Fluctuation (noise) spectroscopy is widely used to investigate the low-frequency dynamics of charge carriers in condensed-matter systems, aiming to (i) improve the performance of micro- and nanoscale electronic devices and sensors, and (ii) use 'noise as a signal' in order to fundamentally investigate the microscopic motion and transitions of charge carriers coupled to the low-lying excitations in solids. Here, we focus on measurements of the ubiquitous 1/f-type fluctuations, often composed by a superposition of (independent or correlated) two-level systems each giving rise to a Lorentzian spectrum, denoted random telegraph noise in the time domain. We briefly review the basic concepts of noise and fluctuations and give a comprehensive overview of state-of-the-art measuring techniques, using both commercially available signal analyzers and custom software in order to perform the spectral analysis of the recorded time signals, and critically evaluate their advantages and drawbacks. We describe how to use fast data acquisition devices in a cross-correlation setup providing additional quantitative information on the magnitude of uncorrelated instrument noise, and demonstrate the mathematical intricacies of extracting the noise spectra. We introduce a new method for high-throughput measurements by automated spectral analysis based on the concept of Continuous Analysis. We demonstrate the potential of this technique, working towards a FAIR data workflow, by measurements of magnetic flux noise within the hysteresis loop of ferromagnetic nanostructures.

We report on the electron spin resonance (ESR), heat capacity, magnetization, nuclear magnetic resonance (NMR), magnetic circular and linear dichroism (XMCD, XMLD), as well as the electrical resistivity of EuMn2P2 single crystals. Antiferromagnetic order of Eu was observed in several quantities at TNEu = 18 K. The temperature dependencies of ESR linewidth and resonance shift show, when approaching the Eu-ordered state, a divergence towards T Eu N , indicating the growing importance of magnetic correlations and the build-up of internal magnetic fields. An additional temperature scale of approximate to 47 K has considerable impact on linewidth, resonance field and intensity. This points to the presence of weak Mn-based ordering. The observed ESR line is interpreted as an Eu2+ resonance, which probes the weak magnetic background of the Mn subsystem. Such a picture is suggested by the line shape which keeps to be Lorentzian across the 47 K scale and by the ESR intensity which can be described by the same Curie-Weiss temperature above and below 47 K. In the same temperature range, anomalies were observed at 48.5 and 51 K in the heat capacity data as well as a pronounced broadening of the NMR signal of the EuMn2P2 samples. In XMCD and XMLD measurements, this weak magnetic order could not be detected in the same temperature range which might be due to the small magnetic moment, with a potential c component or frustration.

The universality class of the Mott metal-insulator transition in electronic systems strongly coupled to lattice degrees of freedom has been the subject of recent debate. While transport scaling exponents as well as critical slowing down of resistance fluctuations point towards Ising-type criticality, a strong involvement of lattice elasticity suggests a mean-field scenario instead. We investigate the low-frequency resistance noise in model system kappa-(ET)2Cu[N(CN)2]Cl (kappa-Cl)-selected because it is known to exhibit critical elasticity-tuned to the Mott end point by hydrostatic pressure. Contrary to previous reports for the related, deuterated kappa compound, we do not find any indication of a critical slowing down, but observe a strong and anisotropic change of the relative resistance noise level in the metallic compared to the insulating phase. We speculate within framework of isostructural criticality that a critical slowing down might emerge in the presence of structural disorder.

We present a comprehensive study of three-dimensional arrays of nanostructured Co3Fe tetrapods consisting of four pillars each with tetrahedral symmetry, prepared by focused electron beam-induced deposition and placed in two distinct orientations with respect to the direction of an external magnetic field. Using ultra-sensitive micro-Hall magnetometry, we obtain angular-dependent magnetic stray field hysteresis loops, which reveal a characteristic yet complex magnetization reversal behavior of the structures during a field sweep. We find that micromagnetic simulations significantly deviate from the experimental findings due to inherent limitations of the method. As an alternative approach, we derive a macrospin model that enables us to elucidate the observed hysteresis curves through the cascading switching dynamics of dipolar-coupled magnetic grains.

Materials exhibiting electronic inhomogeneities at the nanometer scale have enormous potential for applications. Magnetic polarons are one such type of inhomogeneity which link the electronic, magnetic and lattice degrees of freedom in correlated matter and often give rise to colossal magnetoresistance. Here, we investigate single crystals of Eu5In2Sb6 by thermal expansion and magnetostriction along different crystallographic directions. These data provide compelling evidence for the formation of magnetic polarons in Eu5In2Sb6 well above the magnetic ordering temperature. More specifically, our results are consistent with anisotropic polarons with varying extent along the different crystallographic directions. A crossover revealed within the magnetically ordered phase can be associated with a surprising stabilization of ferromagnetic polarons within the global antiferromagnetic order upon decreasing temperature. These findings make Eu5In2Sb6 a rare example of such coexisting and competing magnetic orders and, importantly, shed new light on colossal magnetoresistive behavior beyond manganites. Materials exhibiting electronic inhomogeneities at the nanometer scale, such as magnetic polarons, have great potential for magnetoresistive applications. Here, thermal expansion and magnetostriction measurements on Eu5In2Sb6 single crystals reveal the formation of magnetic polarons well above the magnetic ordering temperature, providing insights on colossal magnetoresistive behavior beyond manganites.

We present a comprehensive study of small-scale three-dimensional (3D) tetrahedral CoFe nanostructure arrays prepared by focused electron beam-induced deposition (FEBID) and placed in two distinct orientations with respect to the direction of an external magnetic field. Using ultra-sensitive micro-Hall magnetometry we obtain angular-dependent magnetic stray field hysteresis loops that show characteristic cascading magnetic switching close to zero magnetic field. By employing micromagnetic simulations we could reproduce the hysteresis loops and identify characteristic field dependent magnetic configurations including a vortex-type groundstate. From this we derive a coarse-graining macrospin model and show that the complex switching behavior can be explained by the reorientation dynamics of non-interacting uniaxial anisotropic magnetic grains modeled as a superposition of Stoner-Wohlfarth particles.

While SmB6 attracts attention as a possible topological Kondo insulator, EuB6 is known to host magnetic polarons that give rise to large magnetoresistive effects above its ferromagnetic order transition. Here, we investigate single crystals of Sm1−xEuxB6 by magnetic and magnetotransport measurements to explore a possible interplay of these two intriguing phenomena, with a focus on the Eu-rich substitutions. Sm0.01Eu0.99B6 exhibits generally similar behavior as EuB6. Interestingly, Sm0.05Eu0.95B6 combines a global antiferromagnetic order with local polaron formation. A pronounced hysteresis is found in the magnetoresistance of Sm0.1Eu0.9B6 at low temperature (T= 1.9 K) and applied magnetic fields between 2.3 and 3.6 T. The latter is in agreement with a phenomenological model that predicts the stabilization of ferromagnetic polarons with an increasing magnetic field within materials with a global antiferromagnetic order.

We present results of the coefficient of thermal expansion for the frustrated quasi -two-dimensional molecular conductor 0-(BEDT-TTF)2RbZn(SCN)4 for temperatures 1.5 T 290 K. A pronounced first -order phase transition anomaly is observed at the combined charge-order/structural transition at 215 K. Furthermore, clear evidence is found for two separate glasslike transitions at Tg = 90-100 K and Tg dagger = 120-130 K, similar to previous findings for 0-(BEDT-TTF)2CsZn(SCN)4 and 0-(BEDT-TTF)2CsCo(SCN)4 reported by Thomas et al. [Phys. Rev. B 105, L041114 (2022)], both of which lack the charge-order/structural transition. Our findings indicate that these glasslike transitions are common features for the 0-(BEDT-TTF)2MM'(SCN)4 family, with M = (Rb, Cs) and M' = (Co, Zn), irrespective of the presence or absence of charge order. These results are consistent with our model calculations on the glasslike dynamics associated with the flexible ethylene end groups of the BEDT-TTF molecules for various 0-(BEDT-TTF)2MM'(SCN)4 salts, predicting two different conformational glass transitions. Moreover, calculations of the hopping integrals show a substantial degree of dependence on the end groups' conformation, suggesting a significant coupling to the electronic degrees of freedom. Our findings support the possibility that the glassy freezing of the ethylene end groups could drive or enhance glassy charge dynamics.

We present results of the coefficient of thermal expansion for the frustrated quasi-two-dimensional molecular conductor $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{RbZn}{(\mathrm{SCN})}_{4}$ for temperatures $1.5\ensuremath{\le}T\ensuremath{\le}290$ K. A pronounced first-order phase transition anomaly is observed at the combined charge-order/structural transition at 215 K. Furthermore, clear evidence is found for two separate glasslike transitions at ${T}_{\mathrm{g}}=90$--100 K and ${T}_{\mathrm{g}}^{\ifmmode\dagger\else\textdagger\fi{}}=120$--130 K, similar to previous findings for $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{CsZn}{(\mathrm{SCN})}_{4}$ and $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{CsCo}{(\mathrm{SCN})}_{4}$ reported by Thomas et al. [Phys. Rev. B 105, L041114 (2022)], both of which lack the charge-order/structural transition. Our findings indicate that these glasslike transitions are common features for the $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}M{M}^{\ensuremath{'}}{(\mathrm{SCN})}_{4}$ family, with $M=(\mathrm{Rb}, \mathrm{Cs})$ and ${M}^{\ensuremath{'}}=(\mathrm{Co}, \mathrm{Zn})$, irrespective of the presence or absence of charge order. These results are consistent with our model calculations on the glasslike dynamics associated with the flexible ethylene end groups of the BEDT-TTF molecules for various $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}M{M}^{\ensuremath{'}}{(\mathrm{SCN})}_{4}$ salts, predicting two different conformational glass transitions. Moreover, calculations of the hopping integrals show a substantial degree of dependence on the end groups' conformation, suggesting a significant coupling to the electronic degrees of freedom. Our findings support the possibility that the glassy freezing of the ethylene end groups could drive or enhance glassy charge dynamics.

The mechanism of the peculiar transport properties around the magnetic ordering temperature of semiconducting antiferromagnetic EuCd 2 P 2 is not yet understood. With a huge peak in the resistivity observed above the N & eacute;el temperature T N = 10 . 6 K, it exhibits a colossal magnetoresistance effect. Recent reports on observations of ferromagnetic contributions above T N as well as metallic behavior below this temperature have motivated us to perform a comprehensive characterization of this material, including its resistivity, heat capacity, magnetic properties, and electronic structure. Our transport measurements revealed quite different temperature dependence of resistivity with the maximum at 14 K instead of previously reported 18 K. Low-field susceptibility data support the presence of static ferromagnetism above T N and show a complex behavior of the material at small applied magnetic fields. Namely, signatures of reorientation of magnetic domains are observed up to T = 16 K. Our magnetization measurements indicate a magnetocrystalline anisotropy which also leads to a preferred alignment of the magnetic clusters above T N . The momentum-resolved photoemission experiments at temperatures from 24 down to 2.5 K indicate the permanent presence of a fundamental band gap without change of the electronic structure when going through T N that is in contradiction with previous results. We performed ab initio band structure calculations which are in good agreement with the measured photoemission data when assuming an antiferromagnetic ground state. Calculations for the ferromagnetic phase show a much smaller band gap, indicating the importance of possible ferromagnetic contributions for the explanation of the colossal magnetoresistance effect in the related EuZn 2 P 2 .

Ferroelectricity, where electronic degrees of freedom determine the polar order—thereby enabling fast switching and phase control—is an important research field in current condensed-matter physics. Using a combination of resistance noise and dielectric spectroscopy we investigate the nature of relaxor-type electronic ferroelectricity in the organic conductor κ-(BETS)2Mn[N(CN)2]3, a system that represents a wider class of materials of correlated electron systems for which functionalities for organic spintronics recently have been discussed. The two complementary spectroscopies reveal a distinct low-frequency dynamics on different length scales, namely (i) an intrinsic relaxation that is typical for relaxor ferroelectrics which classifies the system as a possible new multiferroic, and (ii) two-level processes which we identify as fluctuating polar nanoregions (PNR), i.e., clusters of quantum electric dipoles that fluctuate collectively. The PNR preform above the metal insulator (MI) transition. Upon cooling through TMI, a drastic increase of the low-frequency 1/f-type fluctuations and slowing down of the charge carrier dynamics is accompanied by the onset of strong non-equilibrium dynamics indicating a glassy transition of interacting dipolar clusters. The freezing of PNR and non-equilibrium dynamics is suggested to be a common feature of organic relaxor-type electronic ferroelectrics.

Using a combination of resistance fluctuation (noise) and dielectric spectroscopy we investigate the nature of relaxor-type electronic ferroelectricity in the organic conductor $\kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$, a system representative for a wider class of materials, where strong correlations of electrons on a lattice of dimerized molecules results in an insulating ground state. The two complementary spectroscopies reveal a distinct low-frequency dynamics. By dielectric spectroscopy we detect an intrinsic relaxation that is typical for relaxor ferroelectrics below the metal-to-insulator transition at $T_{\rm{MI}}\sim 25\,$K. Resistance noise spectroscopy reveals fluctuating two-level processes above $T_{\rm MI}$ which strongly couple to the applied electric field, a signature of fluctuating polar nanoregions (PNR), i.e. clusters of quantum electric dipoles fluctuating collectively. The PNR preform above the metal insulator transition. Upon cooling through $T_{\rm MI}$, a drastic increase of the low-frequency $1/f$-type fluctuations and slowing down of the charge carrier dynamics is accompanied by the onset of strong non-equilibrium dynamics indicating a glassy transition of interacting dipolar clusters, the scaling properties of which are consistent with a droplet model. The freezing of nano-scale polar clusters and non-equilibrium dynamics is suggested to be a common feature of organic relaxor-type electronic ferroelectrics and needs to be considered in theoretical models describing these materials.

We investigate single crystals of the trigonal antiferromagnet EuZn2P2 (P (3) over bar m1) by means of electrical transport, magnetization measurements, x-ray magnetic scattering, optical reflectivity, angle-resolved photoemission spectroscopy (ARPES), and ab initio band structure calculations (DFT + U). We find that the electrical resistivity of EuZn2P2 increases strongly upon cooling and can be suppressed in magnetic fields by several orders of magnitude (colossal magnetoresistance effect). Resonant magnetic scattering reveals a magnetic ordering vector of q = (0 0 1/2), corresponding to an A-type antiferromagnetic order, below T-N = 23.7 K. We find that the moments are canted out of the a-a plane by an angle of about 40 degrees +/- 10 degrees and aligned along the [100] direction in the a-a plane. We observe nearly isotropic magnetization behavior for low fields and low temperatures which is consistent with the magnetic scattering results. The magnetization measurements show a deviation from the Curie-Weiss behavior below approximate to 150 K, the temperature below which also the field dependence of the material's resistivity starts to increase. An analysis of the infrared reflectivity spectrum at T = 295 K allows us to resolve the main phonon bands and intraband and interband transitions, and estimate indirect and direct band gaps of E-i(opt) = 0.09 and E-d(opt) = 0.33 eV, respectively, which are in good agreement with the theoretically predicted ones. The experimental band structure obtained by ARPES is nearly T independent above and below T-N. The comparison of the theoretical and experimental data shows a weak intermixing of the Eu 4f states close to the Gamma point with the bands formed by the phosphorous 3p orbitals leading to an induction of a small magnetic moment at the P sites.

The discovery of superconductivity in the quantum critical Kondo-lattice system YbRh2Si2 at an extremely low temperature of 2 mK has inspired efforts to perform high-resolution electrical resistivity measurements down to this temperature range in highly conductive materials. Here we show that control over the sample geometry by microstructuring using focused-ion-beam techniques allows to reach ultra-low temperatures and increase signal-to-noise ratios (SNRs) tenfold, without adverse effects to sample quality. In five experiments we show four-terminal sensing resistance and magnetoresistance measurements which exhibit sharp phase transitions at the Neel temperature, and Shubnikov-de-Haas (SdH) oscillations between 13 T and 18 T where we identified a new SdH frequency of 0.39 kT. The increased SNR allowed resistance fluctuation (noise) spectroscopy that would not be possible for bulk crystals, and confirmed intrinsic 1/f-type fluctuations. Under controlled strain, two thin microstructured samples exhibited a large increase of T-N from 67 mK up to 188 mK while still showing clear signatures of the phase transition and SdH oscillations. Superconducting quantum interference device-based thermal noise spectroscopy measurements in a nuclear demagnetization refrigerator down to 0.95 mK, show a sharp superconducting transition at T-c = 1.2 mK. These experiments demonstrate microstructuring as a powerful tool to investigate the resistance and the noise spectrum of highly conductive correlated metals over wide temperature ranges.

We present a systematic investigation of the low-frequency charge carrier dynamics in different charge states of the organic conductors $\theta$-(BEDT-TTF)$_2$$M$Zn(SCN)$_4$ with $M$=Rb,Tl, which result from quenching or relaxing the charge degrees of freedom on a geometrically frustrated triangular lattice. Due to strong electronic correlations these materials exhibit a charge-ordering transition, which can be kinetically avoided by rapid cooling resulting in a so-called charge-glass state without long-range order. The combination of fluctuation spectroscopy and a heat pulse method allows us to study and compare the resistance fluctuations in the low-resistive quenched and the high-resistive charge-ordered state, revealing striking differences in the respective noise magnitudes. For both compounds, we find strongly enhanced resistance fluctuations right at the metal-insulator transition and a broad noise maximum in the slowly cooled charge-crystal state with partly dominating two-level processes revealing characteristic activation energies.

Combined resistance noise and muon-spin relaxation ($\mu$SR) measurements of the ferromagnetic semiconductor HgCr$_2$Se$_4$ suggest a degree of magnetoelectric coupling and provide evidence for the existence of isolated magnetic polarons. These form at elevated temperatures and undergo a percolation transition with a drastic enhancement of the low-frequency 1/$f$-type charge fluctuations at the insulator-to-metal transition at $\sim 95 - 98$ K in the vicinity of the magnetic ordering temperature $T_C \sim 105 - 107$ K. Upon approaching the percolation threshold from above, the strikingly unusual dynamics of a distinct two-level fluctuator superimposed on the $1/f$ noise can be described by a slowing down of the dynamics of a nanoscale magnetic cluster, a magnetic polaron, when taking into account an effective radius of the polaron depending on the spin correlation length. Coinciding temperature scales found in $\mu$SR and noise measurements suggest changes in the magnetic dynamics over a wide range of frequencies and are consistent with the existence of large polarized and domain-wall-like regions at low temperatures, that result from the freezing of spin dynamics at the magnetic polaron percolation transition.

The low-temperature state of the quantum spin liquid candidate κ-(BEDT-TTF)_{2}Cu_{2}(CN)_{3} emerges via an anomaly at T^{*}∼6 K. Although signatures of this anomaly have been revealed in various quantities, its origin has remained unclear. Here we report inelastic neutron scattering measurements on single crystals of κ-(BEDT-TTF)_{2}Cu_{2}(CN)_{3}, aiming at studying phonon renormalization effects at T^{*}. A drastic change was observed in the phonon damping across T^{*} for a breathing mode of BEDT-TTF dimers at E=4.7 meV. The abrupt change in the phonon damping is attributed to a phase transition into a valence bond solid state based on an effective model describing the spin-charge coupling in this dimer-Mott system.

We present a combined study of thermal expansion and resistance fluctuation spectroscopy measurements exploring the static and dynamic aspects of the charge-glass formation in the quasi-twodimensional organic conductors θ-(BEDT-TTF)2MM (SCN)4 with M = Cs and M ′ = Co,Zn. In these materials, the emergence of a novel charge-glass state so far has been interpreted in purely electronic terms by considering the strong frustration of the Coulomb interactions on a triangular lattice. Contrary to this view, we provide comprehensive evidence for the involvement of a structural glass-like transition at Tg ∼ 90− 100 K. This glassy transition can be assigned to the freezing of structural conformations of the ethylene endgroups in the donor molecule with an activation energy of Ea ≈ 0.32 eV, and the concomitant slowing down of the charge carrier dynamics is well described by a model of non-exponential kinetics. These findings discolse an important aspect of the phase diagram and renders the current understanding of the charge-glass state in the whole family of θ-(BEDT-TTF)2MM (SCN)4 incomplete. Our results suggest that the entanglement of slow structural and charge-cluster dynamics due to the intimate coupling of lattice and electronic degrees of freedom determine the charge-glass formation under geometric frustration.

Inelastic neutron scattering measurements have revealed an abrupt change in the phonon damping at the 6 K anomaly in the quantum spin liquid candidate $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}$Cu${}_{2}$(CN)${}_{3}$. This is interpreted as a phase transition to the valence bond solid state on the basis of an effective model describing the spin-charge coupling in the dimer-Mott system.

We report a comprehensive investigation of the triple perovskite iridate Ba_3CoIr_2O_9. Stabilizing in the hexagonal P6_3/mmc symmetry at room temperature, this system transforms to a monoclinic C2/c symmetry at the magnetic phase transition. On further reduction in temperature, the system partially distorts to an even lower symmetry (P2/c), with both these structurally disparate phases coexisting down to the lowest measured temperatures. The magnetic structure as determined from neutron diffraction data indicates a weakly canted antiferromagnetic structure, which is also supported by first-principles calculations. Theory indicates that the Ir^5+ carries a finite magnetic moment, which is also consistent with the neutron data. This suggests that the putative J=0 state is avoided. Measurements of heat capacity, electrical resistance noise and dielectric susceptibility all point towards the stabilization of a highly correlated ground state in the Ba_3CoIr_2O_9 system.