arXiv subjects

B. Lorenz

Publications and source records attributed to B. Lorenz.

At least 37 records · Page 2Linked to original sources

arXiv2011

The Unusual Superconducting State at 49 K in Electron-Doped CaFe2As2 at Ambient

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe2As2 via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at ~ 49 K, which has a low critical field ~ 4 Oe, and the other at ~ 21 K, with a much higher critical field > 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe2As2 (or Ae122), where Ae = Ca, Sr or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature (Tc) only up to 38 K and pressurization has been reported to produce superconductivity with a Tc up to 30 K. The unusual 49 K phase detected will be discussed.

cond-mat.supr-con↗

arXiv2011

Giant Magnetoelectric Effect in HoAl_3(BO_3)_4

A giant magnetoelectric polarization is found in HoAl$_3$(BO$_3$)$_4$. The polarization in transverse field geometry at 70 kOe reaches 3600 $μC/m^2$ which is significantly higher than reported values of linear magnetoelectric or even multiferroic compounds. The magnetostrictive effect is also measured and compared with the magnetoelectricity. The results show that spin-lattice coupling in HoAl$_3$(BO$_3$)$_4$ is extremely strong and that the magnetic field causes a large polar distortion of the ionic positions in the unit cell.

cond-mat.str-el↗

arXiv2011

Fluid squeeze-out between rough surfaces: comparison of theory with experiment

We study the time dependency of the (average) interfacial separation between an elastic solid with a flat surface and a rigid solid with a randomly rough surface, squeezed together in a fluid. We use an analytical theory describing the fluid flow factors, based on the Persson contact mechanics theory and the Bruggeman effective medium theory, to calculate the removal of the fluid from the contacting interface of the two solids. In order to test this approach, we have performed simple squeeze-out experiments. The experimental results are compared to the theory predictions.

cond-mat.soft↗

arXiv2011

Robust Ferroelectric State in Multiferroic Mn$_{1-x}$Zn$_x$WO$_4$

We report the remarkably robust ferroelectric state in the multiferroic compound Mn$_{1-x}$Zn$_x$WO$_4$. The substitution of the magnetic Mn$^{2+}$ with nonmagnetic Zn$^{2+}$ reduces the magnetic exchange and provides control of the various magnetic and multiferroic states of MnWO$_4$. Only 5 % of Zn substitution results in a complete suppression of the frustrated collinear (paraelectric) low temperature phase. The helical magnetic and ferroelectric phase develops as the ground state. The multiferroic state is stable up to a high level of substitution of more than 50 %. The magnetic, thermodynamic, and dielectric properties as well as the ferroelectric polarization of single crystals of Mn$_{1-x}$Zn$_x$WO$_4$ are studied for different substitutions up to x=0.5. The magnetic phases have been identified in single crystal neutron scattering experiments. The ferroelectric polarization scales with the neutron intensity of the incommensurate peak of the helical phase.

cond-mat.str-el↗

arXiv2010

Magnetic and multiferroic phases of single-crystalline Mn$_{0.85}$Co$_{0.15}$WO$_4$

The magnetic and multiferroic phase diagram of Mn$_{0.85}$Co$_{0.15}$WO$_4$ single crystals is investigated by means of magnetic, heat capacity, dielectric, polarization, and neutron scattering experiments. Three magnetic phase transitions are detected through distinct anomalies in all physical quantities. The ferroelectric polarization is observed only along the b-axis below 10 K but not along the a-axis as recently suggested. The magnetic phases studied by neutron scattering are very complex. Up to four different magnetic structures, partially coexisting at certain temperature ranges, have been identified. Upon decreasing temperature two commensurate phases (AF4, AF1) are followed by an incommensurate phase (AF5) and a second incommensurate phase (AF2) is detected as a minor phase. The ferroelectric polarization is possibly associated with both (AF2 and AF5) phases.

cond-mat.str-el↗

arXiv2010

Magnetoelectricity and Magnetostriction due to the Rare Earth Moment in TmAl$_3$(BO$_3$)$_4$

The magnetic properties, the magnetostriction, and the magnetoelectric effect in the d-electron free rare-earth aluminum borate TmAl$_3$(BO$_3$)$_4$ are investigated between room temperature and 2 K. The magnetic susceptibility reveals a strong anisotropy with the hexagonal c-axis as the hard magnetic axis. Magnetostriction measurements show a large effect of an in-plane field reducing both, the a- and c-axis lattice parameters. The magnetoelectric polarization change in a- and c-directions reaches up to 300 $μ$C/m$^2$ at 70 kOe with the field applied along the a-axis. The magnetoelectric polarization is proportional to the lattice contraction in magnetic field. The results of this investigation prove the existence of a significant coupling between the rare earth magnetic moment and the lattice in $R$Al$_3$(BO$_3$)$_4$ compounds ($R$ = rare earth). They further show that the rare earth moment itself will generate a large magnetoelectric effect which makes it easier to study and to understand the origin of the magnetoelectric interaction in this class of materials.

cond-mat.str-el↗

arXiv2010

Fluid squeeze-out between solids with rough surfaces

We study the fluid squeeze-out from the interface between an elastic solid with a flat surface and a rigid solid with a randomly rough surface. As an application we discuss fluid squeeze-out between a tire tread block and a road surface. Some implications for the leakage of seals are discussed, and experimental data are presented to test the theory.

cond-mat.soft↗

arXiv2010

On the dependence of the leak-rate of seals on the skewness of the surface height probability distribution

Seals are extremely useful devices to prevent fluid leakage. We present experimental result which show that the leak-rate of seals depend sensitively on the skewness in the height probability distribution. The experimental data are analyzed using the critical-junction theory. We show that using the top-power spectrum result in good agreement between theory and experiment.

cond-mat.soft↗

arXiv2009

Leak-rate of seals: effective medium theory and comparison with experiment

Seals are extremely useful devices to prevent fluid leakage. We present an effective medium theory of the leak-rate of rubber seals, which is based on a recently developed contact mechanics theory. We compare the theory with experimental results for seals consisting of silicon rubber in contact with sandpaper and sand-blasted PMMA surfaces.

cond-mat.soft↗

arXiv2009

Magnetoelectric Effect and Spontaneous Polarization in HoFe$_3$(BO$_3$)$_4$ and Ho$_{0.5}$Nd$_{0.5}$Fe$_3$(BO$_3$)$_4$

The thermodynamic, magnetic, dielectric, and magnetoelectric properties of HoFe$_3$(BO$_3$)$_4$ and Ho$_{0.5}$Nd$_{0.5}$Fe$_3$(BO$_3$)$_4$ are investigated. Both compounds show a second order Neél transition above 30 K and a first order spin reorientation transition below 10 K. HoFe$_3$(BO$_3$)$_4$ develops a spontaneous electrical polarization below the Neél temperature (T$_N$) which is diminished in external magnetic fields. No magnetoelectric effect could be observed in HoFe$_3$(BO$_3$)$_4$. In contrast, the solid solution Ho$_{0.5}$Nd$_{0.5}$Fe$_3$(BO$_3$)$_4$ exhibits both, a spontaneous polarization below T$_N$ and a magnetoelectric effect at higher fields that extends to high temperatures. The superposition of spontaneous polarization, induced by the internal magnetic field in the ordered state, and the magnetoelectric polarizations due to the external field results in a complex behavior of the total polarization measured as a function of temperature and field.

cond-mat.str-el↗

arXiv2009

Heat transfer between elastic solids with randomly rough surfaces

We study the heat transfer between elastic solids with randomly rough surfaces. We include both the heat transfer from the area of real contact, and the heat transfer between the surfaces in the noncontact regions. We apply a recently developed contact mechanics theory, which accounts for the hierarchical nature of the contact between solids with roughness on many different length scales. For elastic contact, at the highest (atomic) resolution the area of real contact typically consists of atomic (nanometer) sized regions, and we discuss the implications of this for the heat transfer. For solids with very smooth surfaces, as is typical in many modern engineering applications, the interfacial separation in the non-contact regions will be very small, and for this case we show the importance of the radiative heat transfer associated with the evanescent electromagnetic waves which exist outside of all bodies.

cond-mat.soft↗

arXiv2009

Leak-rate of seals: comparison of theory with experiment

Seals are extremely useful devices to prevent fluid leakage. We present experimental results for the leak-rate of rubber seals, and compare the results to a novel theory, which is based on percolation theory and a recently developed contact mechanics theory. We find good agreement between theory and experiment.

cond-mat.soft↗

arXiv2009

The superconductor KxSr(1-x)Fe2As2: Normal state and superconducting properties

The normal state and superconducting properties are investigated in the phase diagram of K_xSr_{1-x}Fe_2As_2 for 0<x<1. The ground state upper critical field, H_{c2}(0), is extrapolated from magnetic field dependent resistivity measurements. H_{c2}(0) scales with the critical temperature, T_c, of the superconducting transition. In the normal state the Seebeck coefficient is shown to experience a dramatic change near a critical substitution of x=0.3. This is associated with the formation of a spin density wave state above the superconducting transition temperature. The results provide strong evidence for the reconstruction of the Fermi surface with the onset of magnetic order.

cond-mat.supr-con↗

arXiv2009

The Synthesis and Characterization of LiFeAs and NaFeAs

The newest homologous series of superconducting As-pnictides, LiFeAs (Li111) and NaFeAs (Na111) have been synthesized and investigated. Both crystallize with the layered tetragonal anti-PbFCl-type structure in P4/nmm space group. Polycrystalline samples and single-crystals of Li111 and Na111 display superconducting transitions at ~ 18 K and 12-25 K, respectively. No magnetic order has been found in either compound, although a weak magnetic background is clearly in evidence. The origin of the carriers and the stoichiometric compositions of Li111 and Na111 were explored.

cond-mat.supr-con↗

arXiv2009

High Pressure Studies on Fe-Pnictide Superconductors

A review of high-pressure studies on Fe-pnictide superconductors is given. The pressure effects on the magnetic and superconducting transitions are discussed for different classes of doped and undoped FeAs-compounds, ROFeAs (R = rare earth), AeFe2As2 (Ae = Ca, Sr, Ba), and AFeAs (A = Li, Na). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting Tc increases with pressure for underdoped FeAs-pnictides, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. The undoped LaOFeAs and AeFe2As2 become superconducting under pressure although nonhydrostatic pressure conditions seem to play a role in CaFe2As2. The superconductivity in the (undoped) AFeAs is explained as a chemical pressure effect due to the volume contraction caused by the small ionic size of the A-elements. The binary FeSe shows the largest pressure coefficient of Tc in the Se-deficient superconducting phase.

cond-mat.supr-con↗

arXiv2009

Pressure shift of the superconducting T_c of LiFeAs

The effect of hydrostatic pressure on the superconductivity in LiFeAs is investigated up to 1.8 GPa. The superconducting transition temperature, T_c, decreases linearly with pressure at a rate of 1.5 K/GPa. The negative pressure coefficient of T_c and the high ambient pressure T_c indicate that LiFeAs is the high-pressure analogue of the isoelectronic SrFe_2As_2 and BaFe_2As_2.

cond-mat.supr-con↗

arXiv2008

Magnetic switching and phase competition in the multiferroic antiferromagnet $\rm Mn_{1-x}Fe_xWO_4$

Elastic neutron scattering is used to study the spin correlations in the multiferroic $\rm Mn_{1-x}Fe_{x}WO_4$ with $x=0.035, 0.05$ and 0.10. The noncollinear, incommensurate (ICM) magnetic structure associated with the ferroelectric (FE) phase in pure $\rm MnWO_4$ is suppressed at $x=0.035$ and completely absent at $x=0.10$. The ICM spin order and FE phase can be restored by applying a magnetic field along the spin easy-axis. The low-$T$ commensurate magnetic structure extends in both H/T with increasing Fe concentration. The systematic evolution of the magnetic and electric properties indicates that the noncollinear ICM spin order results from competing magnetic interactions and its stabilization can be tuned by the internal ($x$) or external (magnetic field) perturbations.

cond-mat.str-el↗

arXiv2008

Interfacial separation between elastic solids with randomly rough surfaces: comparison of experiment with theory

We study the average separation between an elastic solid and a hard solid with a nominal flat but randomly rough surface, as a function of the squeezing pressure. We present experimental results for a silicon rubber (PDMS) block with a flat surface squeezed against an asphalt road surface. The theory shows that an effective repulse pressure act between the surfaces of the form p proportional to exp(-u/u0), where u is the average separation between the surfaces and u0 a constant of order the root-mean-square roughness, in good agreement with the experimental results.

cond-mat.soft↗