arXiv subjects

W. Scherer

Publications and source records attributed to W. Scherer.

At least 19 recordsLinked to original sources

arXiv2021

Pressure-induced excitations in the out-of-plane optical response of the nodal-line semimetal ZrSiS

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS.

cond-mat.mtrl-sci↗

arXiv2021

Li$_2$Sr[MnN]$_2$: a magnetically ordered, metallic nitride

Li$_2$Sr[MnN]$_2$ single crystals were successfully grown out of Li rich flux. The crystal structure was determined by single crystal X-ray diffraction and revealed almost linear $-$N$-$Mn$-$N$-$Mn$-$ chains as central structural motif. Tetragonal columns of this air and moisture sensitive nitridomanganate were employed for electrical transport, heat capacity, and anisotropic magnetization measurements. Both the electronic and magnetic properties are most remarkable, in particular the linear increase of the magnetic susceptibility with temperature that is reminiscent of underdoped cuprate and Fe-based superconductors. Clear indications for antiferromagnetic ordering at $T_{\rm N} = 290$ K were obtained. Metallic transport behavior is experimentally observed in accordance with electronic band structure calculations.

cond-mat.mtrl-sci↗

arXiv2020

Cooperative cluster-Jahn-Teller effect as a possible route to antiferroelectricity

We report the observation of an antipolar phase in cubic GaNb$_4$S$_8$ driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb$_4$S$_4$ molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at $T_\mathrm{JT}=31$\,K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above $T_\mathrm{JT}$. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb$_4$S$_8$ may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferrolectric.

cond-mat.str-el↗

arXiv2020

Indications for Lifshitz transitions in the nodal-line semimetal ZrSiTe induced by interlayer interaction

The layered material ZrSiTe is currently extensively investigated as a nodal-line semimetal with Dirac-like band crossings protected by nonsymmorphic symmetry close to the Fermi energy. A recent infrared spectroscopy study on ZrSiTe under external pressure found anomalies in the optical response, providing hints for pressure-induced phase transitions at $\approx$4.1 and $\approx$6.5 GPa. By pressure-dependent Raman spectroscopy and x-ray diffraction measurements combined with electronic band structure calculations we find indications for two pressure-induced Lifshitz transitions with major changes in the Fermi surface topology in the absence of lattice symmetry changes. These electronic phase transitions can be attributed to the enhanced interlayer interaction induced by external pressure. Our findings demonstrate the crucial role of the interlayer distance for the electronic properties of layered van der Waals topological materials.

cond-mat.mtrl-sci↗

arXiv2017

The collapsed tetragonal phase as a strongly covalent and fully nonmagnetic state: persistent magnetism with interlayer As-As bond formation in Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_2$As$_2$

A well-known feature of CaFe$_{2}$As$_{2}$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As-As bond. Using detailed X-ray scattering and spectroscopy, we find that Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_{2}$As$_{2}$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As-As bonds. Our density functional theory calculations reveal that the Fe-As bond geometry is critical for stabilizing magnetism and that the pressure-induced drop in the $c$ lattice parameter observed in pure CaFe$_{2}$As$_{2}$ is mostly due to a constriction within the FeAs planes. These phenomena are best understood using an often overlooked explanation for the equilibrium Fe-As bond geometry, which is set by a competition between covalent bonding and exchange splitting between strongly hybridized Fe $3d$ and As $4p$ states. In this framework, the collapsed tetragonal phase emerges when covalent bonding completely wins out over exchange splitting. Thus the collapsed tetragonal phase is properly understood as a strong, covalent phase that is fully nonmagnetic with the As-As bond forming as a byproduct.

cond-mat.str-el↗

arXiv2012

Superconductivity at Tc = 44 K in LixFe2Se2(NH3)y

Following a recent proposal by Burrard-Lucas et al. [unpublished, arXiv: 1203.5046] we intercalated FeSe by Li in liquid ammonia. We report on the synthesis of new LixFe2Se2(NH3)y phases as well as on their magnetic and superconducting properties. We suggest that the superconducting properties of these new hybride materials appear not to be influenced by the presence of electronically-innocent Li(NH2) salt moieties. Indeed, high onset temperatures of 44 K and shielding fractions of almost 80% were only obtained in samples containing exclusively Lix(NH3)y moieties acting simultaneously as electron donors and spacer units. The c-axis of the new intercalated phases is strongly enhanced when compared to the alkali-metal intercalated iron selenides A1-xFe2-ySe2 with A = K, Rb, Cs, Tl with T c = 32 K.

cond-mat.supr-con↗

arXiv2012

Low field extension for magnetometers (TinyBee) used for investigations on low-dimensional superconductors with Bc1 < 5G

In this article a simple and easy to install low magnetic field extension of the SQUID magnetometer Quantum Design MPMS-7 is described. This has been accomplished by complementing the MPMS-7 magnet control system with a laboratory current supply for the low magnetic field region (B \leq 200G). This hard- and software upgrade provides a significant gain in the magnetic field accuracy up to an order of magnitude compared with the standard instrument's setup and is improving the resolution to better than 0.01G below 40G. The field control has been integrated into the Quantum Design MultiVu software for a transparent and user-friendly operation of this extension. The improvements achieved are especially useful, when low magnetic field strengths (B < 1G) are required at high precision. The specific advantages of this application are illustrated by sophisticated magnetic characterisation of lowdimensional superconductors like Sc3CoC4 and SnSe2{Co(η-C5H5)2}x.

cond-mat.supr-con↗

arXiv2010

Possible indicators for low dimensional superconductivity in the quasi-1D carbide Sc3CoC4

The transition metal carbide Sc3CoC4 consists of a quasi-one-dimensional (1D) structure with [CoC4]$_{\inft}$ polyanionic chains embedded in a scandium matrix. At ambient temperatures Sc3CoC4 displays metallic behavior. At lower temperatures, however, charge density wave formation has been observed around 143K which is followed by a structural phase transition at 72K. Below T^onset_c = 4.5K the polycrystalline sample becomes superconductive. From Hc1(0) and Hc2(0) values we could estimate the London penetration depth (λ_L ~= 9750 Angstroem) and the Ginsburg-Landau (GL) coherence length (ξ_GL ~= 187 Angstroem). The resulting GL-parameter (κ ~= 52) classifies Sc3CoC4 as a type II superconductor. Here we compare the puzzling superconducting features of Sc3CoC4, such as the unusual temperature dependence i) of the specific heat anomaly and ii) of the upper critical field H_c2(T) at T_c, and iii) the magnetic hysteresis curve, with various related low dimensional superconductors: e.g., the quasi-1D superconductor (SN)_x or the 2D transition-metal dichalcogenides. Our results identify Sc3CoC4 as a new candidate for a quasi-1D superconductor.

cond-mat.supr-con↗

arXiv2009

Intermediate-valence behavior of the transition-metal oxide CaCu$_3$Ru$_4$O$_{12}$

The transition--metal oxide CaCu$_3$Ru$_4$O$_{12}$ with perovskite--type structure shows characteristic properties of an intermediate--valence system. The temperature--dependent susceptibility exhibits a broad maximum around $150 - 160$ K. At this temperature, neutron powder diffraction reveals a small but significant volume change whereby the crystal structure is preserved. Moreover, the temperature--dependent resistivity changes its slope. NMR Knight shift measurements of Ru reveal a cross--over from high temperature paramagnetic behavior of localized moments to itinerant band states at low temperatures. Additional density--functional theory calculations can relate the structural anomaly with the $d$--electron number. The different experimental and calculational methods result in a mutually consistent description of CaCu$_3$Ru$_4$O$_{12}$ as an intermediate--valent system in the classical sense of having low--energy charge fluctuations.

cond-mat.str-el↗

arXiv2009

Antiferromagnetic behavior in CeCo$_{9}$Ge$_{4}$

We investigate the novel intermetallic ternary compounds \emph{R}Co$_{9}$Ge$_{4}$ with \emph{R} = La and Ce by means of $X$-ray diffraction, susceptibility and specific heat measurements. CeCo$_{9}$Ge$_{4}$ crystallizes in the space group ${I}$ 4/ ${mcm}$ and is characterized by the coexistence of two different magnetic sublattices. The Ce-based sublattice, with an effective moment close to the expected value for a Ce$^{3+}$-ion, exhibits a magnetically ordered ground state with $T_{\mathrm{N}}=12.5$ K. The Co-based sublattice, however, exhibits magnetic moments due to itinerant 3$d$ electrons. The magnetic specific heat contribution of the Ce-sublattice is discussed in terms of a resonance-level model implying the interplay between an antiferromagnetic phase transition and the Kondo-effect and an underlying Schottky-anomaly indicating a crystal field level scheme splitting into three twofold degenerated micro states ($Δ_1 = 69$ K, $Δ_2 = 133$ K).

cond-mat.str-el↗

arXiv2008

Evolution of Quantum Criticality in CeNi_{9-x}Cu_xGe_4

Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi_{9-x}Cu_xGe_4 (0 <= x <= 1) reveal a continuous tuning of the ground state by Ni/Cu substitution from an effectively fourfold degenerate non-magnetic Kondo ground state of CeNi_9Ge_4 (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold degenerate ground state in CeNi_8CuGe_4 with T_N = 175 +- 5 mK. Quantum critical behavior, C/T ~ χ~ -ln(T), is observed for x about 0.4. Hitherto, CeNi_{9-x}Cu_xGe_4 represents the first system where a substitution-driven quantum phase transition is connected not only with changes of the relative strength of Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.

cond-mat.str-el↗

arXiv2008

Amido Rhenium Trioxides: Cases of Hindered Agostic C-H-M Interactions?

The amido rhenium trioxides of composition (iPr2N)ReO3, (iPrCyN)ReO3 and (Cy2N)ReO3 (Cy = cyclohexyl) were synthesized in a one pot reaction starting from Re2O7, Me3SiCl and the corresponding amines (iPr)2NH, (iPr)(Cy)NH, and (Cy)2NH, respectively. In the solid state the amido ligands of all three complexes are asymmetrically coordinated to the ReO3 core allowing for one short Re-H-C contact in each case which might indicate the presence of beta-agostic interaction. However, analysis of the charge density distribution provided us clear-cut criteria that beta-agostic interactions are suppressed by the trans-influence of the oxo-groups. Comparison with structurally related tetra-coordinated d0 titanium amido complexes highlighted a systematic concept how the extent of beta-agostic interactions in these complexes can be controlled by reducing the trans-influence of the co-ligands. We therefore suggest to employ the expression "hindered agostic interactions" in cases where covalent M-H-C are in principle supported by vacant coordination sites at Lewis-acidic metal centers but are actually hindered for electronic reasons. We further point out that electronic hindrance originating from the transinfluence might be overruled by changing the s/p-donor characteristics of ligands.

physics.chem-ph↗

arXiv2005

Poly-MTO, {(CH_3)_{0.92} Re O_3}_\infty, a Conducting Two-Dimensional Organometallic Oxide

Polymeric methyltrioxorhenium, {(CH_{3})_{0.92}ReO_{3}}_{\infty} (poly-MTO), is the first member of a new class of organometallic hybrids which adopts the structural pattern and physical properties of classical perovskites in two dimensions (2D). We demonstrate how the electronic structure of poly-MTO can be tailored by intercalation of organic donor molecules, such as tetrathiafulvalene (TTF) or bis-(ethylendithio)-tetrathiafulvalene (BEDT-TTF), and by the inorganic acceptor SbF$_3$. Integration of donor molecules leads to a more insulating behavior of poly-MTO, whereas SbF$_3$ insertion does not cause any significant change in the resistivity. The resistivity data of pure poly-MTO is remarkably well described by a two-dimensional electron system. Below 38 K an unusual resistivity behavior, similar to that found in doped cuprates, is observed: The resistivity initially increases approximately as $ρ\sim$ ln$(1/T$) before it changes into a $\sqrt{T}$ dependence below 2 K. As an explanation we suggest a crossover from purely two-dimensional charge-carrier diffusion within the \{ReO$_2$\}$_{\infty}$ planes at high temperatures to three-dimensional diffusion at low temperatures in a disorder-enhanced electron-electron interaction scenario (Altshuler-Aronov correction). Furthermore, a linear positive magnetoresistance was found in the insulating regime, which is caused by spatial localization of itinerant electrons at some of the Re atoms, which formally adopt a $5d^1$ electronic configuration. X-ray diffraction, IR- and ESR-studies, temperature dependent magnetization and specific heat measurements in various magnetic fields suggest that the electronic structure of poly-MTO can safely be approximated by a purely 2D conductor.

cond-mat.mtrl-sci↗

arXiv2005

Unusual Non-Fermi Liquid Behavior of Ce$_{1-x}$La$_{x}$Ni$_{9}$Ge$_4$ Analyzed in a Single Impurity Anderson Model with Crystal Field Effects

CeNi$_{9}$Ge$_4$ exhibits unusual non-Fermi liquid behavior with the largest ever recorded value of the electronic specific heat $ΔC/T \cong 5.5$ JK$^{-2}$mol$^{-1}$ without showing any evidence of magnetic order. Specific heat measurements show that the logarithmic increase of the Sommerfeld coefficient flattens off below 200 mK. In marked contrast, the local susceptibility $Δχ$ levels off well above 200 mK and already becomes constant below 1 K. Furthermore, the entropy reaches 2$R$ln2 below 20 K corresponding to a four level system. An analysis of $C$ and $χ$ was performed in terms of an $SU(N=4)$ single impurity Anderson model with additional crystal electric field (CEF) splitting. Numerical renormalization group calculations point to a possible consistent description of the different low temperature scales in $Δc$ and $Δχ$ stemming from the interplay of Kondo effect and crystal field splitting.

cond-mat.str-el↗

arXiv2005

Possible Localization Behavior of the Inherent Conducting Polymer (CH$_3$)$_{0.9}$ReO$_3$

Polymeric methyltrioxorhenium (poly-MTO) represents the first example of an inherent conducting organometallic oxide. It adopts the structural motives and transport properties of some classical perovskites in two dimensions. In this study we present resistivity data down to 30 mK which exhibit a crossover from a metallic (d$ρ$/d$T >$ 0) to an insulating (d$ρ$/d$T <$ 0) behavior at about 30 K. Below 30 K an unusual resistivity behavior, similar to that of some doped cuprate systems, is observed: initially the resistivity increases approximately as $ρ\sim$ log$(1/T$) before it starts to saturate below 2 K. Furthermore, a linear positive magnetoresistance is found (up to 7 T). Temperature dependent magnetization and specific heat measurements in various magnetic fields indicate that the unusual resistivity behavior may be driven by spatial localization of the d$^1$ moments at the Re atoms.

cond-mat.mtrl-sci↗

arXiv2005

Crossover from Single-Ion to Coherent Non-Fermi Liquid Behavior in Ce$_{1-x}$La$_x$Ni$_9$Ge$_4$

We report specific heat and magneto-resistance studies on the compound Ce${}_{1-x}$La${}_x$Ni${}_9$Ge${}_4$ for various concentrations over the entire stoichiometric range. Our data reveal single-ion scaling with Ce-concentration between $x = 0.1$ and 0.95. Furthermore, CeNi${}_9$Ge${}_4$ turns out to have the largest ever recorded value of the electronic specific heat $Δc/T \approx$ 5.5 J $\rm K^{-2}mol^{-1}$ at $T=0.08$ K which was found in Cerium f-electron lattice systems. In the doped samples $Δc/T$ increases logarithmically in the temperature range between 3 K and 50 mK typical for non-Fermi liquid (nFl) behavior, while $ρ$ exhibits a Kondo-like minimum around 30 K, followed by a single-ion local nFl behavior. In contrast to this, CeNi${}_9$Ge${}_4$ flattens out in $Δc/T$ below 300 mK and displays a pronounced maximum in the resistivity curve at 1.5 K indicating a coherent heavy fermion groundstate. These properties render the compound Ce${}_{1-x}$La${}_x$Ni${}_9$Ge${}_4$ a unique system on the borderline between Fermi liquid and nFl physics.

cond-mat.str-el↗

arXiv2005

Design of the electronic structure of poly-MTO

Polymeric methyltrioxorhenium (poly-MTO) is the first member of a new class of organometallic hybrids which adopts the structural motives and physical properties of classical perowskites in two dimensions. In this study we demonstrate how the electronic structure of poly-MTO can be tailored by intercalation of organic donor molecules such as tetrathiafulvalene (TTF). With increasing donor intercalation the metallic behavior of the parent compound, (CH$_{3}$)$_{0.92}$ReO$_{3} \cdot x%$ TTF ($x = 0$) becomes suppressed leading to an insulator at donor concentrations $x$ larger than 50. Specific heat, electric resistance and magnetic susceptibility studies indicate that an increasing amount of TTF causes the itinerant electrons of the poly-MTO matrix to localize.

cond-mat.mtrl-sci↗

arXiv2005

Possible canted antiferromagnetism in UCu$_9$Sn$_4$

We report on the new compound UCu${}_9$Sn${}_4$ which crystallizes in the tetragonal structure \emph{I}4/\emph{mcm} with lattice parameters $a = 8.600{\rmÅ}$ and $c = 12.359{\rmÅ}$. This compound is isotyp to the ferromagnetic systems RECu${}_9$Sn${}_4$ (RE = Ce, Pr, Nd) with Curie temperatures $T{}\rm_C$ = 5.5 K, 10.5 K and 15 K, respectively. UCu${}_9$Sn${}_4$ exhibits an uncommon magnetic behavior resulting in three different electronic phase transitions. Below 105 K the sample undergoes a valence transition accompanied by an entropy change of 0.5 Rln2. At 32 K a small hump in the specific heat and a flattening out in the susceptibility curve probably indicate the onset of helical spin order. To lower temperatures a second transition to antiferromagnetic ordering occurs which develops a small ferromagnetic contribution on lowering the temperature further. These results are strongly hinting for canted antiferromagnetism in UCu${}_9$Sn${}_4$.

cond-mat.str-el↗