The Kondo lattice system is a unique member among the hexaborides due to its multi-phase magnetic phase diagram, dense Kondo behavior, and antiferroquadrupolar ordering. Understanding its surface properties is crucial for interpreting the results of highly surface-sensitive techniques like Scanning Tunneling Microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). In this study, we conducted a comprehensive survey of surfaces prepared by low-temperature in situ cleaving and investigated by low-temperature STM. Two different types of surface reconstructions could be located, but atomically flat, non-reconstructed surface areas could be found only very rarely and were limited in size to a few tens of nanometers. Tunneling spectroscopic measurements within such non-reconstructed clean areas revealed a partially opened gap of about 42 meV at 4.6 K. Its origin is discussed to rely either on Kondo hybridization or on strong correlation effects in the 5-2 band structure. Density functional theory (DFT) calculations show that the common wide bands of itinerant 5 and 2 electrons remain robust for bulk . Spectra obtained on reconstructed surfaces indicated a modified low-energy electronic surface structure. Our work draws attention to the impact of details of the surface structure of complex materials like on its electronic surface properties.

The interplay between magnetism and charge transport is central to understanding colossal magnetoresistance (CMR), a phenomenon well studied in ferromagnets. Recently, antiferromagnetic (AFM) EuCd2P2 has attracted considerable interest due to its remarkable CMR, for which magnetic fluctuations and the formation of ferromagnetic clusters have been proposed as key mechanisms. Here we provide direct evidence that these effects originate from the formation and percolation of magnetic polarons. We employ a complementary set of sensitive probes that allows for a direct comparison of electronic and magnetic properties on multiple time scales revealing pronounced electronic and magnetic phase separation below T* ≈ 2TN. These measurements indicate an inhomogeneous, percolating electronic system below T* and well above the magnetic ordering temperature TN = 11 K. In applied magnetic fields, the onset of the pronounced negative MR in the paramagnetic regime emerges at a universal critical magnetization. The characteristic size of the magnetic polarons near the percolation threshold is estimated to be ~6−10 nm. Our results establish dynamic polaron percolation within an AFM matrix as the microscopic origin of CMR in EuCd2P2, providing a unified framework for magnetotransport in Eu-based correlated semiconductors.

The new binary silicide Pr2Si7 is prepared by high-pressure high-temperature techniques at 9.5 GPa and 1100 K (600 min). The crystal structure is solved and refined using synchrotron X-ray diffraction data. It belongs to the structure type of Ce2Si7 (space group Cmmm, a = 7.1513(2) angstrom, b = 10.0033(2) angstrom, c = 4.5417(1) angstrom, and Z = 2). The characteristic feature of the atomic arrangement is the 3D-polyanion formed by silicon atoms with (highest weight) topological coordination numbers of 4, 5, and 6. Analysis of chemical bonding within the electron-localizability approach reveals six electron localizability indicator (ELI-D) attractors around each of the three symmetrically independent silicon atoms. The bonding picture in the polyanion is characterized by the appearance of lone-pair-like arrangements at silicon species and two types of two-atomic Si-Si bonds. The total of six attractors for each silicon atom documents hypervalent configurations within the polyanionic framework. Magnetic susceptibility measurements indicate an oxidation state +3 for praseodymium (f 2 configuration) with the degenerate states of the free ion being split by crystalline electric fields.

Magnetite (Fe3O4) is a material of ongoing interest in solid state physics due to its enigmatic Verwey transition, while being extensively studied as thin film for spintronic applications. Here, we report on our study of the Verwey transition under magnetic fields in Fe3O4 thin films on spinel substrates Co2-x-yMnxFeyTiO4 and non-magnetic Mg2TiO4. The Verwey transition of these films is highly tunable and anisotropic with applied magnetic fields suggesting strong magnetostriction via an active spin-orbit effect of the Fe2+ ions. An alteration of Fe2+ orbital occupations, however, does not impede the Verwey transition, while tensile strain enhances its transition temperature. This indicates that various types of orbital orderings may occur depending on the thin film conditions. Remarkably, this high magnetic tunability results in a closed magnetoresistance-loop reaching up to 88% at 0.5 Tesla, which is beyond one order of magnitude larger than the reported values of Fe3O4 films. Additionally, the spin-polarized tunneling channel for the magnetoresistance is active in anti-phase boundary-free Fe3O4/Mg2TiO4 thin films. These findings underscore the critical role of utilizing structural defect-free, high-quality thin films in exploring the full potential of Fe3O4 thin films, offering additional perspectives for understanding the Verwey transition of Fe3O4.

Introducing the concept of topology into material science has sparked a revolution from classic electronic and optoelectronic devices to topological quantum devices. The latter has potential for transferring energy and information with unprecedented efficiency. Here, we demonstrate a topological diode effect on the surface of a three-dimensional material, SmB6, a candidate topological Kondo insulator. The diode effect is evidenced by pronounced rectification and photogalvanic effects under electromagnetic modulation and radiation at radio frequency. Our experimental results and modeling suggest that these prominent effects are intimately tied to the spatially inhomogeneous formation of topological surface states (TSS) at the intermediate temperature. This work provides a manner of breaking the mirror symmetry (in addition to the inversion symmetry), resulting in the formation of pn-junctions between puddles of metallic TSS. This effect paves the way for efficient current rectifiers or energy-harvesting devices working down to radio frequency range at low temperature, which could be extended to high temperatures using other topological insulators with large bulk gap.

Antiferromagnetic EuCd_2P_2 has attracted considerable attention due to its unconventional (magneto)transport properties. At a temperature T_ peak significantly above the magnetic ordering temperature T_N = 11K a large peak in resistivity is observed which gets strongly suppressed in magnetic field, resulting in a colossal magnetoresistance (CMR), for which magnetic fluctuations and the formation of ferromagnetic clusters have been proposed as underlying mechanisms. Employing a selection of sensitive probes including fluctuation spectroscopy and third-harmonic resistance, Hall effect, AC susceptibility and μSR measurements, allows for a direct comparison of electronic and magnetic properties on multiple time scales. We find compelling evidence for the formation and percolation of magnetic polarons, which explains the CMR of the system. Large peaks in the weakly-nonlinear transport and the resistance noise power spectral density at zero magnetic field signify an inhomogeneous, percolating electronic system below T^∗≈ 2 T_N with a percolation threshold at T_ peak. In magnetic fields, the onset of large negative MR in the paramagnetic regime occurs at a universal critical magnetization similar to ferromagnetic CMR materials. The size of the magnetic polarons at the percolation threshold is estimated to ∼ 1 - 2nm. The mechanism of magntic cluster formation and percolation in EuCd_2P_2 appears to be rather robust despite large variations in carrier concentration and likely is relevant for other Eu-based antiferromagnetic CMR systems.

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.

The higher dimensionality in the crystal fields of the Ce2MIn8 (M=Rh,Ir) compounds and its interplay with hybridization and disorder are key ingredients to understand the complex phase diagrams by this family, which have been explored extensively by macroscopic techniques. Here, we present an investigation of the crystal-electric field schemes of Ce2Rh1−xIrxIn8 using x-ray absorption spectroscopy. Our full multiplet calculations for the 4f1 configuration of Ce3+ to describe the temperature-dependent linear dichroism in Ce2MIn8 are consistent with a Γ71=1−α2·|∓32〉−|α|·|±52〉 ground state containing a predominant |±3/2〉 contribution that increases further with x. This enhancement is believed to favor superconductivity in Ce-based heavy fermion materials, observed in previous results in the CeMIn5 family. Our recent observations shed light on the unexpected emergence of the ambient-pressure superconducting dome in the center of the composition phase diagram and its subsequent suppression on the Ir-rich side due to the early onset of fluctuations associated with the structurally more disordered state, inferred from previous neutron magnetic diffraction experiments. Published by the American Physical Society 2024

Single crystals of DyRh2Si2 and HoRh2Si2 were investigated by thermal expansion and magnetostriction. The different types of magnetic order can clearly be seen in these measurements, particularly the canting of the moments away from the crystallographic c direction below about 12 K and the spin flip for magnetic field applied along the c direction. For HoRh2Si2, an additional transition just below TN is analyzed by means of the Gr & uuml;neisen ratio and is likely caused by a change in the magnetic structure. Our results nicely corroborate findings from other magnetic and thermodynamic measurements on these materials and provide further evidence suggesting the formation of magnetic domains.

New metastable SmSi3-x (x=0-0.05) is obtained by high-pressure high-temperature synthesis (9.5 GPa, 870-1270 K). Powder diffraction data refinements reveal that the crystal structure of SmSi3 is isotypic to that of YbSi3 (space group I4/mmm, a=7.23634(5) angstrom, c=11.0854(1) angstrom). In the crystal structure, two types of Si2 dumbbells agglomerate into layers, which embed the samarium atoms. At ambient pressure, SmSi3 decomposes exothermally upon heating into Si and SmSi2-x. Single-crystal structure refinements of a specimen SmSi3-x (x=0.05) reveal considerable electron density, which is not accounted for by the YbSi3-type model. The additional maxima can be assigned to disorder which affects the samarium positions and induces silicon vacancies. Scanning transmission electron microscopy experiments evidence that the disorder can be attributed to extended defects. Magnetic measurements on SmSi3-x reveal van Vleck paramagnetic behavior and antiferromagnetic ordering at low temperatures. Computations within the local spin density approximation (LSDA and LSDA+U) on the crystal structure of SmSi3 reproduce the antiferromagnetic coupling as the favored long-range order. Quantum chemical analysis of the chemical bonding in SmSi3 reveals two-center two-electron bonds within the Si-2 dumbbells plus a total of a little less than four electrons in lone pairs at each silicon atom.

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.

Impurity doping at the nanoscale for silicon is becoming less efficient with conventional techniques. Here, an alternative virtual doping method is presented for silicon that can achieve an equivalent carrier density while addressing the primary limitations of traditional doping methods. The doping for silicon is carried out by placing aluminum-induced acceptor states externally in a silicon dioxide dielectric shell. This technique can be referred to as direct modulation doping. The resistivity, carrier density, and mobility are investigated by Hall effect measurements to characterize the carrier transport using the new doping method. The results thereof are compared with carrier transport analysis of conventionally doped silicon at room-temperature, demonstrating a 100% increase in carrier mobility at equal carrier density. The sheet density of hole carriers in silicon due to modulation doping remains nearly constant, approximate to 4.7 x 1012 cm-2 over a wide temperature range from 300 down to 2 K, proving that modulation-doped devices do not undergo carrier freeze-out at cryogenic temperatures. In addition, a mobility enhancement is demonstrated with an increase from 89 cm2 Vs-1 at 300 K to 227 cm2 Vs-1 at 10 K, highlighting the benefits of the new method for creating emerging nanoscale electronic devices or peripheral cryo-electronics to quantum computing.

Scanning Tunneling Microscopy and Spectroscopy (STM/S), with its exceptional surface sensitivity and exquisite energy resolution, is well suited for the investigation of surface states down to atomic length scales. As such, it became an essential tool to probe the surface states of materials, including those with non-trivial topology. One challenge, however, can be the preparation of clean surfaces which allow the study of preferably unchanged surface properties with respect to the bulk amount. Here, we report on the STM/S of two materials, ZrTe2 and TmB4. The former cleaves easily and defects can be examined in detail. However, our STS data can only qualitatively be compared to the results of band structure calculations. In the case of TmB4, the preparation of suitable surfaces is highly challenging, and atomically flat surfaces (likely of B-termination) were only encountered rarely. We found a large density of states (DOS) at the Fermi level EF and a mostly featureless differential conductance near EF. Further efforts are required to relate our results to the electronic structure predicted by ab initio calculations.

Half-Heusler systems host a plethora of different ground states, especially with non-trivial topology. However, there is still a lack of spectroscopic insight into the corresponding band inversion in this family. In this work, we locally explore the half-Heuslers Y$T$Bi ($T =$ Pt and Pd) by means of scanning tunneling microscopy/spectroscopy. From our analysis of the (120) surface plane, we infer that the increase of the spin--orbit coupling upon going from Pd to Pt is the main player in tuning the surface states from trivial to topologically non-trivial. Our measurements unveil a ($2 \times 1$) reconstruction of the (120) surface of both systems. Using density functional theory calculations, we show that the observed different behavior of the local density of states near the Fermi level in these two materials is directly related to the presence of metallic surface states. Our work sheds new light on a well known tunable family of materials and opens new routes to explore the presence of topological states of matter in half-Heusler systems and its microscopic observation.

The intermetallic compound Eu_5In_2Sb_6, an antiferromagnetic material with nonsymmorphic crystalline structure, is investigated by magnetic, electronic transport and specific heat measurements. Being a Zintl phase, insulating behavior is expected. Our thermodynamic and magnetotransport measurements along different crystallographic directions strongly indicate polaron formation well above the magnetic ordering temperatures. Pronounced anisotropies of the magnetic and transport properties even above the magnetic ordering temperature are observed despite the Eu^2+ configuration which testify to complex and competing magnetic interactions between these ions and give rise to intricate phase diagrams discussed in detail. Our results provide a comprehensive framework for further detailed study of this multifaceted compound with possible nontrivial topology.

The intermetallic compound Eu[Formula: see text]In[Formula: see text]Sb[Formula: see text], an antiferromagnetic material with nonsymmorphic crystalline structure, is investigated by magnetic, electronic transport and specific heat measurements. Being a Zintl phase, insulating behavior is expected. Our thermodynamic and magnetotransport measurements along different crystallographic directions strongly indicate polaron formation well above the magnetic ordering temperatures. Pronounced anisotropies of the magnetic and transport properties even above the magnetic ordering temperature are observed despite the Eu[Formula: see text] configuration which testify to complex and competing magnetic interactions between these ions and give rise to intricate phase diagrams discussed in detail. Our results provide a comprehensive framework for further detailed study of this multifaceted compound with possible nontrivial topology.

The new metastable binary rare-earth metal trisilicides RESi3 (RE=Tb, Dy, Er, and Tm) are obtained by high-pressure high-temperature synthesis (9.5 GPa, 823-923 K). Powder diffraction data refinements reveal that their crystal structure is isotypic to that of YbSi3, and the values of cell parameters and cell volumes decrease from Tb to Tm. Magnetic measurements on the compounds indicate Curie-Weiss paramagnetic behavior and antiferromagnetic ordering at low temperatures. The values for T-N follow a de Gennes scaling pointing towards Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. Upon heating at ambient pressure, the RESi3 compounds decompose into Si and RESi2-x.The new high-pressure phases TbSi3, DySi3, ErSi3 and TmSi3 adopt atomic arrangements in which two-dimensional building units of silicon are separated by rare-earth metal atoms. The silicon environment with five short Si-Si contacts in connection with the formal charge Si- is incompatible with the classical 8-N rule. This is confirmed by earlier quantum chemical methods evidencing a mixture of two-atomic Si-Si bonds and multiatomic interactions involving lone-pairs located at silicon atoms. Consequently, the rare-earth metal trisilicides do not belong to the class of electron-precise Zintl phases, which is consistent with their earlier observed character of covalent metals. This result is in concert with the present study revealing that compounds RESi3 (RE=Gd-Tm) follow a de Gennes scaling, which is based on a magnetic coupling via conduction electrons.

The perovskite BaBiO$_3$ crystallizes in a cubic structure and undergoes structural transitions toward lower symmetry phases upon cooling. The two low-temperature monoclinic phases are insulating, and the origin of this unexpected non-metallic character has been under debate. Both monoclinic phases exhibit tilting and breathing distortions, which are connected with the insulating nature of this compound and may have important effects on phononic heat conductivity. Here, we report the first thermal conductivity measurement, $\kappa$(T), in pristine polycrystalline BaBiO$_3$ from 1.5 K to 310 K. At low and intermediate temperatures, we observe features reminiscent of a glass-like behavior, whereas at high-temperatures we find a downturn - typical of a crystalline solid. We compare our findings with available data of other recently investigated perovskite oxides displaying similar temperature dependence.

The intermetallic compound Eu $$_5$$ In $$_2$$ Sb $$_6$$ , an antiferromagnetic material with nonsymmorphic crystalline structure, is investigated by magnetic, electronic transport and specific heat measurements. Being a Zintl phase, insulating behavior is expected. Our thermodynamic and magnetotransport measurements along different crystallographic directions strongly indicate polaron formation well above the magnetic ordering temperatures. Pronounced anisotropies of the magnetic and transport properties even above the magnetic ordering temperature are observed despite the Eu $$^{2+}$$ configuration which testify to complex and competing magnetic interactions between these ions and give rise to intricate phase diagrams discussed in detail. Our results provide a comprehensive framework for further detailed study of this multifaceted compound with possible nontrivial topology.

Inconsistencies in the concentrations of unintentional donor impurities and free charge carriers in GaN/AlGaN layer stacks hosting a two-dimensional electron gas (2DEG) can be attributed to the measurement procedure and solely depend on the way in which the free charge carrier concentration is extracted. Particularly, when the 2DEG acts as the bottom electrode in capacitance versus voltage measurements, unphysically low concentrations of free charges are calculated. This originates from the depletion of the 2DEG and the accompanying disappearance of the bottom electrode. It is shown that, for the case of a defined (non-vanishing) bottom electrode, the levels of donor impurities and resulting free charges consistently match.