Yogesh K. Vohra

University of Maryland, College Park, College Park, MD, United States

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Publications (407)

  • Source
    Gopi K. Samudrala · Samuel L. Moore · Nenad Velisavljevic · [...] · Yogesh K. Vohra
    [Show abstract] [Hide abstract] ABSTRACT: By combining mask-less lithography and chemical vapor deposition (CVD) techniques, a novel two-stage diamond anvil has been fabricated. A nanocrystalline diamond (NCD) micro-anvil in diameter was grown at the center of a [100]-oriented, diamond anvil by utilizing microwave plasma CVD method. The NCD micro-anvil has a diamond grain size of 115 nm and micro-focused Raman and X-ray Photoelectron spectroscopy analysis indicate sp³-bonded diamond content of 72%. These CVD grown NCD micro-anvils were tested in an opposed anvil configuration and the transition metals osmium and tungsten were compressed to high pressures of 264 GPa in a diamond anvil cell.
    Full-text Article · Sep 2016 · AIP Advances
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    [Show abstract] [Hide abstract] ABSTRACT: The bulk modulus of nanocrystalline, fluorite-structured samarium doped ceria, Sm0.2Ce0.8O1.9, has been investigated using synchrotron-based high-pressure X-ray diffraction technique. Experiments were carried out under both quasi-hydrostatic condition with silicon oil pressure transmitting medium (PTM) and nonhydrostatic conditions without PTM. The high pressure structural results indicate that the highly defected ionic conductor is stable up to 20 GPa and has a lower bulk modulus than what has been reported for undoped-CeO2. The isothermal bulk modulus of Sm0.2Ce0.8O1.9 is ~ 150–190 GPa compared to ~ 210–220 GPa for CeO2. The collected data experimentally verifies the effect of Sm3 + dopant and oxygen vacancy defect formation on bulk modulus in doped CeO2. The effect of modulus on misfit dislocation formation and dopant ion segregation is discussed in relation to a fundamental understanding of the strain effect in this important family of fast ionic conductors, with potential application as oxygen vacancy conducting solid state electrolytes.
    Full-text Article · Sep 2016 · Solid State Ionics
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    [Show abstract] [Hide abstract] ABSTRACT: Extreme magnetoresistance (XMR) in topological semimetals is a recent discovery which attracts attention due to its robust appearance in a growing number of materials. To search for a relation between XMR and superconductivity, we study the effect of pressure on LaBi taking advantage of its simple structure and simple composition. By increasing pressure we observe the disappearance of XMR followed by the appearance of superconductivity at P=3.5 GPa.The suppression of XMR is correlated with increasing zero-field resistance instead of decreasing in-field resistance. At higher pressures, P=11 GPa, we find a structural transition from the face center cubic lattice to a primitive tetragonal lattice in agreement with theoretical predictions. We discuss the relationship between extreme magnetoresistance, superconductivity, and structural transition in LaBi.
    Full-text Article · Jul 2016
  • [Show abstract] [Hide abstract] ABSTRACT: The material USb2 is a correlated, moderately heavy-electron compound within the uranium dipnictide (UX2) series. It is antiferromagnetic with a relatively high transition temperature TN=204K and a large U-U separation. While the uranium atoms in the lighter dipnictides are considered to be localized, those of USb2 exhibit hybridization and itineracy, promoting uncertainty as to the continuity of the magnetic order within the UX2. We have explored the evolution of the magnetic order by employing magnetotransport measurements as a function of pressure and temperature. We find that the TN in USb2 is enhanced, moving towards that of its smaller sibling UAs2. But, long before reaching a TN as high as UAs2, the antiferromagnetism of USb2 is abruptly destroyed in favor of another magnetic ground state. We identify this pressure-induced ground state as being ferromagnetic based on the appearance of a strong anomalous Hall effect in the transverse resistance in magnetic field. With pressure, this emergent ferromagnetic state is suppressed and ultimately destroyed in favor of a non-Fermi-liquid ground state.
    Article · May 2016
  • [Show abstract] [Hide abstract] ABSTRACT: A boron-doped designer heater anvil is used in conjunction with powderx-ray diffraction to collect structural information on a sample of quasi-hydrostatically loaded gadolinium metal up to pressures above 8 GPa and 600 K. The heater anvil consists of a natural diamond anvil that has been surface modified with a homoepitaxially grown chemical-vapor-deposited layer of conducting boron-doped diamond, and is used as a DC heating element. Internally insulating both diamond anvils with sapphire support seats allows for heating and cooling of the high-pressure area on the order of a few tens of seconds. This device is then used to scan the phase diagram of the sample by oscillating the temperature while continuously increasing the externally applied pressure and collecting in situ time-resolved powderdiffraction images. In the pressure-temperature range covered in this experiment, the gadolinium sample is observed in its hcp, αSm, and dhcp phases. Under this temperature cycling, the hcp → αSm transition proceeds in discontinuous steps at points along the expected phase boundary. From these measurements (representing only one hour of synchrotron x-ray collection time), a single-experiment equation of state and phase diagram of each phase of gadolinium is presented for the range of 0–10 GPa and 300–650 K.
    Article · Apr 2016 · Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: Using high-pressure magnetotransport techniques we have discovered superconductivity in Bi2Te, a member of the infinitely adaptive (Bi2)m(Bi2Te3)n series, whose end members, Bi and Bi2Te3, can be tuned to display topological surface states or superconductivity. Bi2Te has a maximum Tc=8.6K at P=14.5GPa and goes through multiple high-pressure phase transitions, ultimately collapsing into a bcc structure that suggests a universal behavior across the series. High-pressure magnetoresistance and Hall measurements suggest a semimetal to metal transition near 5.4 GPa, which accompanies the hexagonal to intermediate phase transition seen via x-ray diffraction measurements. In addition, the linearity of Hc2(T) exceeds the Werthamer-Helfand-Hohenberg limit, even in the extreme spin-orbit scattering limit, yet is consistent with other strong spin-orbit materials. Considering these results in combination with similar reports on strong spin-orbit scattering materials seen in the literature, we suggest the need for a new theory that can address the unconventional nature of their superconducting states.
    Article · Mar 2016
  • Source
    Kathryn M. Schoenrock · Julie B. Schram · Charles D. Amsler · [...] · Yogesh K. Vohra
    [Show abstract] [Hide abstract] ABSTRACT: Climate change impacts were investigated in two species of crustose Antarctic macroalgae that may be natural competitors in their habitat. The seawater parameters oceanic pH and temperature were modified to near future projections for the western Antarctic Peninsula in microcosm experiments. Experiments included two crustose algae, the calcified coralline alga Clathromorphum obtectulum and the fleshy encrusting rhodophyte Hildenbrandia sp., and were run for six weeks. Treatments reflected near future ocean conditions under climate change predictions: increased temperature (3.5°C×pH8.1), increased pCO2 (1.5°C×pH7.8), combined factors (3.5°C×pH7.8), and ambient conditions (1.5°C×pH8.1). The physiological responses of the algae were evaluated through photosynthetic parameters (slope to saturation of photo centers (α), saturating irradiance (Ek), maximum electron transport rate (ETRmax), maximum quantum yield of photosystem II (Fv/Fm)), growth, chlorophyll a concentration, and for C. obtectulum calcium carbonate content and Mg/Ca ratio. No negative impacts of elevated temperature or increased pCO2 were observed in either species. The fleshy alga decreased in size in low pH and high temperature treatments alone, but increased growth significantly when these factors were combined. Photosynthetic parameters were depressed by increased temperature in the calcified species and pH in the fleshy species but no significant differences were observed in other parameters in either species. This indicates that Hildenbrandia sp. may have a competitive advantage for space in the subtidal environment in near future oceanic conditions. However because benthic ecology in this geographic region is not well understood it is uncertain how these results will ultimately impact the community.
    Full-text Article · Jan 2016 · Journal of Experimental Marine Biology and Ecology
  • Spencer J Smith · Jeffrey M Montgomery · Yogesh K Vohra
    [Show abstract] [Hide abstract] ABSTRACT: High-pressure high-temperature (HPHT) Raman spectroscopy studies have been performed on the organic crystal paracetamol in a diamond anvil cell utilizing boron-doped heating diamond anvil. Isobaric measurements were conducted at pressures up to 8.5 GPa and temperature up to 520 K in five different experiments. Solid state phase transitions from monoclinic Form I → orthorhombic Form II were observed at various pressures and temperatures as well as transitions from Form II → unknown Form IV. The melting temperature for paracetamol was observed to increase with increasing pressures to 8.5 GPa. This new data is combined with previous ambient temperature high-pressure Raman and x-ray diffraction data to create the first HPHT phase diagram of paracetamol.
    Article · Jan 2016 · Journal of Physics Condensed Matter
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: Electrospinning technique was utilized to engineer a small-diameter (id = 4 mm) tubular graft. The tubular graft was made from biocompatible and biodegradable polymers polycaprolactone (PCL) and poliglecaprone with 3:1 (PCL:PGC) ratio. Enzymatic degradation effect on the mechanical properties and fiber morphology in the presence of lipase enzyme were observed. Significant changes in tensile strength (1.86-1.49 MPa) and strain (245-205 %) were noticed after 1 month in vitro degradation. The fiber breakage was clearly evident through scanning electron microscopy (SEM) after 4 weeks in vitro degradation. Then, the graft was coated with a collagenous protein matrix to impart bioactivity. Human umbilical vein endothelial cells (HUVECs) and aortic artery smooth muscle cells (AoSMCs) attachment on the coated graft were observed in static condition. Further, HUVECs were seeded on the lumen surface of the grafts and exposed to laminar shear stress for 12 h to understand the cell attachment. The coated graft was aged in PBS solution (pH 7.3) at 37 °C for 1 month to understand the coating stability. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) suggested the erosion of the protein matrix from the coated graft under in vitro condition.
    Full-text Article · Nov 2015
  • Source
    Ryan L. Stillwell · Jason R. Jeffries · Scott K. McCall · [...] · Yogesh K. Vohra
    [Show abstract] [Hide abstract] ABSTRACT: We have performed the first high-pressure magnetotransport and x-ray diffraction measurements on ferromagnetic LaCo5, confirming the theoretically predicted electronic topological transition driving the magneto-elastic collapse seen in the related compound YCo5. Our x-ray diffraction results show an anisotropic lattice collapse of the c-axis near 10 GPa that is also commensurate with a change in the majority charge carriers evident from high-pressure Hall effect measurements. The coupling of the electronic, magnetic and lattice degrees of freedom is further substantiated by the evolution of the anomalous Hall effect, which couples to the magnetization of the ordered state of LaCo5.
    Full-text Article · Nov 2015 · Physical Review B
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: Using high-pressure magnetotransport techniques we have discovered superconductivity in Bi2Te, a member of the infinitely adaptive (Bi2)m(Bi2Te3)n series, whose end members, Bi and Bi2Te3, can be tuned to display topological surface states or superconductivity. Bi2Te has a maximum Tc= 8.6 K at P= 14.5 GPa and goes through multiple high pressure phase transitions, ultimately collapsing into a bcc structure that suggests a universal behavior across the series. High-pressure magnetoresistance and Hall measurements suggest a semi-metal to metal transition near 5.4 GPa, which accompanies the hexagonal to intermediate phase transition seen via x-ray diffraction measurements. In addition, the linearity of Hc2(T) exceeds the Werthamer-Helfand-Hohenberg limit, even in the extreme spin-orbit scattering limit, yet is consistent with other strong spin-orbit materials. Considering these results in combination with similar reports on strong spin-orbit scattering materials seen in the literature, we suggest the need for a new theory that can address the unconventional nature of their superconducting states.
    Full-text Article · Nov 2015
  • Source
    [Show abstract] [Hide abstract] ABSTRACT: We report thermodynamic and transport properties, and also theoretical calculations, for Cu-based compound Ca2Cu6P5 and compare with CaCu(2-x)P2. Both materials have layers of edge-sharing copper pnictide tetrahedral CuP4, similar to Fe-As and Fe-Se layers (with FeAs4, FeSe4) in the iron-based superconductors. Despite the presence of this similar transition-metal pnictide layer, we find that both Ca2Cu6P5 and CaCu(2-x)P2 have temperature-independent magnetic susceptibility and show metallic behavior with no evidence of either magnetic ordering or superconductivity down to 1.8 K. CaCu(2-x)P2 is slightly off-stoichiometric, with delta = 0.14. Theoretical calculations suggest that unlike Fe 3d-based magnetic materials with a large density of states (DOS) at the Fermi surface, Cu have comparatively low DOS, with the majority of the 3d spectral weight located well below Fermi level. The room-temperature resistivity value of Ca2Cu6P5 is only 9 micro ohm-cm, due to a substantial plasma frequency and an inferred electron-phonon coupling lambda of 0.073 (significantly smaller than that of metallic Cu). Also, microscopy result shows that Cu-Cu distance along the c-axis within the double layers can be very short (2.5 A), even shorter than metallic elemental copper bond (2.56 A). The value of dp over dT for CaCu(2-x)P2 at 300 K is approximately three times larger than in Ca2Cu6P5, which suggests the likelihood of stronger electron-phonon coupling. This study shows that the details of Cu-P layers and bonding are important for their transport characteristics. In addition, it emphasizes the remarkable character of the DOS of '122' iron-based materials, despite much structural similarities.
    Full-text Article · Sep 2015 · Journal of Alloys and Compounds
  • [Show abstract] [Hide abstract] ABSTRACT: The pressure-temperature (P-T) phase diagram of 1,1-diamino-2,2-dinitroethylene (FOX-7) was determined by in situ synchrotron infrared radiation spectroscopy with resistively heated diamond anvil cell (DAC) technique. The stability of high P-T FOX-7 polymorphs is established from ambient pressure up to 10 GPa and temperatures until decomposition. The phase diagram indicates two near isobaric phase boundaries at ~2 GPa (α → I) and ~5 GPa (I → II) that persists from 25oC until the onset of decomposition at ~300oC. In addition, the ambient pressure, high temperature α→β phase transition (~111oC) lies along a steep boundary (~100 oC/GPa) with a α-β-δ triple point at ~1 GPa and 300 oC. A 0.9 GPa isobaric temperature ramping measurement indicates a limited stability range for the γ-phase between 0.5 - 0.9 GPa and 180 - 260oC terminating in a β-γ-δ triple point. With increasing pressure, the δ-phase exhibited a small negative dT/dP slope (up to ~0.2 GPa) before turning over to a positive 70oC/GPa slope, at higher pressures. The decomposition boundary (~55oC/GPa) was identified through the emergence of spectroscopic signatures of the characteristic decomposition products as well as trapped inclusions within the solid KBr pressure media.
    Article · Aug 2015 · The Journal of Physical Chemistry A
  • Vinoy Thomas · Danna Nozik · Harsh Patel · [...] · Yogesh K. Vohra
    [Show abstract] [Hide abstract] ABSTRACT: Tubular grafts were fabricated from blends of polycaprolactone (PCL) and poly(glycolide-co-caprolactone) (PGC) polymers and coated with an extracellular matrix containing collagens, laminin, and proteoglycans, but not growth factors (HuBiogel™). Multifunctional scaffolds from polymer blends and membrane proteins provide the necessary biomechanics and biological functions for tissue regeneration. Two crosslinking agents, a natural crosslinker namely genipin (Gp) and a carbodiimide reagent namely 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), were used for further stabilizing the protein matrix and the effect of crosslinking was evaluated for structural, morphological, mechanical properties using SEM, DSC and DMA. SEM images and fiber diameter distribution showed fiber-size between 0.2 μm to 1 μm with the majority of fiber diameters being under 500 nm, indicating upper range of protein fiber-sizes (for example, collagen fibers in extracellular matrix are in 50 to 500 nm diameter range). HB coating did not affect the mechanical properties, but increased its hydrophilicity of the graft. Overall data showed that PCL/PGC blends with 3:1 mass ratio exhibited mechanical properties comparable to those of human native arteries (tensile strength of 1-2 MPa and Young’s modulus of <10 MPa). Additionally, the effect of crosslinking on coating stability was investigated to assure the retention of proteins on scaffold for effective cell-matrix interactions.
    Conference Paper · Jun 2015
  • Yogesh K. Vohra · Gopi K. Samudrala · Samuel L. Moore · [...] · Nenad Velisavljevic
    [Show abstract] [Hide abstract] ABSTRACT: Ultra-high static pressures have been achieved in the laboratory using a two-stage micro-ball nanodiamond anvils as well as a two-stage micro-paired diamond anvils machined using a focused ion-beam system. The two-stage diamond anvils’ designs implemented thus far suffer from a limitation of one diamond anvil sliding past another anvil at extreme conditions. We describe a new method of fabricating two-stage diamond micro-anvils using a tungsten mask on a standard diamond anvil followed by microwave plasma chemical vapor deposition (CVD) homoepitaxial diamond growth. A prototype two-stage diamond anvil with 300 µm culet and with a CVD diamond second stage of 50 µm in diameter was fabricated. We have carried out preliminary high pressure X-ray diffraction studies on a sample of rare-earth metal lutetium sample with a copper pressure standard to 86 GPa. The micro-anvil grown by CVD remained intact during indentation of gasket as well as on decompression from the highest pressure of 86 GPa.
    Article · Jun 2015 · High Pressure Research
  • Source
    Gopi K. Samudrala · Samuel L. Moore · Yogesh K. Vohra
    [Show abstract] [Hide abstract] ABSTRACT: Electrical and magnetic sensors can be lithographically fabricated on top of diamond substrates and encapsulated in a protective layer of chemical vapor deposited single crystalline diamond. This process when carried out on single crystal diamond anvils employed in high pressure research is termed as designer diamond anvil fabrication. These designer diamond anvils allow researchers to study electrical and magnetic properties of materials under extreme conditions without any possibility of damaging the sensing elements. We describe a novel method for the fabrication of designer diamond anvils with the use of maskless lithography and chemical vapor deposition in this paper. This method can be utilized to produce diamond based sensors which can function in extreme environments of high pressures, high and low temperatures, corrosive and high radiation conditions. We demonstrate applicability of these diamonds under extreme environments by performing electrical resistance measurements during superconducting transition in rare earth doped iron-based compounds under high pressures to 12 GPa and low temperatures to 10 K.
    Full-text Article · May 2015 · Materials
  • J R Jeffries · N P Butch · Y K Vohra · S T Weir
    [Show abstract] [Hide abstract] ABSTRACT: The group V-VI compounds—like Bi2Se3, Sb2Te3, or Bi2Te3—have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and crystal structure of a pseudobinary (Bi,Sb)2(Te,Se)3 compound. Similar to some of its sister compounds, the (Bi,Sb)2(Te,Se)3 pseudobinary compound undergoes multiple, pressure-induced phase transformations that result in metallization, the onset of a close-packed crystal structure, and the development of distinct superconducting phases.
    Article · Mar 2015 · Journal of Physics Conference Series
  • Samuel L. Moore · Yogesh K. Vohra
    [Show abstract] [Hide abstract] ABSTRACT: Chemical Vapor Deposited (CVD) diamond growth on (111)-diamond surfaces has received increased attention lately because of the use of N-V related centers in quantum computing as well as application of these defect centers in sensing nano-Tesla strength magnetic fields. We have carried out a detailed study of homoepitaxial diamond deposition on (111)-single crystal diamond (SCD) surfaces using a 1.2 kW microwave plasma CVD (MPCVD) system employing methane/hydrogen/nitrogen/oxygen gas phase chemistry. We have utilized Type Ib (111)-oriented single crystal diamonds as seed crystals in our study. The homoepitaxially grown diamond films were analyzed by Raman spectroscopy, Photoluminescence Spectroscopy (PL), X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The nitrogen concentration in the plasma was carefully varied between 0 and 1500 ppm while a ppm level of silicon impurity is present in the plasma from the quartz bell jar. The concentration of N-V defect centers with PL zero phonon lines (ZPL) at 575nm and 637nm and the Si-defect center with a ZPL at 737nm were experimentally detected from a variation in CVD growth conditions and were quantitatively studied. Altering nitrogen and oxygen concentration in the plasma was observed to directly affect N-V and Si-defect incorporation into the (111)-oriented diamond lattice and these findings are presented.
    Article · Jan 2015 · MRS Online Proceeding Library Archive
  • Danna Nozik · Harsh Patel · Yogesh Vohra · Vinoy Thomas
    [Show abstract] [Hide abstract] ABSTRACT: There is a huge demand for small-diameter vascular grafts, as the majority of vascular disease cases involve small-caliber blood vessels. Recently, electrospinning has gained attention as a valuable technique for the fabrication of scaffolds for blood vessel engineering, as electrospinning produces nanofibers that closely approximate the structure of native extracellular matrix (ECM). Accordingly, electrospun scaffolds were fabricated in a 3D tubular structure from a blend of the synthetic polymers, viscoelastic and durable polycaprolactone (PCL) and relatively fast-degrading shape memory poliglecaprone (PGC). The scaffolds were coated with a physiological biomatrix, HuBiogelTM. The biohybrid graft was found to exhibit mechanical properties comparable to those of native blood vessels, and the HuBiogelTM coating imparted bioactivity. The coating was crosslinked using EDC and the natural crosslinker genipin (Gp) to improve its stability in physiological conditions. This study evaluated the effects of EDC- and Gp-crosslinking on the scaffold mechanical, structural, and morphological properties. Additionally, coating stability was studied to assure the presence of collagenous biomatrix on the scaffold for effective cell-matrix interactions. Mechanical testing showed little difference between EDC- and Gp-crosslinked scaffolds; both retained mechanical properties in the range of native human arteries (tensile strength 1-2 MPa, tensile modulus 9-12 MPa). SEM imaging revealed that while crosslinking with EDC resulted in an increase in fiber diameter compared to uncrosslinked scaffolds, Gp-crosslinking did not affect fiber diameter; the majority of fibers in EDC and Gp-crosslinked scaffolds had diameters ranging from approximately 0.9 – 1.4 microns and 0.7 – 1.2 microns, respectively. This is in the upper range of fiber diameters in native extracellular matrix. Coating stability studies using picrosirius red (PSR) stain showed that EDC-crosslinked scaffolds were more effective than Gp-crosslinked scaffolds in enhancing the stability of the biomatrix coating.
    Conference Paper · Nov 2014
  • [Show abstract] [Hide abstract] ABSTRACT: Blends of poliglecaprone (PGC) and polycaprolactone (PCL) of varying compositions were electrospun into tubular conduits and their mechanical, morphological, thermal and in vitro degradation properties were evaluated under simulated physiological conditions. Generally, mechanical strength, modulus and hydrophilic nature were enhanced by the addition of PGC to PCL. In vitro degradation study in PBS (pH of 7.3) was carried out for up to one month to understand the hydrolytic degradation effect on the mechanical properties in both longitudinal and circumferential directions. Pure PCL and 4:1PCL/PGC blend scaffolds exhibited considerable elastic stiffening after a one-month long in vitro degradation. FT-IR and DSC techniques were used to understand the degradation behavior and the changes in structure and crystallinity of the polymeric blends. 3:1 PCL/PGC blend was concluded to be a judicious blend composition for tubular graft based on overall results from mechanical properties and performance after one month in vitro degradation study.
    Article · Nov 2014 · Polymer International

Publication Stats

8k Citations


  • 2013
    • University of Maryland, College Park
      • Department of Physics
      College Park, MD, United States
  • 1995-2007
    • University of Alabama at Birmingham
      • Department of Physics
      Birmingham, Alabama, United States
    • University of Alabama
      Tuscaloosa, Alabama, United States
  • 1978-2001
    • Bhabha Atomic Research Centre
      • Chemistry Division
      Mumbai, State of Maharashtra, India
  • 1985
    • Cornell University
      • Department of Materials Science and Engineering
      Итак, New York, United States
  • 1982
    • Universität Paderborn
      Paderborn, North Rhine-Westphalia, Germany