arXiv subjects

Markos Skoulatos

Publications and source records attributed to Markos Skoulatos.

8 recordsLinked to original sources

arXiv2020

Tetragonal mixed system $Cs_2CuCl_{4-x}Br_x$ complemented by the tetragonal phase realisation of $Cs_2CuCl_4$

Realisation of the tetragonal phase of $Cs_2CuCl_4$ is possible using specific crystal growth conditions at a temperature below $281K$. This work deals with the comparison of the magnetic susceptibility and the magnetization of this new tetragonal compound with the magnetic behaviour of tetragonal $Cs_2CuCl_{2.9}Br_{1.1}$, $Cs_2CuCl_{2.5}Br_{1.5}$, $Cs_2CuCl_{2.2}Br_{1.8}$ and presents consistent results for such quasi $2-D$ antiferromagnets. Structural investigation at low temperature for $Cs_2CuCl_{2.2}Br_{1.8}$ shows no phase transition. The structure remains in the tetragonal symmetry $I4/mmm$. Furthermore, several magnetic reflections corresponding to the propagation vector $k = (0, 0, 0)$ are observed for this tetragonal compound through neutron diffraction experiments below the magnetic phase transition at $T_N = 11.3K$ confirming its antiferromagnetic nature.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic phase diagram of the triangular antiferromagnetic $Cs_2CuCl_{4-x}Br_x$ mixed system

The novel magnetic phase diagram of the $Cs_2CuCl_{4-x}Br_x$ mixed system is established by means of single crystal neutron diffraction in the lowest temperature region and zero magnetic field. Two long-range ordered magnetic phases exist in this mixed system depending on the Cl/Br concentration. In the rich Cl concentration range, the ordered magnetic state occurs below the ordering temperature $T_N = 0.51(1)K$ for $Cs_2CuCl_3Br_1$ and at $Cs_2CuCl_{2.6}Br_{1.4}$ below $T_N = 0.24(2)K$. Magnetic order with a temperature-independent position $(0, 0.573(1), 0)$ below the ordering temperature $T_N = 0.63(1)K$ appears in the rich Br concentration for $Cs_2CuCl_{0.6}Br_{3.4}$. Between the rich Cl and rich Br concentration ranges (two magnetic phases), there is a range of x without magnetic order down to $50mK$. A suggestion about the magnetic exchange paths in the $bc$-layer for different regimes is presented, which can be controlled depending on the preferred Br-occupation in the [CuX4] tetrahedra. The density functional theory (DFT) calculations of the exchange coupling constants J, J^' for some ordered compositions of the mixed system $Cs_2CuCl_{4-x}Br_x$ indicate that these are not frustrated.

cond-mat.mtrl-sci↗

arXiv2018

Non-reciprocal magnons in non-centrosymmetric MnSi

Using two cold-neutron triple-axis spectrometers we have succeeded in fully mapping out the field-dependent evolution of the non-reciprocal magnon dispersion relations in all magnetic phases of MnSi. The non-reciprocal nature of the dispersion manifests itself in a full asymmetry (non-reciprocity) of the dynamical structure factor $S(q, E, μ_0 H_{int})$ with respect to flipping either the direction of the applied magnetic field $μ_0 H_{int}$, the reduced momentum transfer $q$, or the energy transfer $E$.

cond-mat.str-el↗

arXiv2018

Giant pressure dependence and dimensionality switching in a metal-organic quantum antiferromagnet

We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [(CuF$_2$(H$_2$O)$_2$)$_2$pyrazine]. At zero pressure, this material realizes a quasi-two-dimensional (Q2D) spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 upon compression. Above 18 kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the Q2D lattice into weakly coupled chains (Q1D). We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction results with quantitative modeling using spin-polarized density functional theory.

cond-mat.mtrl-sci↗

arXiv2017

Spin excitation anisotropy in optimal-isovalent-doped superconductor BaFe2(As0.7P0.3)2

We use neutron polarization analysis to study spin excitation anisotropy in the optimal-isovalent-doped superconductor BaFe2(As0.7P0.3)2 (Tc = 30 K). Different from optimally hole and electron-doped BaFe2As2, where there is a clear spin excitation anisotropy in the paramagnetic tetragonal state well above Tc, we find no spin excitation anisotropy for energies above 2 meV in the normal state of BaFe2(As0.7P0.3)2. Upon entering the superconducting state, significant spin excitation anisotropy develops at the antiferromagnetic (AF) zone center QAF = (1, 0, L = odd), while magnetic spectrum is isotropy at the zone boundary Q = (1, 0, L = even). By comparing temperature, wave vector, and polarization dependence of the spin excitation anisotropy in BaFe2(As0.7P0.3)2 and hole-doped Ba0.67K0.33Fe2As2 (Tc = 38 K), we conclude that such anisotropy arises from spin-orbit coupling and is associated with the nearby AF order and superconductivity.

cond-mat.supr-con↗

arXiv2017

Impurities in weakly coupled quantum spin chains Sr$_{2}$CuO$_{3}$ and SrCuO$_{2}$

We study the effect of non-magnetic Zn$^{2+}$ (spin-$0$) and magnetic Ni$^{2+}$ (spin-$1$) impurities on the ground state and low-lying excitations of the quasi-one-dimensional spin-$1/2$ Heisenberg antiferromagnet Sr$_{2}$CuO$_{3}$ using inelastic neutron scattering, specific heat and bulk magnetization measurements. We show that 1 \% Ni$^{2+}$ doping in Sr$_2$CuO$_3$ results in a sizable spin gap in the spinon excitations, analogous to the case of Ni-doped SrCuO$_2$ previously reported [ref. 1]. However, a similar level of Zn$^{2+}$ doping in SrCuO$_2$, investigated here for comparison, did not reveal any signs of a spin gap. Magnetic ordering temperature was found to be suppressed in the presence of both Zn$^{2+}$ and Ni$^{2+}$ impurities, however, the rate of suppression due to Ni$^{2+}$ was found to be much more pronounced than for Zn$^{2+}$. Effect of magnetic field on the ordering temperature is investigated. We found that with increasing magnetic field, not only the magnetic ordering temperature gradually increases but the size of specific heat anomaly associated with the magnetic ordering also progressively enhances, which can be qualitatively understood as due to the field induced suppression of quantum fluctuations.

cond-mat.str-el↗

arXiv2016

Effects of quantum impurity spins on the magnetic properties of zigzag and linear spin chains

We investigated the magnetic ground state and low-energy excitations of the spin chains compounds SrCuO$_{2}$ (zigzag chains) and Sr$_{2}$CuO$_{3}$ (linear chains) in the presence of quantum impurities induced by lightly doping ($\leq 1 \%$) with Zn$^{2+}$ ($S = 0$), Co$^{2+}$ ($S =1/2$) and Ni$^{2+}$ ($S = 1$) impurities at the Cu$^{2+}$ site. We show that the ground states and the nature of low-lying excitations (i.e., gapped or gapless) depend on the spin state and symmetry of the defects. For Ni doped chains a spin gap is observed but for Zn and Co doping the excitations remain gapless. Co-doped chains exhibit magnetic order with critical temperatures significantly enhanced compared to those of the pristine compounds. In the specific case of 1 \% Co impurities, the linear chains exhibit long-range order below 11 K, while the zigzag chain is characterized by a quasi-long range ordered phase below 6 K with correlation lengths of about 12\textit{a} and 40\textit{c} units along the crystal axes \textit{a} and \textit{c}, respectively. The different magnetic behaviours of these two compounds with comparable intra- and interchain couplings underpin the role of spin frustration in the zigzag chains.

cond-mat.str-el↗

arXiv2015

Structural and magnetic phase transitions near optimal superconductivity in BaFe$_2$(As$_{1-x}$P$_x$)$_2$

We use nuclear magnetic resonance (NMR), high-resolution x-ray and neutron scattering to study structural and magnetic phase transitions in phosphorus-doped BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at $x=0.3$. However, we show that the tetragonal-to-orthorhombic structural ($T_s$) and paramagnetic to antiferromagnetic (AF, $T_N$) transitions in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ are always coupled and approach to $T_N\approx T_s \ge T_c$ ($\approx 29$ K) for $x=0.29$ before vanishing abruptly for $x\ge 0.3$. These results suggest that AF order in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ disappears in a weakly first order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.

cond-mat.supr-con↗