arXiv subjects

F. Hellman

Publications and source records attributed to F. Hellman.

16 recordsLinked to original sources

arXiv2024

Nematicity of a Magnetic Helix

A system that possesses translational symmetry but breaks orientational symmetry is known as a nematic phase. While there are many examples of nematic phases in a wide range of contexts, such as in liquid crystals, complex oxides, and superconductors, of particular interest is the magnetic analogue, where the spin, charge, and orbital degrees of freedom of the electron are intertwined. The difficulty of spin nematics is the unambiguous realization and characterization of the phase. Here we present an entirely new type of magnetic nematic phase, which replaces the basis of individual spins with magnetic helices. The helical basis allows for the direct measurement of the order parameters with soft X-ray scattering and a thorough characterization of the nematic phase and its thermodynamic transitions. We discover two distinct nematic phases with unique spatio-temporal correlation signatures. Using coherent X-ray methods, we find that near the phase boundary between the two nematic phases, fluctuations coexist on the timescale of both seconds and sub-nanoseconds. Additionally, we have determined that the fluctuations occur simultaneously with a reorientation of the magnetic helices, indicating that there is spontaneous symmetry breaking and new degrees of freedom become available. Our results provide a novel framework for characterizing exotic phases and the phenomena presented can be mapped onto a broad class of physical systems.

cond-mat.mtrl-sci↗

arXiv2021

Structural tunability and origin of two-level systems in amorphous silicon

Amorphous silicon films prepared by electron beam evaporation have systematically and substantially greater atomic density for higher thickness, higher growth temperature, and slower deposition rate, reaching the density of crystalline Si when films of thickness greater than ~300 nm are grown at 425 $^{\circ}$C and at <1 $\r{A}$/sec. A combination of spectroscopic techniques provide insight into atomic disorder, local strains, dangling bonds, and nanovoids. Electron diffraction shows that the short-range order of the amorphous silicon is similar at all growth temperatures, but fluctuation electron microscopy shows that films grown above room-temperature show a form of medium-range order not previously observed in amorphous silicon. Atomic disorder and local strain obtained from Raman spectroscopy reduce with increasing growth temperature and show a non-monotonic dependence on thickness. Dangling bond density decreases with increasing growth temperature and is only mildly dependent on thickness. Positron annihilation Doppler broadening spectroscopy and electron energy loss spectroscopy show that nanovoids, and not density variations within the network, are responsible for reduced atomic density. Specific heat and mechanical loss measurements, which quantify the density of tunneling two-level systems, in combination with the structural data, suggest that two-level systems in amorphous silicon films are associated with nanovoids and their surroundings; which are in essence loosely bonded regions where atoms are less constrained.

cond-mat.mtrl-sci↗

arXiv2021

Decoupling between propagating acoustic waves and two-level systems in hydrogenated amorphous silicon

Specific heat measurements of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large density of two-level systems at low temperature. Annealing at 200 {\deg}C, well below the growth temperature, does not significantly affect the already-low internal friction or the sound velocity, but irreversibly reduces the non-Debye specific heat by an order of magnitude at 2 K, indicating a large reduction of the density of two-level systems. Comparison of the specific heat to the internal friction suggests that the two-level systems are uncharacteristically decoupled from acoustic waves, both before and after annealing. Analysis yields an anomalously low value of the coupling constant, which increases upon annealing but still remains anomalously low. The results suggest that the coupling constant value is lowered by the presence of hydrogen.

cond-mat.mtrl-sci↗

arXiv2020

Creation and confirmation of Hopfions in magnetic multilayer systems

Topological solitons have been studied for decades in classical field theories, and have started recently to impact condensed matter physics. Among those solitons, magnetic skyrmions are two-dimensional particle-like objects with a continuous winding of the magnetization, and magnetic Hopfions are three-dimensional topological solitons that can be formed from a closed loop of a twisted skyrmion string. Whereas intense research is underway with magnetic skyrmions towards a fundamental understanding and potential applications in advanced storage and logic devices, the experimental creation and confirmation of magnetic Hopfions has been elusive so far. Theoretical models suggest that Hopfions can be stabilized in frustrated or chiral magnetic systems, and that target skymions can be transformed into Hopfions by adapting their perpendicular magnetic anisotropy. Here, we present experimental evidence of magnetic Hopfions that were created in magnetic Ir/Co/Pt multilayers shaped into nanoscale disks, which are known to host target skyrmions. The three-dimensional spin texture, which distinguishes magnetic Hopfions from target skyrmions was confirmed by combining two advanced element-specific magnetic X-ray microscopy techniques with about 20-30nm lateral resolution, using X-ray magnetic circular dichroism effect as magnetic contrast mechanism in surface-sensitive X-ray photoemission electron microscopy and bulk-sensitive soft x-ray transmission microscopy. We anticipate that these results will stimulate further investigations of Hopfions with different topologies and their potential application in three-dimensional spintronics devices.

cond-mat.mes-hall↗

arXiv2020

Origin of Mechanical and Dielectric Losses from Two-Level Systems in Amorphous Silicon

Amorphous silicon contains tunneling two-level systems, which are the dominant energy loss mechanisms for amorphous solids at low temperatures. These two-level systems affect both mechanical and electromagnetic oscillators and are believed to produce thermal and electromagnetic noise and energy loss. However, it is unclear whether the two-level systems that dominate mechanical and dielectric losses are the same; the former relies on phonon-TLS coupling, with an elastic field coupling constant, $\gamma$, while the latter depends on a TLS dipole moment, $p_0$, which couples to the electromagnetic field. Mechanical and dielectric loss measurements as well as structural characterization were performed on amorphous silicon thin films grown by electron beam deposition with a range of growth parameters. Samples grown at 425 $^{\circ}$C show a large reduction of mechanical loss (34 times) and a far smaller reduction of dielectric loss (2.3 times) compared to those grown at room temperature. Additionally, mechanical loss shows lower loss per unit volume for thicker films, while dielectric loss shows lower loss per unit volume for thinner films. Analysis of these results indicate that mechanical loss correlates with atomic density, while dielectric loss correlates with dangling bond density, suggesting a different origin for these two energy dissipation processes in amorphous silicon.

cond-mat.mtrl-sci↗

arXiv2019

Itinerant ferromagnetism and intrinsic anomalous Hall effect in amorphous iron-germanium

The amorphous iron-germanium system ($a$-Fe$_x$Ge$_{1-x}$) lacks long-range structural order and hence lacks a meaningful Brillouin zone. The magnetization of \aFeGe is well explained by the Stoner model for Fe concentrations $x$ above the onset of magnetic order around $x=0.4$, indicating that the local order of the amorphous structure preserves the spin-split density of states of the Fe-$3d$ states sufficiently to polarize the electronic structure despite $\mathbf{k}$ being a bad quantum number. Measurements reveal an enhanced anomalous Hall resistivity $\rho_{xy}^{\mathrm{AH}}$ relative to crystalline FeGe; this $\rho_{xy}^{\mathrm{AH}}$ is compared to density functional theory calculations of the anomalous Hall conductivity to resolve its underlying mechanisms. The intrinsic mechanism, typically understood as the Berry curvature integrated over occupied $\mathbf{k}$-states but shown here to be equivalent to the density of curvature integrated over occupied energies in aperiodic materials, dominates the anomalous Hall conductivity of $a$-Fe$_x$Ge$_{1-x}$ ($0.38 \leq x \leq 0.61$). The density of curvature is the sum of spin-orbit correlations of local orbital states and can hence be calculated with no reference to $\mathbf{k}$-space. This result and the accompanying Stoner-like model for the intrinsic anomalous Hall conductivity establish a unified understanding of the underlying physics of the anomalous Hall effect in both crystalline and disordered systems.

cond-mat.mes-hall↗

arXiv2018

Two-level systems and growth-induced metastability in hydrogenated amorphous silicon

Specific heat measurements from 2 to 300 K of hydrogenated amorphous silicon prepared by hot-wire chemical vapor deposition show a large excess specific heat at low temperature, significantly larger than the Debye specific heat calculated from the sound velocity. The as-prepared films have a Schottky anomaly that is associated with metastable hydrogen in the amorphous network, as well as large linear and excess cubic term commonly associated with tunneling two-level systems in amorphous solids. Annealing at 200 {\deg}C, a temperature that enables hydrogen mobility but not evaporation, irreversibly reduces the heat capacity, eliminating the Schottky anomaly and leaving a reduced linear heat capacity. A non-monotonic dependence on growth temperature and H content is observed in all effects, except for sound velocity, which suggests that the tunneling two-level systems and the Schottky anomaly are associated with atomic hydrogen and require low density regions to form, while sound velocity is associated with the silicon network and improves with increasing growth temperature.

cond-mat.mtrl-sci↗

arXiv2015

Two-Level Systems in Evaporated Amorphous Silicon

In $e$-beam evaporated amorphous silicon ($a$-Si), the densities of two-level systems (TLS), $n_{0}$ and $\overline{P}$, determined from specific heat $C$ and internal friction $Q^{-1}$ measurements, respectively, have been shown to vary by over three orders of magnitude. Here we show that $n_{0}$ and $\overline{P}$ are proportional to each other with a constant of proportionality that is consistent with the measurement time dependence proposed by Black and Halperin and does not require the introduction of additional anomalous TLS. However, $n_{0}$ and $\overline{P}$ depend strongly on the atomic density of the film ($n_{\rm Si}$) which depends on both film thickness and growth temperature suggesting that the $a$-Si structure is heterogeneous with nanovoids or other lower density regions forming in a dense amorphous network. A review of literature data shows that this atomic density dependence is not unique to $a$-Si. These findings suggest that TLS are not intrinsic to an amorphous network but require a heterogeneous structure to form.

cond-mat.dis-nn↗

arXiv2007

Quenched magnetic moment in Mn-doped amorphous Si (\textit{a}-Mn$_{x}$Si$_{1-x}$) across the metal-insulator transition

The magnetic and electrical transport properties of Mn-doped amorphous silicon (\textit{a-}Mn$_{x}$Si$_{1-x}$) thin films have been measured. The magnetic susceptibility obeys the Curie-Weiss law for a wide range of $x$ (0.005-0.175) and the saturation moment is small. While all Mn atoms contribute to the electrical transport, only a small fraction (interstitial Mn$^{2+}$ states with $J$=$S$=5/2) contribute to the magnetization. The majority of the Mn atoms do not possess any magnetic moment, contrary to what is predicted by the Ludwig-Woodbury model for Mn in crystalline silicon. Unlike \textit{a-}Gd$_{x}$Si$_{1-x}$ films which have an enormous \textit{negative} magnetoresistance, \textit{a-}Mn$_{x}$Si$_{1-x}$ films have only a small \textit{positive} magnetoresistance, which can be understood by this quenching of the Mn moment.

cond-mat.mtrl-sci↗

arXiv2007

Microstructure, magneto-transport and magnetic properties of Gd-doped magnetron-sputtered amorphous carbon

The magnetic rare earth element gadolinium (Gd) was doped into thin films of amorphous carbon (hydrogenated \textit{a}-C:H, or hydrogen-free \textit{a}-C) using magnetron co-sputtering. The Gd acted as a magnetic as well as an electrical dopant, resulting in an enormous negative magnetoresistance below a temperature ($T'$). Hydrogen was introduced to control the amorphous carbon bonding structure. High-resolution electron microscopy, ion-beam analysis and Raman spectroscopy were used to characterize the influence of Gd doping on the \textit{a-}Gd$_x$C$_{1-x}$(:H$_y$) film morphology, composition, density and bonding. The films were largely amorphous and homogeneous up to $x$=22.0 at.%. As the Gd doping increased, the $sp^{2}$-bonded carbon atoms evolved from carbon chains to 6-member graphitic rings. Incorporation of H opened up the graphitic rings and stabilized a $sp^{2}$-rich carbon-chain random network. The transport properties not only depended on Gd doping, but were also very sensitive to the $sp^{2}$ ordering. Magnetic properties, such as the spin-glass freezing temperature and susceptibility, scaled with the Gd concentration.

cond-mat.mtrl-sci↗

arXiv2006

Excess Modes and Enhanced Scattering in Rare-Earth Doped Amorphous Silicon Thin Films

We report specific heat and thermal conductivity of gadolinium- and yttrium-doped amorphous silicon thin films measured using silicon-nitride membrane-based microcalorimeters. Addition of gadolinium or yttrium to the amorphous silicon network reduces the thermal conductivity over a wide temperature range while significantly increasing the specific heat. This result indicates that a large number of non-propagating states are added to the vibrational spectrum that are most likely caused either by localized vibration of the dopant atom in a Si cage, or softening of the material forming the cage structures. High-resolution cross-sectional electron micrographs reveal columnar features in Gd-doped material which do not appear in pure amorphous silicon. Scattering from both the nanoscaled columns and the filled-cage structures play a role in the reduced thermal conductivity in the rare-earth doped amorphous semiconductor. The overall result is an amorphous solid with a large bump in $C/T^{3}$ and no plateau in thermal conductivity.

cond-mat.dis-nn↗

arXiv2003

Variation of the density of states in amorphous GdSi at the metal-insulator transition

We performed detailed conductivity and tunneling mesurements on the amorphous, magnetically doped material $α$-Gd$_x$Si$_{1-x}$ (GdSi), which can be driven through the metal-insulator transition by the application of an external magnetic field. Conductivity increases linearly with field near the transition and slightly slower on the metallic side. The tunneling conductance, proportional to the density of states $N(E)$, undergoes a gradual change with increasing field, from insulating, showing a soft gap at low bias, with a slightly weaker than parabolic energy dependence, i.e. $N(E) \sim E^c$, $c \lesssim 2$, towards metallic behavior, with $E^d$, $0.5 \lt d \lt 1$ energy dependence. The density of states at the Fermi level appears to be zero at low fields, as in an insulator, while the sample shows already small, but metal-like conductivity. We suggest a possible explanation to the observed effect.

cond-mat.str-el↗

arXiv2003

Thermodynamic properties of excess-oxygen-doped La2CuO4.11 near a simultaneous transition to superconductivity and long-range magnetic order

We have measured the specific heat and magnetization {\it versus} temperature in a single crystal sample of superconducting La$_{2}$CuO$_{4.11}$ and in a sample of the same material after removing the excess oxygen, in magnetic fields up to 15 T. Using the deoxygenated sample to subtract the phonon contribution, we find a broad peak in the specific heat, centered at 50 K. This excess specific heat is attributed to fluctuations of the Cu spins possibly enhanced by an interplay with the charge degrees of freedom, and appears to be independent of magnetic field, up to 15 T. Near the superconducting transition $T_{c}$($H$=0)= 43 K, we find a sharp feature that is strongly suppressed when the magnetic field is applied parallel to the crystallographic c-axis. A model for 3D vortex fluctuations is used to scale magnetization measured at several magnetic fields. When the magnetic field is applied perpendicular to the c-axis, the only observed effect is a slight shift in the superconducting transition temperature.

cond-mat.str-el↗

arXiv2002

Tricritical Point and the Doping Dependence of the Order of the Ferromagnetic Phase Transition of La1-xCaxMnO3

We report the doping dependence of the order of the ferromagnetic metal to paramagnetic insulator phase transition in La1-xCaxMnO3. At x = 0.33, magnetization and specific heat data show a first order transition, with an entropy change (2.3 J/molK) accounted for by both volume expansion and the discontinuity of M ~ 1.7 Bohr magnetons via the Clausius-Clapeyron equation. At x = 0.4, the data show a continuous transition with tricritical point exponents alpha = 0.48+/- 0.06, beta = 0.25+/- 0.03, gamma = 1.03+/- 0.05, and delta = 5.0 +/- 0.8. This tricritical point separates first order (x<0.4) from second order (x>0.4) transitions.

cond-mat.str-el↗

arXiv2001

Enhancement of the electronic contribution to the low temperature specific heat of Fe/Cr magnetic multilayer

We measured the low temperature specific heat of a sputtered $(Fe_{23Å}/Cr_{12Å})_{33}$ magnetic multilayer, as well as separate $1000Å$ thick Fe and Cr films. Magnetoresistance and magnetization measurements on the multilayer demonstrated antiparallel coupling between the Fe layers. Using microcalorimeters made in our group, we measured the specific heat for $4<T<30 K$ and in magnetic fields up to $8 T$ for the multilayer. The low temperature electronic specific heat coefficient of the multilayer in the temperature range $4<T<14 K$ is $γ_{ML}=8.4 mJ/K^{2}g-at$. This is significantly larger than that measured for the Fe or Cr films (5.4 and $3.5 mJ/K^{2}mol$ respectively). No magnetic field dependence of $γ_{ML}$ was observed up to $8 T$. These results can be explained by a softening of the phonon modes observed in the same data and the presence of an Fe-Cr alloy phase at the interfaces.

cond-mat.mtrl-sci↗

arXiv2001

Charge dynamics and metal-insulator transition in Si$_{1-x}$Gd$_{x}$ and Si$_{1-x}$Y$_{x}$ alloys

Carrier dynamics in amorphous a-Si$_{1-x}$RE$_{x}$ (RE=Gd, Y) films has been studied in the doping regime close to the metal-insulator transition by means of infrared spectroscopy. Optical constants throughout the entire intra-gap region ($\hbar ω<$ 1 eV) have been found to be anomalously sensitive to changes of temperature and/or magnetic field. The observed behavior is consistent with the model of hopping transport where the interaction of carriers with both the lattice and large core spin of Gd ions is taken into account.

cond-mat.str-el↗