations reveal small effective masses, with a particularly small value $m^*$ $=$ 0.046$m_0$ for the α branch of Zr$_2$Te$_2$P. Electronic structure calculations are in good agreement with quantum oscillation results and illustrate the effect of a stronger spin-orbit interaction going from Zr to Hf. These results suggest that by using appropriate tuning parameters this class of materials may deepen the pool of novel Dirac phenomena.

cond-mat.mtrl-sci↗

arXiv2015

Oscillons and oscillating kinks in the Abelian-Higgs model

We study the classical dynamics of the Abelian Higgs model employing an asymptotic multiscale expansion method, which uses the ratio of the Higgs to the gauge field amplitudes as a small parameter. We derive an effective nonlinear Schrödinger equation for the gauge field, and a linear equation for the scalar field containing the gauge field as a nonlinear source. This equation is used to predict the existence of oscillons and oscillating kinks for certain regimes of the ratio of the Higgs to the gauge field masses. Results of numerical simulations are found to be in very good agreement with the analytical findings, and show that the oscillons are robust, while kinks are unstable. It is also demonstrated that oscillons emerge spontaneously as a result of the onset of the modulational instability of plane wave solutions of the model. Connections of the obtained solutions with the phenomenology of superconductors is discussed.

hep-th↗

arXiv2015

Role of spin-orbit coupling and evolution of the electronic structure of WTe$_2$ under an external magnetic field

Here, we present a detailed study on the temperature and angular dependence of the Shubnikov-de-Haas (SdH) effect in the semi-metal WTe$_2$. This compound was recently shown to display a very large non-saturating magnetoresistance which was attributed to nearly perfectly compensated densities of electrons and holes. We observe four fundamental SdH frequencies and attribute them to spin-orbit split, electron- and hole-like, Fermi surface (FS) cross-sectional areas. Their angular dependence seems consistent with ellipsoidal FSs with volumes suggesting a modest excess in the density of electrons with respect to that of the holes. We show that density functional theory (DFT) calculations fail to correctly describe the FSs of WTe$_2$. When their cross-sectional areas are adjusted to reflect the experimental data, the resulting volumes of the electron/hole FSs obtained from the DFT calculations would imply a pronounced imbalance between the densities of electrons and holes. We find evidence for field-dependent Fermi surface cross-sectional areas by fitting the oscillatory component superimposed onto the magnetoresistivity signal to several Lifshitz-Kosevich components. We also observe a pronounced field-induced renormalization of the effective masses. Taken together, our observations suggest that the electronic structure of WTe$_2$ evolves with the magnetic field. This evolution might be a factor contributing to its pronounced magnetoresistivity.

cond-mat.mtrl-sci↗

arXiv2013

Oscillons and oscillating kinks in the Abelian-Higgs model

We study the classical dynamics of the Abelian Higgs model employing an asymptotic multiscale expansion method, which uses the ratio of the Higgs to the gauge field amplitudes as a small parameter. We derive an effective nonlinear Schrodinger equation for the gauge field, and a linear equation for the scalar field containing the gauge field as a nonlinear source. This equation is used to predict the existence of oscillons and oscillating kinks for certain regimes of the ratio of the Higgs to the gauge field masses. Results of direct numerical simulations are found to be in very good agreement with the analytical findings, and show that the oscillons are robust, while kinks are unstable. It is also demonstrated that oscillons emerge spontaneously as a result of the onset of the modulational instability of plane wave solutions of the model. Connections of the results with the phenomenology of superconductors is discussed.

hep-th↗

arXiv2012

Signatures of the superfluid to Mott insulator transition in equilibrium and in dynamical ramps

We investigate the equilibrium and dynamical properties of the Bose-Hubbard model and the related particle-hole symmetric spin-1 model in the vicinity of the superfluid to Mott insulator quantum phase transition. We employ the following methods: exact-diagonalization, mean field (Gutzwiller), cluster mean-field, and mean-field plus Gaussian fluctuations. In the first part of the paper we benchmark the four methods by analyzing the equilibrium problem and give numerical estimates for observables such as the density of double occupancies and their correlation function. In the second part, we study parametric ramps from the superfluid to the Mott insulator and map out the crossover from the regime of fast ramps, which is dominated by local physics, to the regime of slow ramps with a characteristic universal power law scaling, which is dominated by long wavelength excitations. We calculate values of several relevant physical observables, characteristic time scales, and an optimal protocol needed for observing universal scaling.

cond-mat.quant-gas↗

arXiv2011

Nearly frozen Coulomb Liquids

We show that very long range repulsive interactions of a generalized Coulomb-like form $V(R)\sim R^{-α}$, with $α<d$ ($d$-dimensionality), typically introduce very strong frustration, resulting in extreme fragility of the charge-ordered state. An \textquotedbl{}almost frozen\textquotedbl{} liquid then survives in a broad dynamical range above the (very low) melting temperature $T_{c}$ which is proportional to $α$. This \textquotedbl{}pseudogap\textquotedbl{} phase is characterized by unusual insulating-like, but very weakly temperature dependent transport, similar to experimental findings in certain low carrier density systems.

cond-mat.str-el↗

arXiv2010

Characterizing Complex Particle Morphologies Through Shape Matching: Descriptors, Applications, and Algorithms

Many standard structural quantities, such as order parameters and correlation functions, exist for common condensed matter systems, such as spherical and rod-like particles. However, these structural quantities are often insufficient for characterizing the unique and highly complex structures often encountered in the emerging field of nano and microscale self-assembly, or other disciplines involving complex structures such as computational biology. Computer science algorithms known as "shape matching" methods pose a unique solution to this problem by providing robust metrics for quantifying the similarity between pairs of arbitrarily complex structures. This pairwise matching operation, either implicitly or explicitly, lies at the heart of most standard structural characterization schemes for particle systems. By substituting more robust "shape descriptors" into these schemes we extend their applicability to structures formed from more complex building blocks. Here, we describe several structural characterization schemes and shape descriptors that can be used to obtain various types of structural information about particle systems. We demonstrate the application of shape matching algorithms to a variety of example problems, for topics including local and global structure identification and classification, automated phase diagram mapping, and the construction of spatial and temporal correlation functions. The methods are applicable to a wide range of systems, both simulated and experimental, provided particle positions are known or can be accurately imaged.

cond-mat.soft↗

arXiv2010

Magnetic structure of an imbalanced Fermi gas in an optical lattice

We analyze the repulsive fermionic Hubbard model on square and cubic lattices with spin imbalance and in the presence of a parabolic confinement. We analyze the magnetic structure as a function of the repulsive interaction strength and polarization. In the first part of the paper we perform unrestricted Hartree-Fock calculations for the 2D case and find that above a critical interaction strength $U_c$ the system turns ferromagnetic at the edge of the trap, in agreement with the ferromagnetic Stoner instability of a homogeneous system away from half-filling. For $U<U_c$ we find a canted antiferromagnetic structure in the Mott region in the center and a partially polarized compressible edge. The antiferromagnetic order in the Mott plateau is perpendicular to the direction of the imbalance. In this regime the same qualitative behavior is expected for 2D and 3D systems. In the second part of the paper we give a general discussion of magnetic structures above $U_c$. We argue that spin conservation leads to nontrivial textures, both in the ferromagnetic polarization at the edge and for the Neel order in the Mott plateau. We discuss differences in magnetic structures for 2D and 3D cases.

cond-mat.quant-gas↗

arXiv2009

Is the nature of magnetic order in copper-oxides and in iron-pnictides different?

We use the results of first-principles electronic structure calculations and a strong coupling perturbation approach, together with general theoretical arguments, to illustrate the differences in super-exchange interactions between the copper-oxides and iron-pnictides. We show that the two magnetic ground states can be understood in a simple manner within the same theoretical foundation. Contrary to the emerging view that magnetic order in the iron-pnictides is of itinerant nature, we argue that the observed magnetic moment is small because of frustration introduced by the electrons of the Fe orbitals as they compete to impose their preferred magnetic ordering.

cond-mat.str-el↗

arXiv1999

Reply to Comment on "Stripes and the t-J Model"

This is a Reply to the Comment of S.R. White and D.J. Scalapino [cond-mat/9907243] on our recent paper ``Stripes and the t-J Model'' [Physical Review Letters 83, 132 (1999) and cond-mat/9812022].

cond-mat.str-el↗

arXiv1999

Classical Phase Fluctuations in High Temperature Superconductors

Phase fluctuations of the superconducting order parameter play a larger role in the cuprates than in conventional BCS superconductors because of the low superfluid density of a doped insulator. In this paper, we analyze an XY model of classical phase fluctuations in the high temperature superconductors using a low-temperature expansion and Monte Carlo simulations. In agreement with experiment, the value of the superfluid density at temperature T=0 is a quite robust predictor of Tc, and the evolution of the superfluid density with T, including its T-linear behavior at low temperature, is insensitive to microscopic details.

cond-mat.supr-con↗